386INTEL.PT1

INTEL 80386 PROGRAMMER'S REFERENCE MANUAL 1986

Intel Corporation makes no warranty for the use of its products and
assumes no responsibility for any errors which may appear in this document
nor does it make a commitment to update the information contained herein.

Intel retains the right to make changes to these specifications at any
time, without notice.

Contact your local sales office to obtain the latest specifications before
placing your order.

The following are trademarks of Intel Corporation and may only be used to
identify Intel Products:

Above, BITBUS, COMMputer, CREDIT, Data Pipeline, FASTPATH, Genius, i, Œ,
ICE, iCEL, iCS, iDBP, iDIS, IýICE, iLBX, im, iMDDX, iMMX, Inboard,
Insite, Intel, intel, intelBOS, Intel Certified, Intelevision,
inteligent Identifier, inteligent Programming, Intellec, Intellink,
iOSP, iPDS, iPSC, iRMK, iRMX, iSBC, iSBX, iSDM, iSXM, KEPROM, Library
Manager, MAPNET, MCS, Megachassis, MICROMAINFRAME, MULTIBUS, MULTICHANNEL,
MULTIMODULE, MultiSERVER, ONCE, OpenNET, OTP, PC BUBBLE, Plug-A-Bubble,
PROMPT, Promware, QUEST, QueX, Quick-Pulse Programming, Ripplemode, RMX/80,
RUPI, Seamless, SLD, SugarCube, SupportNET, UPI, and VLSiCEL, and the
combination of ICE, iCS, iRMX, iSBC, iSBX, iSXM, MCS, or UPI and a numerical
suffix, 4-SITE.

MDS is an ordering code only and is not used as a product name or
trademark. MDS(R) is a registered trademark of Mohawk Data Sciences
Corporation.

Additional copies of this manual or other Intel literature may be obtained
from:

Intel Corporation
Literature Distribution
Mail Stop SC6-59
3065 Bowers Avenue
Santa Clara, CA 95051

(c)INTEL CORPORATION 1987    CG-5/26/87


Customer Support

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Customer Support is Intel's complete support service that provides Intel
customers with hardware support, software support, customer training, and
consulting services. For more information contact your local sales offices.

After a customer purchases any system hardware or software product,
service and support become major factors in determining whether that
product will continue to meet a customer's expectations. Such support
requires an international support organization and a breadth of programs
to meet a variety of customer needs. As you might expect, Intel's customer
support is quite extensive. It includes factory repair services and
worldwide field service offices providing hardware repair services,
software support services, customer training classes, and consulting
services.

Hardware Support Services

Intel is committed to providing an international service support package
through a wide variety of service offerings available from Intel Hardware
Support.

Software Support Services

Intel's software support consists of two levels of contracts. Standard
support includes TIPS (Technical Information Phone Service), updates and
subscription service (product-specific troubleshooting guides and COMMENTS
Magazine). Basic support includes updates and the subscription service.
Contracts are sold in environments which represent product groupings
(i.e., iRMX environment).

Consulting Services

Intel provides field systems engineering services for any phase of your
development or support effort. You can use our systems engineers in a
variety of ways ranging from assistance in using a new product, developing
an application, personalizing training, and customizing or tailoring an
Intel product to providing technical and management consulting. Systems
Engineers are well versed in technical areas such as microcommunications,
real-time applications, embedded microcontrollers, and network services.
You know your application needs; we know our products. Working together we
can help you get a successful product to market in the least possible time.

Customer Training

Intel offers a wide range of instructional programs covering various
aspects of system design and implementation. In just three to ten days a
limited number of individuals learn more in a single workshop than in
weeks of self-study. For optimum convenience, workshops are scheduled
regularly at Training Centers woridwide or we can take our workshops to
you for on-site instruction. Covering a wide variety of topics, Intel's
major course categories include: architecture and assembly language,
programming and operating systems, bitbus and LAN applications.

Training Center Locations

To obtain a complete catalog of our workshops, call the nearest Training
Center in your area.

Boston                    (617) 692-1000
Chicago                   (312) 310-5700
San Francisco             (415) 940-7800
Washington D.C.           (301) 474-2878
Isreal                    (972) 349-491-099
Tokyo                     03-437-6611
Osaka (Call Tokyo)        03-437-6611
Toronto, Canada           (416) 675-2105
London                    (0793) 696-000
Munich                    (089) 5389-1
Paris                     (01) 687-22-21
Stockholm                 (468) 734-01-00
Milan                     39-2-82-44-071
Benelux (Rotterdam)       (10) 21-23-77
Copenhagen                (1) 198-033
Hong Kong                 5-215311-7


Table of Contents

Chapter 1  Introduction to the 80386

1.1  Organization of This Manual
      1.1.1  Part I ÄÄ Applications Programming
      1.1.2  Part II ÄÄ Systems Programming
      1.1.3  Part III ÄÄ Compatibility
      1.1.4  Part IV ÄÄ Instruction Set
      1.1.5  Appendices

1.2  Related Literature
1.3  Notational Conventions
      1.3.1  Data-Structure Formats
      1.3.2  Undefined Bits and Software Compatibility
      1.3.3  Instruction Operands
      1.3.4  Hexadecimal Numbers
      1.3.5  Sub- and Super-Scripts

                     PART I    APPLICATIONS PROGRAMMING                   

Chapter 2  Basic Programming Model

2.1  Memory Organization and Segmentation
      2.1.1  The"Flat" Model
      2.1.2  The Segmented Model

2.2  Data Types
2.3  Registers
      2.3.1  General Registers
      2.3.2  Segment Registers
      2.3.3  Stack Implementation
      2.3.4  Flags Register
              2.3.4.1  Status Flags
              2.3.4.2  Control Flag
              2.3.4.3  Instruction Pointer

2.4  Instruction Format
2.5  Operand Selection
      2.5.1  Immediate Operands
      2.5.2  Register Operands
      2.5.3  Memory Operands
              2.5.3.1  Segment Selection
              2.5.3.2  Effective-Address Computation

2.6  Interrupts and Exceptions

Chapter 3  Applications Instruction Set

3.1  Data Movement Instructions
      3.1.1  General-Purpose Data Movement Instructions
      3.1.2  Stack Manipulation Instructions
      3.1.3  Type Conversion Instructions

3.2  Binary Arithmetic Instructions
      3.2.1  Addition and Subtraction Instructions
      3.2.2  Comparison and Sign Change Instruction
      3.2.3  Multiplication Instructions
      3.2.4  Division Instructions

3.3  Decimal Arithmetic Instructions
      3.3.1  Packed BCD Adjustment Instructions
      3.3.2  Unpacked BCD Adjustment Instructions

3.4  Logical Instructions
      3.4.1  Boolean Operation Instructions
      3.4.2  Bit Test and Modify Instructions
      3.4.3  Bit Scan Instructions
      3.4.4  Shift and Rotate Instructions
              3.4.4.1  Shift Instructions
              3.4.4.2  Double-Shift Instructions
              3.4.4.3  Rotate Instructions
              3.4.4.4  Fast"bit-blt" Using Double Shift
                         Instructions
              3.4.4.5  Fast Bit-String Insert and Extract

      3.4.5  Byte-Set-On-Condition Instructions
      3.4.6  Test Instruction

3.5  Control Transfer Instructions
      3.5.1  Unconditional Transfer Instructions
              3.5.1.1  Jump Instruction
              3.5.1.2  Call Instruction
              3.5.1.3  Return and Return-From-Interrupt Instruction

      3.5.2  Conditional Transfer Instructions
              3.5.2.1  Conditional Jump Instructions
              3.5.2.2  Loop Instructions
              3.5.2.3  Executing a Loop or Repeat Zero Times

      3.5.3  Software-Generated Interrupts

3.6  String and Character Translation Instructions
      3.6.1  Repeat Prefixes
      3.6.2  Indexing and Direction Flag Control
      3.6.3  String Instructions

3.7  Instructions for Block-Structured Languages
3.8  Flag Control Instructions
      3.8.1  Carry and Direction Flag Control Instructions
      3.8.2  Flag Transfer Instructions

3.9  Coprocessor Interface Instructions
3.10 Segment Register Instructions
      3.10.1  Segment-Register Transfer Instructions
      3.10.2  Far Control Transfer Instructions
      3.10.3  Data Pointer Instructions

3.11  Miscellaneous Instructions
       3.11.1  Address Calculation Instruction
       3.11.2  No-Operation Instruction
       3.11.3  Translate Instruction

                       PART II    SYSTEMS PROGRAMMING                     

Chapter 4  Systems Architecture

4.1  Systems Registers
      4.1.1  Systems Flags
      4.1.2  Memory-Management Registers
      4.1.3  Control Registers
      4.1.4  Debug Register
      4.1.5  Test Registers

4.2  Systems Instructions

Chapter 5  Memory Management

5.1  Segment Translation
      5.1.1  Descriptors
      5.1.2  Descriptor Tables
      5.1.3  Selectors
      5.1.4  Segment Registers

5.2  Page Translation
      5.2.1  Page Frame
      5.2.2  Linear Address
      5.2.3  Page Tables
      5.2.4  Page-Table Entries
              5.2.4.1  Page Frame Address
              5.2.4.2  Present Bit
              5.2.4.3  Accessed and Dirty Bits
              5.2.4.4  Read/Write and User/Supervisor Bits

      5.2.5  Page Translation Cache

5.3  Combining Segment and Page Translation
      5.3.1  "Flat" Architecture
      5.3.2  Segments Spanning Several Pages
      5.3.3  Pages Spanning Several Segments
      5.3.4  Non-Aligned Page and Segment Boundaries
      5.3.5  Aligned Page and Segment Boundaries
      5.3.6  Page-Table per Segment

Chapter 6  Protection

6.1  Why Protection?
6.2  Overview of 80386 Protection Mechanisms
6.3  Segment-Level Protection
      6.3.1  Descriptors Store Protection Parameters
              6.3.1.1  Type Checking
              6.3.1.2  Limit Checking
              6.3.1.3  Privilege Levels

      6.3.2  Restricting Access to Data
              6.3.2.1  Accessing Data in Code Segments

      6.3.3  Restricting Control Transfers
      6.3.4  Gate Descriptors Guard Procedure Entry Points
              6.3.4.1  Stack Switching
              6.3.4.2  Returning from a Procedure

      6.3.5  Some Instructions are Reserved for Operating System
              6.3.5.1  Privileged Instructions
              6.3.5.2  Sensitive Instructions

      6.3.6  Instructions for Pointer Validation
              6.3.6.1  Descriptor Validation
              6.3.6.2  Pointer Integrity and RPL

6.4  Page-Level Protection
      6.4.1  Page-Table Entries Hold Protection Parameters
              6.4.1.1  Restricting Addressable Domain
              6.4.1.2  Type Checking

      6.4.2  Combining Protection of Both Levels of Page Tables
      6.4.3  Overrides to Page Protection

6.5  Combining Page and Segment Protection

Chapter 7  Multitasking

7.1  Task State Segment
7.2  TSS Descriptor
7.3  Task Register
7.4  Task Gate Descriptor
7.5  Task Switching
7.6  Task Linking
      7.6.1  Busy Bit Prevents Loops
      7.6.2  Modifying Task Linkages

7.7  Task Address Space
      7.7.1  Task Linear-to-Physical Space Mapping
      7.7.2  Task Logical Address Space

Chapter 8    Input/Output

8.1  I/O Addressing
      8.1.1  I/O Address Space
      8.1.2  Memory-Mapped I/O

8.2  I/O Instructions
      8.2.1  Register I/O Instructions
      8.2.2  Block I/O Instructions

8.3  Protection and I/O
      8.3.1  I/O Privilege Level
      8.3.2  I/O Permission Bit Map

Chapter 9  Exceptions and Interrupts

9.1  Identifying Interrupts
9.2  Enabling and Disabling Interrupts
      9.2.1  NMI Masks Further NMls
      9.2.2  IF Masks INTR
      9.2.3  RF Masks Debug Faults
      9.2.4  MOV or POP to SS Masks Some Interrupts and Exceptions

9.3  Priority Among Simultaneous Interrupts and Exceptions
9.4  Interrupt Descriptor Table
9.5  IDT Descriptors
9.6  Interrupt Tasks and Interrupt Procedures
      9.6.1  Interrupt Procedures
              9.6.1.1  Stack of Interrupt Procedure
              9.6.1.2  Returning from an Interrupt Procedure
              9.6.1.3  Flags Usage by Interrupt Procedure
              9.6.1.4  Protection in Interrupt Procedures

      9.6.2  Interrupt Tasks

9.7  Error Code
9.8  Exception Conditions
      9.8.1  Interrupt 0 ÄÄ Divide Error
      9.8.2  Interrupt 1 ÄÄ Debug Exceptions
      9.8.3  Interrupt 3 ÄÄ Breakpoint
      9.8.4  Interrupt 4 ÄÄ Overflow
      9.8.5  Interrupt 5 ÄÄ Bounds Check
      9.8.6  Interrupt 6 ÄÄ Invalid Opcode
      9.8.7  Interrupt 7 ÄÄ Coprocessor Not Available
      9.8.8  Interrupt 8 ÄÄ Double Fault
      9.8.9  Interrupt 9 ÄÄ Coprocessor Segment Overrun
      9.8.10 Interrupt 10 ÄÄ Invalid TSS
      9.8.11 Interrupt 11 ÄÄ Segment Not Present
      9.8.12 Interrupt 12 ÄÄ Stack Exception
      9.8.13 Interrupt 13 ÄÄ General Protection Exception
      9.8.14 Interrupt 14 ÄÄ Page Fault
              9.8.14.1  Page Fault during Task Switch
              9.8.14.2  Page Fault with Inconsistent Stack Pointer

      9.8.15 Interrupt 16 ÄÄ Coprocessor Error

9.9  Exception Summary

9.10  Error Code Summary

Chapter 10  Initialization

10.1  Processor State after Reset
10.2  Software Initialization for Real-Address Mode
       10.2.1  Stack
       10.2.2  Interrupt Table
       10.2.3  First Instructions

10.3  Switching to Protected Mode
10.4  Software Initialization for Protected Mode
       10.4.1  Interrupt Descriptor Table
       10.4.2  Stack
       10.4.3  Global Descriptor Table
       10.4.4  Page Tables
       10.4.5  First Task

10.5  Initialization Example
10.6  TLB Testing
       10.6.1  Structure of the TLB
       10.6.2  Test Registers
       10.6.3  Test Operations

Chapter 11  Coprocessing and Multiprocessing

11.1  Coprocessing
       11.1.1  Coprocessor Identification
       11.1.2  ESC and WAIT Instructions
       11.1.3  EM and MP Flags
       11.1.4  The Task-Switched Flag
       11.1.5  Coprocessor Exceptions
                11.1.5.1  Interrupt 7 ÄÄ Coprocessor Not Available
                11.1.5.2  Interrupt 9 ÄÄ Coprocessor Segment Overrun
                11.1.5.3  Interrupt 16 ÄÄ Coprocessor Error

11.2  General Multiprocessing
       11.2.1  LOCK and the LOCK# Signal
       11.2.2  Automatic Locking
       11.2.3  Cache Considerations

Chapter 12  Debugging

12.1  Debugging Features of the Architecture
12.2  Debug Registers
       12.2.1  Debug Address Registers (DRO-DR3)
       12.2.2  Debug Control Register (DR7)
       12.2.3  Debug Status Register (DR6)
       12.2.4  Breakpoint Field Recognition

12.3  Debug Exceptions
       12.3.1  Interrupt 1 ÄÄ Debug Exceptions
                12.3.1.1  Instruction Address Breakpoint
                12.3.1.2  Data Address Breakpoint
                12.3.1.3  General Detect Fault
                12.3.1.4  Single-Step Trap
                12.3.1.5  Task Switch Breakpoint

       12.3.2  Interrupt 3 ÄÄ Breakpoint Exception

                          PART III    COMPATIBILITY                       

Chapter 13  Executing 80286 Protected-Mode Code

13.1  80286 Code Executes as a Subset of the 80386
13.2  Two Ways to Execute 80286 Tasks
13.3  Differences from 80286
       13.3.1  Wraparound of 80286 24-Bit Physical Address Space
       13.3.2  Reserved Word of Descriptor
       13.3.3  New Descriptor Type Codes
       13.3.4  Restricted Semantics of LOCK
       13.3.5  Additional Exceptions

Chapter 14  80386 Real-Address Mode

14.1  Physical Address Formation
14.2  Registers and Instructions
14.3  Interrupt and Exception Handling
14.4  Entering and Leaving Real-Address Mode
       14.4.1  Switching to Protected Mode

14.5  Switching Back to Real-Address Mode
14.6  Real-Address Mode Exceptions
14.7  Differences from 8086
14.8  Differences from 80286 Real-Address Mode
       14.8.1  Bus Lock
       14.8.2  Location of First Instruction
       14.8.3  Initial Values of General Registers
       14.8.4  MSW Initialization

Chapter 15  Virtual 8088 Mode

15.1  Executing 8086 Code
       15.1.1  Registers and Instructions
       15.1.2  Linear Address Formation

15.2  Structure of a V86 Task
       15.2.1  Using Paging for V86 Tasks
       15.2.2  Protection within a V86 Task

15.3  Entering and Leaving V86 Mode
       15.3.1  Transitions Through Task Switches
       15.3.2  Transitions Through Trap Gates and Interrupt Gates

15.4  Additional Sensitive Instructions
       15.4.1  Emulating 8086 Operating System Calls
       15.4.2  Virtualizing the Interrupt-Enable Flag

15.5  Virtual I/O
       15.5.1  I/O-Mapped I/O
       15.5.2  Memory-Mapped I/O
       15.5.3  Special I/O Buffers

15.6  Differences from 8086
15.7  Differences from 80286 Real-Address Mode

Chapter 16  Mixing 16-Bit and 32-Bit Code

16.1  How the 80386 Implements 16-Bit and 32-Bit Features
16.2  Mixing 32-Bit and 16-Bit Operations
16.3  Sharing Data Segments among Mixed Code Segments
16.4  Transferring Control among Mixed Code Segments
       16.4.1  Size of Code-Segment Pointer
       16.4.2  Stack Management for Control Transfers
                16.4.2.1  Controlling the Operand-Size for a CALL
                16.4.2.2  Changing Size of Call

       16.4.3  Interrupt Control Transfers
       16.4.4  Parameter Translation
       16.4.5  The Interface Procedure

                         PART IV    INSTRUCTION SET                       

Chapter 17  80386 Instruction Set

17.1  Operand-Size and Address-Size Attributes
       17.1.1  Default Segment Attribute
       17.1.2  Operand-Size and Address-Size Instruction Prefixes
       17.1.3  Address-Size Attribute for Stack

17.2  Instruction Format
       17.2.1  ModR/M and SIB Bytes
       17.2.2  How to Read the Instruction Set Pages
                17.2.2.1  Opcode
                17.2.2.2  Instruction
                17.2.2.3  Clocks
                17.2.2.4  Description
                17.2.2.5  Operation
                17.2.2.6  Description
                17.2.2.7  Flags Affected
                17.2.2.8  Protected Mode Exceptions
                17.2.2.9  Real Address Mode Exceptions
                17.2.2.10 Virtual-8086 Mode Exceptions

Instruction Sets

AAA
AAD
AAM
AAS
ADC
ADD
AND
ARPL
BOUND
BSF
BSR
BT
BTC
BTR
BTS
CALL
CBW/CWDE
CLC
CLD
CLI
CLTS
CMC
CMP
CMPS/CMPSB/CMPSW/CMPSD
CWD/CDQ
DAA
DAS
DEC
DIV
ENTER
HLT
IDIV
IMUL
IN
INC
INS/INSB/INSW/INSD
INT/INTO
IRET/IRETD
Jcc
JMP
LAHF
LAR
LEA
LEAVE
LGDT/LIDT
LGS/LSS/LDS/LES/LFS
LLDT
LMSW
LOCK
LODS/LODSB/LODSW/LODSD
LOOP/LOOPcond
LSL
LTR
MOV
MOV
MOVS/MOVSB/MOVSW/MOVSD
MOVSX
MOVZX
MUL
NEG
NOP
NOT
OR
OUT
OUTS/OUTSB/OUTSW/OUTSD
POP
POPA/POPAD
POPF/POPFD
PUSH
PUSHA/PUSHAD
PUSHF/PUSHFD
RCL/RCR/ROL/ROR
REP/REPE/REPZ/REPNE/REPNZ
RET
SAHF
SAL/SAR/SHL/SHR
SBB
SCAS/SCASB/SCASW/SCASD
SETcc
SGDT/SIDT
SHLD
SHRD
SLDT
SMSW
STC
STD
STI
STOS/STOSB/STOSW/STOSD
STR
SUB
TEST
VERR,VERW
WAIT
XCHG
XLAT/XLATB
XOR

Appendix A  Opcode Map

Appendix B  Complete Flag Cross-Reference

Appendix C  Status Flag Summary

Appendix D  Condition Codes


Figures

1-1     Example Data Structure

2-1     Two-Component Pointer
2-2     Fundamental Data Types
2-3     Bytes, Words, and Doublewords in Memory
2-4     80386 Data Types
2-5     80386 Applications Register Set
2-6     Use of Memory Segmentation
2-7     80386 Stack
2-8     EFLAGS Register
2-9     Instruction Pointer Register
2-10    Effective Address Computation

3-1     PUSH
3-2     PUSHA
3-3     POP
3-4     POPA
3-5     Sign Extension
3-6     SAL and SHL
3-7     SHR
3-8     SAR
3-9     Using SAR to Simulate IDIV
3-10    Shift Left Double
3-11    Shift Right Double
3-12    ROL
3-13    ROR
3-14    RCL
3-15    RCR
3-16    Formal Definition of the ENTER Instruction
3-17    Variable Access in Nested Procedures
3-18    Stack Frame for MAIN at Level 1
3-19    Stack Frame for Prooedure A
3-20    Stack Frame for Procedure B at Level 3 Called from A
3-21    Stack Frame for Procedure C at Level 3 Called from B
3-22    LAHF and SAHF
3-23    Flag Format for PUSHF and POPF

4-1     Systems Flags of EFLAGS Register
4-2     Control Registers

5-1     Address Translation Overview
5-2     Segment Translation
5-3     General Segment-Descriptor Format
5-4     Format of Not-Present Descriptor
5-5     Descriptor Tables
5-6     Format of a Selector
5-7     Segment Registers
5-8     Format of a Linear Address
5-9     Page Translation
5-10    Format of a Page Table Entry
5-11    Invalid Page Table Entry
5-12    80386 Addressing Mechanism
5-13    Descriptor per Page Table

6-1     Protection Fields of Segment Descriptors
6-2     Levels of Privilege
6-3     Privilege Check for Data Access
6-4     Privilege Check for Control Transfer without Gate
6-5     Format of 80386 Call Gate
6-6     Indirect Transfer via Call Gate
6-7     Privilege Check via Call Gate
6-8     Initial Stack Pointers of TSS
6-9     Stack Contents after an Interievel Call
6-10    Protection Fields of Page Table Entries

7-1     80386 32-Bit Task State Segment
7-2     TSS Descriptor for 32-Bit TSS
7-3     Task Register
7-4     Task Gate Descriptor
7-5     Task Gate Indirectly Identifies Task
7-6     Partially-Overlapping Linear Spaces

8-1     Memory-Mapped I/O
8-2     I/O Address Bit Map

9-1     IDT Register and Table
9-2     Pseudo-Descriptor Format for LIDT and SIDT
9-3     80386 IDT Gate Descriptors
9-4     Interrupt Vectoring for Procedures
9-5     Stack Layout after Exception of Interrupt
9-6     Interrupt Vectoring for Tasks
9-7     Error Code Format
9-8     Page-Fault Error Code Format
9-9     CR2 Format

10-1    Contents of EDX after RESET
10-2    Initial Contents of CRO
10-3    TLB Structure
10-4    Test Registers

12-1    Debug Registers

14-1    Real-Address Mode Address Formation

15-1    V86 Mode Address Formation
15-2    Entering and Leaving an 8086 Program
15-3    PL 0 Stack after Interrupt in V86 Task

16-1    Stack after Far 16-Bit and 32-Bit Calls

17-1    80386 Instruction Format
17-2    ModR/M and SIB Byte Formats
17-3    Bit Offset for BIT[EAX, 21]
17-4    Memory Bit Indexing


Tables

2-1      Default Segment Register Selection Rules
2-2      80386 Reserved Exceptions and Interrupts

3-1      Bit Test and Modify Instructions
3-2      Interpretation of Conditional Transfers

6-1      System and Gate Descriptor Types
6-2      Useful Combinations of E, G, and B Bits
6-3      Interievel Return Checks
6-4      Valid Descriptor Types for LSL
6-5      Combining Directory and Page Protection

7-1      Checks Made during a Task Switch
7-2      Effect of Task Switch on BUSY, NT, and Back-Link

9-1      Interrupt and Exception ID Assignments
9-2      Priority Among Simultaneous Interrupts and Exceptions
9-3      Double-Fault Detection Classes
9-4      Double-Fault Definition
9-5      Conditions That Invalidate the TSS
9-6      Exception Summary
9-7      Error-Code Summary

10-1     Meaning of D, U, and W Bit Pairs

12-1     Breakpeint Field Recognition Examples
12-2     Debug Exception Conditions

14-1     80386 Real-Address Mode Exceptions
14-2     New 80386 Exceptions

17-1     Effective Size Attributes
17-2     16-Bit Addressing Forms with the ModR/M Byte
17-3     32-Bit Addressing Forms with the ModR/M Byte
17-4     32-Bit Addressing Forms with the SIB Byte
17-5     Task Switch Times for Exceptions
17-6     80386 Exceptions


Chapter 1  Introduction to the 80386

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

The 80386 is an advanced 32-bit microprocessor optimized for multitasking
operating systems and designed for applications needing very high
performance. The 32-bit registers and data paths support 32-bit addresses
and data types. The processor can address up to four gigabytes of physical
memory and 64 terabytes (2^(46) bytes) of virtual memory. The on-chip
memory-management facilities include address translation registers,
advanced multitasking hardware, a protection mechanism, and paged virtual
memory. Special debugging registers provide data and code breakpoints even
in ROM-based software.


1.1  Organization of This Manual

This book presents the architecture of the 80386 in five parts:

   Part I      ÄÄ Applications Programming
   Part II     ÄÄ Systems Programming
   Part III    ÄÄ Compatibility
   Part IV     ÄÄ Instruction Set
   Appendices

These divisions are determined in part by the architecture itself and in
part by the different ways the book will be used. As the following table
indicates, the latter two parts are intended as reference material for
programmers actually engaged in the process of developing software for the
80386. The first three parts are explanatory, showing the purpose of
architectural features, developing terminology and concepts, and describing
instructions as they relate to specific purposes or to specific
architectural features.

Explanation                Part I   ÄÄ Applications Programming
                           Part II  ÄÄ Systems Programming
                           Part III ÄÄ Compatibility

Reference                  Part IV  ÄÄ Instruction Set
                           Appendices

The first three parts follow the execution modes and protection features of
the 80386 CPU. The distinction between applications features and systems
features is determined by the protection mechanism of the 80386. One purpose
of protection is to prevent applications from interfering with the operating
system; therefore, the processor makes certain registers and instructions
inaccessible to applications programs. The features discussed in Part I are
those that are accessible to applications; the features in Part II are
available only to systems software that has been given special privileges or
in unprotected systems.

The processing mode of the 80386 also determines the features that are
accessible. The 80386 has three processing modes:

   1.  Protected Mode.
   2.  Real-Address Mode.
   3.  Virtual 8086 Mode.

Protected mode is the natural 32-bit environment of the 80386 processor. In
this mode all instructions and features are available.

Real-address mode (often called just "real mode") is the mode of the
processor immediately after RESET. In real mode the 80386 appears to
programmers as a fast 8086 with some new instructions. Most applications of
the 80386 will use real mode for initialization only.

Virtual 8086 mode (also called V86 mode) is a dynamic mode in the sense
that the processor can switch repeatedly and rapidly between V86 mode and
protected mode. The CPU enters V86 mode from protected mode to execute an
8086 program, then leaves V86 mode and enters protected mode to continue
executing a native 80386 program.

The features that are available to applications programs in protected mode
and to all programs in V86 mode are the same. These features form the
content of Part I. The additional features that are available to systems
software in protected mode form Part II. Part III explains real-address
mode and V86 mode, as well as how to execute a mix of 32-bit and 16-bit
programs.

Available in All Modes             Part I ÄÄ Applications Programming

Available in Protected             Part II ÄÄ Systems Programming
Mode Only

Compatibility Modes                Part III ÄÄ Compatibility


1.1.1  Part I ÄÄ Applications Programming

This part presents those aspects of the architecture that are customarily
used by applications programmers.

Chapter 2 ÄÄ Basic Programming Model: Introduces the models of memory
organization. Defines the data types. Presents the register set used by
applications. Introduces the stack. Explains string operations. Defines the
parts of an instruction. Explains addressing calculations. Introduces
interrupts and exceptions as they may apply to applications programming.

Chapter 3 ÄÄ Application Instruction Set: Surveys the instructions commonly
used for applications programming. Considers instructions in functionally
related groups; for example, string instructions are considered in one
section, while control-transfer instructions are considered in another.
Explains the concepts behind the instructions. Details of individual
instructions are deferred until Part IV, the instruction-set reference.


1.1.2  Part II ÄÄ Systems Programming

This part presents those aspects of the architecture that are customarily
used by programmers who write operating systems, device drivers, debuggers,
and other software that supports applications programs in the protected mode
of the 80386.

Chapter 4 ÄÄ Systems Architecture: Surveys the features of the 80386 that 
are used by systems programmers. Introduces the remaining registers and data
structures of the 80386 that were not discussed in Part I. Introduces the
systems-oriented instructions in the context of the registers and data
structures they support. Points to the chapter where each register, data
structure, and instruction is considered in more detail.

Chapter 5 ÄÄ Memory Management: Presents details of the data structures,
registers, and instructions that support virtual memory and the concepts of
segmentation and paging. Explains how systems designers can choose a model
of memory organization ranging from completely linear ("flat") to fully
paged and segmented.

Chapter 6 ÄÄ Protection: Expands on the memory management features of the
80386 to include protection as it applies to both segments and pages.
Explains the implementation of privilege rules, stack switching, pointer
validation, user and supervisor modes. Protection aspects of multitasking
are deferred until the following chapter.

Chapter 7 ÄÄ Multitasking: Explains how the hardware of the 80386 supports
multitasking with context-switching operations and intertask protection.

Chapter 8 ÄÄ Input/Output: Reveals the I/O features of the 80386, including
I/O instructions, protection as it relates to I/O, and the I/O permission
map.

Chapter 9 ÄÄ Exceptions and Interrupts: Explains the basic interrupt
mechanisms of the 80386. Shows how interrupts and exceptions relate to
protection. Discusses all possible exceptions, listing causes and including
information needed to handle and recover from the exception.

Chapter 10 ÄÄ Initialization: Defines the condition of the processor after
RESET or power-up. Explains how to set up registers, flags, and data
structures for either real-address mode or protected mode. Contains an
example of an initialization program.

Chapter 11 ÄÄ Coprocessing and Multiprocessing: Explains the instructions
and flags that support a numerics coprocessor and multiple CPUs with shared
memory.

Chapter 12 ÄÄ Debugging: Tells how to use the debugging registers of the
80386.


1.1.3  Part III ÄÄ Compatibility

Other parts of the book treat the processor primarily as a 32-bit machine,
omitting for simplicity its facilities for 16-bit operations. Indeed, the
80386 is a 32-bit machine, but its design fully supports 16-bit operands and
addressing, too. This part completes the picture of the 80386 by explaining
the features of the architecture that support 16-bit programs and 16-bit
operations in 32-bit programs. All three processor modes are used to
execute 16-bit programs: protected mode can directly execute 16-bit 80286
protected mode programs, real mode executes 8086 programs and real-mode
80286 programs, and virtual 8086 mode executes 8086 programs in a
multitasking environment with other 80386 protected-mode programs. In
addition, 32-bit and 16-bit modules and individual 32-bit and 16-bit
operations can be mixed in protected mode.

Chapter 13 ÄÄ Executing 80286 Protected-Mode Code: In its protected mode,
the 80386 can execute complete 80286 protected-mode systems, because 80286
capabilities are a subset of 80386 capabilities.

Chapter 14 ÄÄ 80386 Real-Address Mode: Explains the real mode of the 80386
CPU. In this mode the 80386 appears as a fast real-mode 80286 or fast 8086
enhanced with additional instructions.

Chapter 15 ÄÄ Virtual 8086 Mode: The 80386 can switch rapidly between its
protected mode and V86 mode, giving it the ability to multiprogram 8086
programs along with "native mode" 32-bit programs.

Chapter 16 ÄÄ Mixing 16-Bit and 32-Bit Code: Even within a program or task,
the 80386 can mix 16-bit and 32-bit modules. Furthermore, any given module
can utilize both 16-bit and 32-bit operands and addresses.


1.1.4  Part IV ÄÄ Instruction Set

Parts I, II, and III present overviews of the instructions as they relate
to specific aspects of the architecture, but this part presents the
instructions in alphabetical order, providing the detail needed by
assembly-language programmers and programmers of debuggers, compilers,
operating systems, etc. Instruction descriptions include algorithmic
description of operation, effect of flag settings, effect on flag settings,
effect of operand- or address-size attributes, effect of processor modes,
and possible exceptions.


1.1.5  Appendices

The appendices present tables of encodings and other details in a format
designed for quick reference by assembly-language and systems programmers.


1.2  Related Literature

The following books contain additional material concerning the 80386
microprocessor:

  þ  Introduction to the 80386, order number 231252

  þ  80386 Hardware Reference Manual, order number 231732

  þ  80386 System Software Writer's Guide, order number 231499

  þ  80386 High Performance 32-bit Microprocessor with Integrated Memory
     Management (Data Sheet), order number 231630


1.3  Notational Conventions

This manual uses special notations for data-structure formats, for symbolic
representation of instructions, for hexadecimal numbers, and for super- and
sub-scripts. Subscript characters are surrounded by {curly brackets}, for
example 10{2} = 10 base 2. Superscript characters are preceeded by a caret
and enclosed within (parentheses), for example 10^(3) = 10 to the third
power. A review of these notations will make it easier to read the
manual.

1.3.1  Data-Structure Formats

In illustrations of data structures in memory, smaller addresses appear at
the lower-right part of the figure; addresses increase toward the left and
upwards. Bit positions are numbered from right to left. Figure 1-1
illustrates this convention.


1.3.2  Undefined Bits and Software Compatibility

In many register and memory layout descriptions, certain bits are marked as
undefined. When bits are marked as undefined (as illustrated in Figure
1-1), it is essential for compatibility with future processors that
software treat these bits as undefined. Software should follow these
guidelines in dealing with undefined bits:

  þ  Do not depend on the states of any undefined bits when testing the
     values of registers that contain such bits. Mask out the undefined bits
     before testing.

  þ  Do not depend on the states of any undefined bits when storing them in
     memory or in another register.

  þ  Do not depend on the ability to retain information written into any
     undefined bits.

  þ  When loading a register, always load the undefined bits as zeros or
     reload them with values previously stored from the same register.

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
NOTE
  Depending upon the values of undefined register bits will make software
  dependent upon the unspecified manner in which the 80386 handles these
  bits. Depending upon undefined values risks making software incompatible
  with future processors that define usages for these bits. AVOID ANY
  SOFTWARE DEPENDENCE UPON THE STATE OF UNDEFINED 80386 REGISTER BITS.
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ


Figure 1-1.  Example Data Structure

 GREATEST                 DATA STRUCTURE
 ADDRESS
  31              23              15              7             0 ÄÄBIT
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»   OFFSET
 º                                                               º28
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                                               º24
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                                               º20
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                                               º16
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                                               º12
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                                               º8
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                           UNDEFINED                           º4
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  SMALLEST
 º    BYTE 3          BYTE 2          BYTE 1          BYTE 0     º0 ADDRESS
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                                                    BYTE OFFSETÄÄÄÙ


1.3.3  Instruction Operands

When instructions are represented symbolically, a subset of the 80386
Assembly Language is used. In this subset, an instruction has the following
format:

label: prefix mnemonic argument1, argument2, argument3

where:

  þ  A label is an identifier that is followed by a colon.

  þ  A prefix is an optional reserved name for one of the instruction
     prefixes.

  þ  A mnemonic is a reserved name for a class of instruction opcodes that
     have the same function.

  þ  The operands argument1, argument2, and argument3 are optional. There
     may be from zero to three operands, depending on the opcode.  When
     present, they take the form of either literals or identifiers for data
     items.  Operand identifiers are either reserved names of registers or
     are assumed to be assigned to data items declared in another part of
     the program (which may not be shown in the example). When two operands
     are present in an instruction that modifies data, the right operand is
     the source and the left operand is the destination.

For example:

LOADREG: MOV EAX, SUBTOTAL

In this example LOADREG is a label, MOV is the mnemonic identifier of an
opcode, EAX is the destination operand, and SUBTOTAL is the source operand.

1.3.4  Hexadecimal Numbers

Base 16 numbers are represented by a string of hexadecimal digits followed
by the character H.  A hexadecimal digit is a character from the set (0, 1,
2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F).  In some cases, especially in
examples of program syntax, a leading zero is added if the number would
otherwise begin with one of the digits A-F.  For example, 0FH is equivalent
to the decimal number 15.

1.3.5  Sub- and Super-Scripts

This manual uses special notation to represent sub- and super-script
characters. Sub-script characters are surrounded by {curly brackets}, for
example 10{2} = 10 base 2. Super-script characters are preceeded by a
caret and enclosed within (parentheses), for example 10^(3) = 10 to the
third power.


                     PART I    APPLICATIONS PROGRAMMING                    


Chapter 2  Basic Programming Model

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

This chapter describes the 80386 application programming environment as
seen by assembly language programmers when the processor is executing in
protected mode. The chapter introduces programmers to those features of the
80386 architecture that directly affect the design and implementation of
80386 applications programs. Other chapters discuss 80386 features that
relate to systems programming or to compatibility with other processors of
the 8086 family.

The basic programming model consists of these aspects:

  þ  Memory organization and segmentation
  þ  Data types
  þ  Registers
  þ  Instruction format
  þ  Operand selection
  þ  Interrupts and exceptions

Note that input/output is not included as part of the basic programming
model. Systems designers may choose to make I/O instructions available to
applications or may choose to reserve these functions for the operating
system. For this reason, the I/O features of the 80386 are discussed in Part
II.

This chapter contains a section for each aspect of the architecture that is
normally visible to applications.


2.1  Memory Organization and Segmentation

The physical memory of an 80386 system is organized as a sequence of 8-bit
bytes. Each byte is assigned a unique address that ranges from zero to a
maximum of 2^(32) -1 (4 gigabytes).

80386 programs, however, are independent of the physical address space.
This means that programs can be written without knowledge of how much
physical memory is available and without knowledge of exactly where in
physical memory the instructions and data are located.

The model of memory organization seen by applications programmers is
determined by systems-software designers. The architecture of the 80386
gives designers the freedom to choose a model for each task. The model of
memory organization can range between the following extremes:

  þ  A "flat" address space consisting of a single array of up to 4
     gigabytes.

  þ  A segmented address space consisting of a collection of up to 16,383
     linear address spaces of up to 4 gigabytes each.

Both models can provide memory protection. Different tasks may employ
different models of memory organization. The criteria that designers use to
determine a memory organization model and the means that systems programmers
use to implement that model are covered in Part IIÄÄSystems Programming.


2.1.1  The "Flat" Model

In a "flat" model of memory organization, the applications programmer sees
a single array of up to 2^(32) bytes (4 gigabytes). While the physical
memory can contain up to 4 gigabytes, it is usually much smaller; the
processor maps the 4 gigabyte flat space onto the physical address space by
the address translation mechanisms described in Chapter 5. Applications
programmers do not need to know the details of the mapping.

A pointer into this flat address space is a 32-bit ordinal number that may
range from 0 to 2^(32) -1. Relocation of separately-compiled modules in this
space must be performed by systems software (e.g., linkers, locators,
binders, loaders).


2.1.2  The Segmented Model

In a segmented model of memory organization, the address space as viewed by
an applications program (called the logical address space) is a much larger
space of up to 2^(46) bytes (64 terabytes). The processor maps the 64
terabyte logical address space onto the physical address space (up to 4
gigabytes) by the address translation mechanisms described in Chapter 5.
Applications programmers do not need to know the details of this mapping.

Applications programmers view the logical address space of the 80386 as a
collection of up to 16,383 one-dimensional subspaces, each with a specified
length. Each of these linear subspaces is called a segment. A segment is a
unit of contiguous address space. Segment sizes may range from one byte up
to a maximum of 2^(32) bytes (4 gigabytes).

A complete pointer in this address space consists of two parts (see Figure
2-1):

  1.  A segment selector, which is a 16-bit field that identifies a
      segment.

  2.  An offset, which is a 32-bit ordinal that addresses to the byte level
      within a segment.

During execution of a program, the processor associates with a segment
selector the physical address of the beginning of the segment. Separately
compiled modules can be relocated at run time by changing the base address
of their segments. The size of a segment is variable; therefore, a segment
can be exactly the size of the module it contains.


2.2  Data Types

Bytes, words, and doublewords are the fundamental data types (refer to
Figure 2-2). A byte is eight contiguous bits starting at any logical
address. The bits are numbered 0 through 7; bit zero is the least
significant bit.

A word is two contiguous bytes starting at any byte address. A word thus
contains 16 bits. The bits of a word are numbered from 0 through 15; bit 0
is the least significant bit. The byte containing bit 0 of the word is
called the low byte; the byte containing bit 15 is called the high byte.

Each byte within a word has its own address, and the smaller of the
addresses is the address of the word. The byte at this lower address
contains the eight least significant bits of the word, while the byte at the
higher address contains the eight most significant bits.

A doubleword is two contiguous words starting at any byte address. A
doubleword thus contains 32 bits. The bits of a doubleword are numbered from
0 through 31; bit 0 is the least significant bit. The word containing bit 0
of the doubleword is called the low word; the word containing bit 31 is
called the high word.

Each byte within a doubleword has its own address, and the smallest of the
addresses is the address of the doubleword. The byte at this lowest address
contains the eight least significant bits of the doubleword, while the byte
at the highest address contains the eight most significant bits. Figure 2-3
illustrates the arrangement of bytes within words anddoublewords.

Note that words need not be aligned at even-numbered addresses and
doublewords need not be aligned at addresses evenly divisible by four. This
allows maximum flexibility in data structures (e.g., records containing
mixed byte, word, and doubleword items) and efficiency in memory
utilization. When used in a configuration with a 32-bit bus, actual
transfers of data between processor and memory take place in units of
doublewords beginning at addresses evenly divisible by four; however, the
processor converts requests for misaligned words or doublewords into the
appropriate sequences of requests acceptable to the memory interface. Such
misaligned data transfers reduce performance by requiring extra memory
cycles. For maximum performance, data structures (including stacks) should
be designed in such a way that, whenever possible, word operands are aligned
at even addresses and doubleword operands are aligned at addresses evenly
divisible by four. Due to instruction prefetching and queuing within the
CPU, there is no requirement for instructions to be aligned on word or
doubleword boundaries. (However, a slight increase in speed results if the
target addresses of control transfers are evenly divisible by four.)

Although bytes, words, and doublewords are the fundamental types of
operands, the processor also supports additional interpretations of these
operands. Depending on the instruction referring to the operand, the
following additional data types are recognized:

Integer:
A signed binary numeric value contained in a 32-bit doubleword,16-bit word,
or 8-bit byte. All operations assume a 2's complement representation. The
sign bit is located in bit 7 in a byte, bit 15 in a word, and bit 31 in a
doubleword. The sign bit has the value zero for positive integers and one
for negative. Since the high-order bit is used for a sign, the range of an
8-bit integer is -128 through +127; 16-bit integers may range from -32,768
through +32,767; 32-bit integers may range from -2^(31) through +2^(31) -1.
The value zero has a positive sign.

Ordinal:
An unsigned binary numeric value contained in a 32-bit doubleword,
16-bit word, or 8-bit byte. All bits are considered in determining
magnitude of the number. The value range of an 8-bit ordinal number
is 0-255; 16 bits can represent values from 0 through 65,535; 32 bits
can represent values from 0 through 2^(32) -1.

Near Pointer:
A 32-bit logical address. A near pointer is an offset within a segment.
Near pointers are used in either a flat or a segmented model of memory
organization.

Far Pointer:
A 48-bit logical address of two components: a 16-bit segment selector
component and a 32-bit offset component. Far pointers are used by
applications programmers only when systems designers choose a
segmented memory organization.

String:
A contiguous sequence of bytes, words, or doublewords. A string may
contain from zero bytes to 2^(32) -1 bytes (4 gigabytes).

Bit field:
A contiguous sequence of bits. A bit field may begin at any bit position
of any byte and may contain up to 32 bits.

Bit string:
A contiguous sequence of bits. A bit string may begin at any bit position
of any byte and may contain up to 2^(32) -1 bits.

BCD:
A byte (unpacked) representation of a decimal digit in the range0 through
9. Unpacked decimal numbers are stored as unsigned byte quantities. One
digit is stored in each byte. The magnitude of the number is determined from
the low-order half-byte; hexadecimal values 0-9 are valid and are
interpreted as decimal numbers. The high-order half-byte must be zero for
multiplication and division; it may contain any value for addition and
subtraction.

Packed BCD:
A byte (packed) representation of two decimal digits, each in the range
0 through 9. One digit is stored in each half-byte. The digit in the
high-order half-byte is the most significant. Values 0-9 are valid in each
half-byte. The range of a packed decimal byte is 0-99.

Figure 2-4 graphically summarizes the data types supported by the 80386.


Figure 2-1.  Two-Component Pointer

                                                  
                                   º               º
                                   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹Ŀ
       32            0             º               º ³
      ÉÍÍÍÍÍÍÍØÍÍÍÍÍÍÍ»   ÉÍÍÍ»    ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ ³
      º    OFFSET     ÇÄÄĶ + ÇÄÄĺ    OPERAND    º ³
      ÈÍÍÍÍÍÍÍØÍÍÍÍÍÍͼ   ÈÍÍͼ    ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ ÃÄ SELECTED SEGMENT
                                  º               º ³
           16    0          ³      º               º ³
          ÉÍÍÍÍÍÍÍ»         ³      º               º ³
          ºSEGMENTÇÄÄÄÄÄÄÄÄÄùÄÄÄÄÄÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ÄÙ
          ÈÍÍÍÍÍÍͼ                º               º
                                   º               º
                                   º               º
                                                  


Figure 2-2.  Fundamental Data Types

  7              0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º     BYTE      º  BYTE
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

  15              7              0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º   HIGH BYTE   ³   LOW BYTE    º  WORD
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
    address n+1      address n

  31              23              15              7             0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º           HIGH WORD           ³            LOW WORD          º DOUBLEWORD
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
    address n+3     address n+2     address n+1      address n


Figure 2-3.  Bytes, Words, and Doublewords in Memory

                  MEMORY
       BYTE       VALUES
All values in hexadecimal
     ADDRESS   ÉÍÍÍÍÍÍÍÍÍÍ»
              Eº          º
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÄ¿
              Dº    7A    º  ÃÄ DOUBLE WORD AT ADDRESS A
               ÌÍÍÍÍÍÍÍÍÍ͹Ŀ³  CONTAINS 7AFE0636
              Cº    FE    º ³³
               ÌÍÍÍÍÍÍÍÍÍ͹ ÃÄ WORD AT ADDRESS B
              Bº    06    º ³³ CONTAINS FE06
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÙ³
              Aº    36    º  ³
               ÌÍÍÍÍÍÍÍÍÍ͹Í͵
              9º    1F    º  ÃÄ WORD AT ADDRESS 9
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÄÙ  CONTAINS IF
              8º          º
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÄ¿
              7º    23    º  ³
               ÌÍÍÍÍÍÍÍÍÍ͹  ÃÄ WORD AT ADDRESS 6
              6º    OB    º  ³  CONTAINS 23OB
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÄÙ
              5º          º
               ÌÍÍÍÍÍÍÍÍÍ͹
              4º          º
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÄ¿
              3º    74    º  ³
               ÌÍÍÍÍÍÍÍÍÍ͹ĿÃÄ WORD AT ADDRESS 2
              2º    CB    º ³³  CONTAINS 74CB
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÄÙ
              1º    31    º ÃÄÄ WORD AT ADDRESS 1
               ÌÍÍÍÍÍÍÍÍÍ͹ÄÙ   CONTAINS CB31
              0º          º
               ÈÍÍÍÍÍÍÍÍÍͼ


Figure 2-4.  80386 Data Types

                                                           +1       0
          7       0              7       0            15 14    8 7      0
     BYTE ÉÑÑÑÑÑÑÑ»         BYTE ÉÑÑÑÑÑÑÑ»         WORD ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
  INTEGER º³  ³   º      ORDINAL º   ³   º      INTEGER º³  ³   ³   ³   º
          ÈÏÍÍÍÍÍͼ              ÈÍÍÍÍÍÍͼ              ÈÏÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
  SIGN BITÙÀÄÄÄÄÄÄÙ              ÀÄÄÄÄÄÄÄÙ      SIGN BITÙÀMSB           ³
           MAGNITUDE             MAGNITUDE              ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                                                            MAGNITUDE


            +1       0                       +3     +2      +1       0
         15              0               31            16 15             0
    WORD ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»    DOUBLEWORD ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
 ORDINAL º³  ³   ³   ³   º       INTEGER º³  ³   ³   ³   ³   ³   ³   ³   º
         ÈÏÍÍÍÍÍÍÏÍÍÍÍÍÍͼ               ÈÏÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
         ³               ³       SIGN BITÙÀMSB                           ³
         ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ               ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
             MAGNITUDE                              MAGNITUDE


                           +3      +2      +1       0
                        31                              0
             DOUBLEWORD ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
                ORDINAL º   ³   ³   ³   ³   ³   ³   ³   º
                        ÈÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
                        ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                                    MAGNITUDE

                           +N              +1       0
                        7       0       7      0 7      0
           BINARY CODED ÉÑÑÑÑÑÑÑ»       ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
          DECIMAL (BCD) º   ³   º    º   ³   ³   ³   º
                        ÈÍÍÍÍÍÍͼ       ÈÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
                           BCD             BCD     BCD
                         DIGIT N         DIGIT 1  DIGIT 0

                           +N              +1       0
                        7       0       7      0 7      0
                 PACKED ÉÑÑÑÑÑÑÑ»       ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
                    BCD º   ³   º    º   ³   ³   ³   º
                        ÈÍÍÍÍÍÍͼ       ÈÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
                        ÀÄÄÄÙ                       ÀÄÄÄÙ
                        MOST                        LEAST
                        SIGNIFICANT           SIGNIFICANT
                        DIGIT                       DIGIT

                           +N              +1       0
                        7       0       7      0 7      0
                   BYTE ÉÑÑÑÑÑÑÑ»       ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
                 STRING º   ³   º    º   ³   ³   ³   º
                        ÈÍÍÍÍÍÍͼ       ÈÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ

                                              -2 GIGABYTES
                +2 GIGABYTES                           210
     BIT ÉÑÑÑÑÍÍÍÍÍÍÍÍÍÍÍÍÑÑÍÍÍÍÍÍÍ  ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÑÑÑ»
  STRING º³³³³            ³³                         ³³³³º
         ÈÏÏÏÏÍÍÍÍÍÍÍÍÍÍÍÍÏÏÍÍÍÍÍÍÍÍ  ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÏÏϼ
                         BIT 0

               +3      +2      +1       0
            31                              0
NEAR 32-BIT ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
    POINTER º   ³   ³   ³   ³   ³   ³   ³   º
            ÈÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
            ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                             OFFSET

              +5      +4      +3      +2      +1       0
           48                                              0
FAR 48-BIT ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
   POINTER º   ³   ³   ³   ³   ³   ³   ³   ³   ³   ³   ³   º
           ÈÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
           ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                SELECTOR                 OFFSET

                  +5      +4      +3      +2      +1       0
    32-BIT ÉÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ»
 BIT FIELD º   ³   ³   ³   ³   ³   ³   ³   ³   ³   ³   ³   º
           ÈÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍͼ
                   ³ÄÄÄÄÄÄÄÄÄ BIT FIELD ÄÄÄÄÄÄÄÄij
                             1 TO 32 BITS


2.3  Registers

The 80386 contains a total of sixteen registers that are of interest to the
applications programmer. As Figure 2-5 shows, these registers may be
grouped into these basic categories:

  1.  General registers. These eight 32-bit general-purpose registers are
      used primarily to contain operands for arithmetic and logical
      operations.

  2.  Segment registers. These special-purpose registers permit systems
      software designers to choose either a flat or segmented model of
      memory organization. These six registers determine, at any given time,
      which segments of memory are currently addressable.

  3.  Status and instruction registers. These special-purpose registers are
      used to record and alter certain aspects of the 80386 processor state.


2.3.1  General Registers

The general registers of the 80386 are the 32-bit registers EAX, EBX, ECX,
EDX, EBP, ESP, ESI, and EDI. These registers are used interchangeably to
contain the operands of logical and arithmetic operations. They may also be
used interchangeably for operands of address computations (except that ESP
cannot be used as an index operand).

As Figure 2-5 shows, the low-order word of each of these eight registers
has a separate name and can be treated as a unit. This feature is useful for
handling 16-bit data items and for compatibility with the 8086 and 80286
processors. The word registers are named AX, BX, CX, DX, BP, SP, SI, and DI.

Figure 2-5 also illustrates that each byte of the 16-bit registers AX, BX,
CX, and DX has a separate name and can be treated as a unit. This feature is
useful for handling characters and other 8-bit data items. The byte
registers are named AH, BH, CH, and DH (high bytes); and AL, BL, CL, and DL
(low bytes).

All of the general-purpose registers are available for addressing
calculations and for the results of most arithmetic and logical
calculations; however, a few functions are dedicated to certain registers.
By implicitly choosing registers for these functions, the 80386 architecture
can encode instructions more compactly. The instructions that use specific
registers include: double-precision multiply and divide, I/O, string
instructions, translate, loop, variable shift and rotate, and stack
operations.


2.3.2  Segment Registers

The segment registers of the 80386 give systems software designers the
flexibility to choose among various models of memory organization.
Implementation of memory models is the subject of Part II ÄÄ Systems
Programming. Designers may choose a model in which applications programs do
not need to modify segment registers, in which case applications programmers
may skip this section.

Complete programs generally consist of many different modules, each
consisting of instructions and data. However, at any given time during
program execution, only a small subset of a program's modules are actually
in use. The 80386 architecture takes advantage of this by providing
mechanisms to support direct access to the instructions and data of the
current module's environment, with access to additional segments on demand.

At any given instant, six segments of memory may be immediately accessible
to an executing 80386 program. The segment registers CS, DS, SS, ES, FS, and
GS are used to identify these six current segments. Each of these registers
specifies a particular kind of segment, as characterized by the associated
mnemonics ("code," "data," or "stack") shown in Figure 2-6. Each register
uniquely determines one particular segment, from among the segments that
make up the program, that is to be immediately accessible at highest speed.

The segment containing the currently executing sequence of instructions is
known as the current code segment; it is specified by means of the CS
register. The 80386 fetches all instructions from this code segment, using
as an offset the contents of the instruction pointer. CS is changed
implicitly as the result of intersegment control-transfer instructions (for
example, CALL and JMP), interrupts, and exceptions.

Subroutine calls, parameters, and procedure activation records usually
require that a region of memory be allocated for a stack. All stack
operations use the SS register to locate the stack. Unlike CS, the SS
register can be loaded explicitly, thereby permitting programmers to define
stacks dynamically.

The DS, ES, FS, and GS registers allow the specification of four data
segments, each addressable by the currently executing program. Accessibility
to four separate data areas helps programs efficiently access different
types of data structures; for example, one data segment register can point
to the data structures of the current module, another to the exported data
of a higher-level module, another to a dynamically created data structure,
and another to data shared with another task. An operand within a data
segment is addressed by specifying its offset either directly in an
instruction or indirectly via general registers.

Depending on the structure of data (e.g., the way data is parceled into one
or more segments), a program may require access to more than four data
segments. To access additional segments, the DS, ES, FS, and GS registers
can be changed under program control during the course of a program's
execution. This simply requires that the program execute an instruction to
load the appropriate segment register prior to executing instructions that
access the data.

The processor associates a base address with each segment selected by a
segment register. To address an element within a segment, a 32-bit offset is
added to the segment's base address. Once a segment is selected (by loading
the segment selector into a segment register), a data manipulation
instruction only needs to specify the offset. Simple rules define which
segment register is used to form an address when only an offset is
specified.


Figure 2-5.  80386 Applications Register Set

                              GENERAL REGISTERS

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                                  EAX       AH       AX      AL        º
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                  EDX       DH       DX      DL        º
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                  ECX       CH       CX      CL        º
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                  EBX       BH       BX      BL        º
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                  EBP                BP                º
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                  ESI                SI                º
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                  EDI                DI                º
 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
 º                                  ESP                SP                º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


                      15                7               0
                     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
                     º         CS (CODE SEGMENT)         º
                     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
                     º         SS (STACK SEGMENT)        º
    SEGMENT          ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
    REGISTERS        º         DS (DATA SEGMENT)         º
                     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
                     º         ES (DATA SEGMENT)         º
                     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
                     º         FS (DATA SEGMENT)         º
                     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
                     º         GS (DATA SEGMENT)         º
                     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


                       STATUS AND INSTRUCTION REGISTERS

     31               23                15                7              0
   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
   º                                EFLAGS                                 º
   ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
   º                        EIP (INSTRUCTION POINTER)                      º
   ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 2-6.  Use of Memory Segmentation

     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»                                ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º     MODULE     º                                º     MODULE     º
     º       A        ºÄÄ¿                        ÚÄĺ       A        º
     º      CODE      º   ³                        ³   º      DATA      º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ   ³  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»  ³   ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                          ÀÄĶ    CS (CODE)     º  ³
                             ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ³
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»   ÚÄĶ    SS (STACK)    º  ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º                º   ³  ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ³   º      DATA      º
     º     STACK      ºÄÄÙ  º    DS (DATA)     ÇÄÄÙÚĺ   STRUCTURE    º
     º                º      ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹   ³  º       1        º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ      º    ES (DATA)     ÇÄÄÄÙ  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                             ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                          ÚÄĶ    FS (DATA)     º
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»   ³  ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹      ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º      DATA      º   ³  º    GS (DATA)     ÇÄÄ¿   º      DATA      º
     º   STRUCTURE    ºÄÄÙ  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ  ÀÄĺ   STRUCTURE    º
     º       2        º                                º       3        º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ                                ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


2.3.3  Stack Implementation

Stack operations are facilitated by three registers:

  1.  The stack segment (SS) register. Stacks are implemented in memory. A
      system may have a number of stacks that is limited only by the maximum
      number of segments. A stack may be up to 4 gigabytes long, the maximum
      length of a segment. One stack is directly addressable at a timeÄÄthe
      one located by SS. This is the current stack, often referred to simply
      as "the" stack. SS is used automatically by the processor for all
      stack operations.

  2.  The stack pointer (ESP) register. ESP points to the top of the
      push-down stack (TOS). It is referenced implicitly by PUSH and POP
      operations, subroutine calls and returns, and interrupt operations.
      When an item is pushed onto the stack (see Figure 2-7), the processor
      decrements ESP, then writes the item at the new TOS. When an item is
      popped off the stack, the processor copies it from TOS, then
      increments ESP. In other words, the stack grows down in memory toward
      lesser addresses.

  3.  The stack-frame base pointer (EBP) register. The EBP is the best
      choice of register for accessing data structures, variables and
      dynamically allocated work space within the stack. EBP is often used
      to access elements on the stack relative to a fixed point on the stack
      rather than relative to the current TOS. It typically identifies the
      base address of the current stack frame established for the current
      procedure. When EBP is used as the base register in an offset
      calculation, the offset is calculated automatically in the current
      stack segment (i.e., the segment currently selected by SS). Because
      SS does not have to be explicitly specified, instruction encoding in
      such cases is more efficient. EBP can also be used to index into
      segments addressable via other segment registers.


Figure 2-7.  80386 Stack

     31                         0
     ÉÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍÍ» ÄÄÄÄÄÄÄBOTTOM OF STACK
     º                           º       (INITIAL ESP VALUE)
     ÇÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍ͹
     º                           º
     ÌÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍ͹        
     º                           º        ³POP
     ÌÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍ͹        ³
     º                           º        ³
     ÌÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍ͹        ³      TOP OF     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º                           º ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ     ESP     º
     ÌÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍÍØÍÍÍÍÍ͹        ³      STACK      ÈÍÍÍÍÍÍÍÍÍÍÍÍͼ
     º                           º        ³
     º                           º        ³
     º                           º        ³PUSH
     º                           º        


2.3.4  Flags Register

The flags register is a 32-bit register named EFLAGS. Figure 2-8 defines
the bits within this register. The flags control certain operations and
indicate the status of the 80386.

The low-order 16 bits of EFLAGS is named FLAGS and can be treated as a
unit. This feature is useful when executing 8086 and 80286 code, because
this part of EFLAGS is identical to the FLAGS register of the 8086 and the
80286.

The flags may be considered in three groups: the status flags, the control
flags, and the systems flags. Discussion of the systems flags is delayed
until Part II.


Figure 2-8.  EFLAGS Register  

                                              16-BIT FLAGS REGISTER
                                                         A
                                     ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
  31                  23                  15               7            0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÑÍÑÍÑÍÑÍØÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍ»
 º                                   ³V³R³ ³N³ IO³O³D³I³T³S³Z³ ³A³ ³P³ ³Cº
 º 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ³ ³ ³0³ ³   ³ ³ ³ ³ ³ ³ ³0³ ³0³ ³1³ º
 º                                   ³M³F³ ³T³ PL³F³F³F³F³F³F³ ³F³ ³F³ ³Fº
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÑÏÑÏÍÏÑÏÍØÍÏÑÏÑÏÑÏÑÏÑÏÑÏÍÏÑÏÍÏÑÏÍÏѼ
                                      ³ ³   ³  ³  ³ ³ ³ ³ ³ ³   ³   ³   ³
       VIRTUAL 8086 MODEÄÄÄXÄÄÄÄÄÄÄÄÄÄÙ ³   ³  ³  ³ ³ ³ ³ ³ ³   ³   ³   ³
             RESUME FLAGÄÄÄXÄÄÄÄÄÄÄÄÄÄÄÄÙ   ³  ³  ³ ³ ³ ³ ³ ³   ³   ³   ³
        NESTED TASK FLAGÄÄÄXÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ  ³  ³ ³ ³ ³ ³ ³   ³   ³   ³
     I/O PRIVILEGE LEVELÄÄÄXÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ  ³ ³ ³ ³ ³ ³   ³   ³   ³
                OVERFLOWÄÄÄSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³ ³ ³ ³   ³   ³   ³
          DIRECTION FLAGÄÄÄCÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³ ³ ³   ³   ³   ³
        INTERRUPT ENABLEÄÄÄXÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³ ³   ³   ³   ³
               TRAP FLAGÄÄÄSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³   ³   ³   ³
               SIGN FLAGÄÄÄSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³   ³   ³   ³
               ZERO FLAGÄÄÄSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ   ³   ³   ³
         AUXILIARY CARRYÄÄÄSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ   ³   ³
             PARITY FLAGÄÄÄSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ   ³
              CARRY FLAGÄÄÄSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

          S = STATUS FLAG, C = CONTROL FLAG, X = SYSTEM FLAG

          NOTE: 0 OR 1 INDICATES INTEL RESERVED. DO NOT DEFINE


2.3.4.1  Status Flags

The status flags of the EFLAGS register allow the results of one
instruction to influence later instructions. The arithmetic instructions use
OF, SF, ZF, AF, PF, and CF. The SCAS (Scan String), CMPS (Compare String),
and LOOP instructions use ZF to signal that their operations are complete.
There are instructions to set, clear, and complement CF before execution of
an arithmetic instruction. Refer to Appendix C for definition of each
status flag.


2.3.4.2  Control Flag

The control flag DF of the EFLAGS register controls string instructions.

DF (Direction Flag, bit 10)

  Setting DF causes string instructions to auto-decrement; that is, to
  process strings from high addresses to low addresses. Clearing DF causes
  string instructions to auto-increment, or to process strings from low
  addresses to high addresses.


2.3.4.3  Instruction Pointer

The instruction pointer register (EIP) contains the offset address,
relative to the start of the current code segment, of the next sequential
instruction to be executed. The instruction pointer is not directly visible
to the programmer; it is controlled implicitly by control-transfer
instructions, interrupts, and exceptions.

As Figure 2-9 shows, the low-order 16 bits of EIP is named IP and can be
used by the processor as a unit. This feature is useful when executing
instructions designed for the 8086 and 80286 processors.


Figure 2-9.  Instruction Pointer Register

                                                 16-BIT IP REGISTER
                                       ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
    31                23                15                7               0
   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
   º                       EIP (INSTRUCTION POINTER)                       º
   ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


2.4  Instruction Format

The information encoded in an 80386 instruction includes a specification of
the operation to be performed, the type of the operands to be manipulated,
and the location of these operands. If an operand is located in memory, the
instruction must also select, explicitly or implicitly, which of the
currently addressable segments contains the operand.

80386 instructions are composed of various elements and have various
formats. The exact format of instructions is shown in Appendix B; the
elements of instructions are described below. Of these instruction elements,
only one, the opcode, is always present. The other elements may or may not
be present, depending on the particular operation involved and on the
location and type of the operands. The elements of an instruction, in order
of occurrence are as follows:

  þ  Prefixes ÄÄ one or more bytes preceding an instruction that modify the
     operation of the instruction. The following types of prefixes can be
     used by applications programs:

     1.  Segment override ÄÄ explicitly specifies which segment register an
         instruction should use, thereby overriding the default
         segment-register selection used by the 80386 for that instruction.

     2.  Address size ÄÄ switches between 32-bit and 16-bit address 
         generation.

     3.  Operand size ÄÄ switches between 32-bit and 16-bit operands.

     4.  Repeat ÄÄ used with a string instruction to cause the instruction
         to act on each element of the string.

  þ  Opcode ÄÄ specifies the operation performed by the instruction. Some
     operations have several different opcodes, each specifying a different
     variant of the operation.

  þ  Register specifier ÄÄ an instruction may specify one or two register
     operands. Register specifiers may occur either in the same byte as the
     opcode or in the same byte as the addressing-mode specifier.

  þ  Addressing-mode specifier ÄÄ when present, specifies whether an operand
     is a register or memory location; if in memory, specifies whether a
     displacement, a base register, an index register, and scaling are to be
     used.

  þ  SIB (scale, index, base) byte ÄÄ when the addressing-mode specifier
     indicates that an index register will be used to compute the address of
     an operand, an SIB byte is included in the instruction to encode the
     base register, the index register, and a scaling factor.

  þ  Displacement ÄÄ when the addressing-mode specifier indicates that a
     displacement will be used to compute the address of an operand, the
     displacement is encoded in the instruction. A displacement is a signed
     integer of 32, 16, or eight bits. The eight-bit form is used in the
     common case when the displacement is sufficiently small. The processor
     extends an eight-bit displacement to 16 or 32 bits, taking into
     account the sign.

  þ  Immediate operand ÄÄ when present, directly provides the value of an
     operand of the instruction. Immediate operands may be 8, 16, or 32 bits
     wide. In cases where an eight-bit immediate operand is combined in some
     way with a 16- or 32-bit operand, the processor automatically extends
     the size of the eight-bit operand, taking into account the sign.


2.5  Operand Selection

An instruction can act on zero or more operands, which are the data
manipulated by the instruction. An example of a zero-operand instruction is
NOP (no operation). An operand can be in any of these locations:

  þ  In the instruction itself (an immediate operand)

  þ  In a register (EAX, EBX, ECX, EDX, ESI, EDI, ESP, or EBP in the case
     of 32-bit operands; AX, BX, CX, DX, SI, DI, SP, or BP in the case of
     16-bit operands; AH, AL, BH, BL, CH, CL, DH, or DL in the case of 8-bit
     operands; the segment registers; or the EFLAGS register for flag
     operations)

  þ  In memory

  þ  At an I/O port

Immediate operands and operands in registers can be accessed more rapidly
than operands in memory since memory operands must be fetched from memory.
Register operands are available in the CPU. Immediate operands are also
available in the CPU, because they are prefetched as part of the
instruction.

Of the instructions that have operands, some specify operands implicitly;
others specify operands explicitly; still others use a combination of
implicit and explicit specification; for example:

Implicit operand: AAM

  By definition, AAM (ASCII adjust for multiplication) operates on the
  contents of the AX register.

Explicit operand: XCHG EAX, EBX

  The operands to be exchanged are encoded in the instruction after the
  opcode.

Implicit and explicit operands: PUSH COUNTER

  The memory variable COUNTER (the explicit operand) is copied to the top of
  the stack (the implicit operand).

Note that most instructions have implicit operands. All arithmetic
instructions, for example, update the EFLAGS register.

An 80386 instruction can explicitly reference one or two operands.
Two-operand instructions, such as MOV, ADD, XOR, etc., generally overwrite
one of the two participating operands with the result. A distinction can
thus be made between the source operand (the one unaffected by the
operation) and the destination operand (the one overwritten by the result).

For most instructions, one of the two explicitly specified operandsÄÄeither
the source or the destinationÄÄcan be either in a register or in memory.
The other operand must be in a register or be an immediate source operand.
Thus, the explicit two-operand instructions of the 80386 permit operations
of the following kinds:

  þ  Register-to-register
  þ  Register-to-memory
  þ  Memory-to-register
  þ  Immediate-to-register
  þ  Immediate-to-memory

Certain string instructions and stack manipulation instructions, however,
transfer data from memory to memory. Both operands of some string
instructions are in memory and are implicitly specified. Push and pop stack
operations allow transfer between memory operands and the memory-based
stack.


2.5.1  Immediate Operands

Certain instructions use data from the instruction itself as one (and
sometimes two) of the operands. Such an operand is called an immediate
operand. The operand may be 32-, 16-, or 8-bits long. For example:

SHR PATTERN, 2

One byte of the instruction holds the value 2, the number of bits by which
to shift the variable PATTERN.

TEST PATTERN, 0FFFF00FFH

A doubleword of the instruction holds the mask that is used to test the
variable PATTERN.


2.5.2  Register Operands

Operands may be located in one of the 32-bit general registers (EAX, EBX,
ECX, EDX, ESI, EDI, ESP, or EBP), in one of the 16-bit general registers
(AX, BX, CX, DX, SI, DI, SP, or BP), or in one of the 8-bit general
registers (AH, BH, CH, DH, AL, BL, CL,or DL).

The 80386 has instructions for referencing the segment registers (CS, DS,
ES, SS, FS, GS). These instructions are used by applications programs only
if systems designers have chosen a segmented memory model.

The 80386 also has instructions for referring to the flag register. The
flags may be stored on the stack and restored from the stack. Certain
instructions change the commonly modified flags directly in the EFLAGS
register. Other flags that are seldom modified can be modified indirectly
via the flags image in the stack.


2.5.3  Memory Operands

Data-manipulation instructions that address operands in memory must specify
(either directly or indirectly) the segment that contains the operand and
the offset of the operand within the segment. However, for speed and compact
instruction encoding, segment selectors are stored in the high speed segment
registers. Therefore, data-manipulation instructions need to specify only
the desired segment register and an offset in order to address a memory
operand.

An 80386 data-manipulation instruction that accesses memory uses one of the
following methods for specifying the offset of a memory operand within its
segment:

  1.  Most data-manipulation instructions that access memory contain a byte
      that explicitly specifies the addressing method for the operand. A
      byte, known as the modR/M byte, follows the opcode and specifies
      whether the operand is in a register or in memory. If the operand is
      in memory, the address is computed from a segment register and any of
      the following values: a base register, an index register, a scaling
      factor, a displacement. When an index register is used, the modR/M
      byte is also followed by another byte that identifies the index
      register and scaling factor. This addressing method is the
      mostflexible.

  2.  A few data-manipulation instructions implicitly use specialized
      addressing methods:

      þ   For a few short forms of MOV that implicitly use the EAX register,
          the offset of the operand is coded as a doubleword in the
          instruction. No base register, index register, or scaling factor
          are used.

      þ   String operations implicitly address memory via DS:ESI, (MOVS,
          CMPS, OUTS, LODS, SCAS) or via ES:EDI (MOVS, CMPS, INS, STOS).

      þ   Stack operations implicitly address operands via SS:ESP
          registers; e.g., PUSH, POP, PUSHA, PUSHAD, POPA, POPAD, PUSHF,
          PUSHFD, POPF, POPFD, CALL, RET, IRET, IRETD, exceptions, and
          interrupts.


2.5.3.1  Segment Selection

Data-manipulation instructions need not explicitly specify which segment
register is used. For all of these instructions, specification of a segment
register is optional. For all memory accesses, if a segment is not
explicitly specified by the instruction, the processor automatically chooses
a segment register according to the rules of Table 2-1. (If systems
designers have chosen a flat model of memory organization, the segment
registers and the rules that the processor uses in choosing them are not
apparent to applications programs.)

There is a close connection between the kind of memory reference and the
segment in which that operand resides. As a rule, a memory reference implies
the current data segment (i.e., the implicit segment selector is in DS).
However, ESP and EBP are used to access items on the stack; therefore, when
the ESP or EBP register is used as a base register, the current stack
segment is implied (i.e., SS contains the selector).

Special instruction prefix elements may be used to override the default
segment selection. Segment-override prefixes allow an explicit segment
selection. The 80386 has a segment-override prefix for each of the segment
registers. Only in the following special cases is there an implied segment
selection that a segment prefix cannot override:

  þ  The use of ES for destination strings in string instructions.
  þ  The use of SS in stack instructions.
  þ  The use of CS for instruction fetches.


Table 2-1. Default Segment Register Selection Rules

Memory Reference Needed  Segment     Implicit Segment Selection Rule
                         Register
                         Used

Instructions             Code (CS)   Automatic with instruction prefetch
Stack                    Stack (SS)  All stack pushes and pops. Any
                                     memory reference that uses ESP or
                                     EBP as a base register.
Local Data               Data (DS)   All data references except when
                                     relative to stack or string 
                                     destination.
Destination Strings      Extra (ES)  Destination of string instructions.


2.5.3.2  Effective-Address Computation

The modR/M byte provides the most flexible of the addressing methods, and
instructions that require a modR/M byte as the second byte of the
instruction are the most common in the 80386 instruction set. For memory
operands defined by modR/M, the offset within the desired segment is
calculated by taking the sum of up to three components:

  þ  A displacement element in the instruction.

  þ  A base register.

  þ  An index register. The index register may be automatically multiplied
     by a scaling factor of 2, 4, or 8.

The offset that results from adding these components is called an effective
address. Each of these components of an effective address may have either a
positive or negative value. If the sum of all the components exceeds 2^(32),
the effective address is truncated to 32 bits.Figure 2-10 illustrates the
full set of possibilities for modR/M addressing.

The displacement component, because it is encoded in the instruction, is
useful for fixed aspects of addressing; for example:

  þ  Location of simple scalar operands.
  þ  Beginning of a statically allocated array.
  þ  Offset of an item within a record.

The base and index components have similar functions. Both utilize the same
set of general registers. Both can be used for aspects of addressing that
are determined dynamically; for example:

  þ  Location of procedure parameters and local variables in stack.

  þ  The beginning of one record among several occurrences of the same
     record type or in an array of records.

  þ  The beginning of one dimension of multiple dimension array.

  þ  The beginning of a dynamically allocated array.

The uses of general registers as base or index components differ in the
following respects:

  þ  ESP cannot be used as an index register.

  þ  When ESP or EBP is used as the base register, the default segment is
     the one selected by SS. In all other cases the default segment is DS.

The scaling factor permits efficient indexing into an array in the common
cases when array elements are 2, 4, or 8 bytes wide. The shifting of the
index register is done by the processor at the time the address is evaluated
with no performance loss. This eliminates the need for a separate shift or
multiply instruction.

The base, index, and displacement components may be used in any
combination; any of these components may be null. A scale factor can be used
only when an index is also used. Each possible combination is useful for
data structures commonly used by programmers in high-level languages and
assembly languages. Following are possible uses for some of the various
combinations of address components.

DISPLACEMENT

   The displacement alone indicates the offset of the operand. This
   combination is used to directly address a statically allocated scalar
   operand. An 8-bit, 16-bit, or 32-bit displacement can be used.

BASE

   The offset of the operand is specified indirectly in one of the general
   registers, as for "based" variables.

BASE + DISPLACEMENT

   A register and a displacement can be used together for two distinct
   purposes:

   1.  Index into static array when element size is not 2, 4, or 8 bytes.
       The displacement component encodes the offset of the beginning of
       the array. The register holds the results of a calculation to
       determine the offset of a specific element within the array.

   2.  Access item of a record. The displacement component locates an
       item within record. The base register selects one of several
       occurrences of record, thereby providing a compact encoding for
       this common function.

   An important special case of this combination, is to access parameters
   in the procedure activation record in the stack.  In this case, EBP is
   the best choice for the base register, because when EBP is used as a
   base register, the processor automatically uses the stack segment
   register (SS) to locate the operand, thereby providing a compact
   encoding for this common function.

(INDEX * SCALE) + DISPLACEMENT

   This combination provides efficient indexing into a static array when
   the element size is 2, 4, or 8 bytes. The displacement addresses the
   beginning of the array, the index register holds the subscript of the
   desired array element, and the processor automatically converts the
   subscript into an index by applying the scaling factor.

BASE + INDEX + DISPLACEMENT

   Two registers used together support either a two-dimensional array (the
   displacement determining the beginning of the array) or one of several
   instances of an array of records (the displacement indicating an item
   in the record).

BASE + (INDEX * SCALE) + DISPLACEMENT

   This combination provides efficient indexing of a two-dimensional array
   when the elements of the array are 2, 4, or 8 bytes wide.


Figure 2-10.  Effective Address Computation

      SEGMENT +    BASE   +    (INDEX * SCALE)  +     DISPLACEMENT

                 Ú     ¿
                 ³ --- ³     Ú     ¿     Ú   ¿
      Ú    ¿     ³ EAX ³     ³ EAX ³     ³ 1 ³
      ³ CS ³     ³ ECX ³     ³ ECX ³     ³   ³     Ú                     ¿
      ³ SS ³     ³ EDX ³     ³ EDX ³     ³ 2 ³     ³     NO DISPLACEMENT ³
     Ä´ DS ÃÄ + Ä´ EBX ÃÄ + Ä´ EBX ÃÄ * Ä´   ÃÄ + Ä´  8-BIT DISPLACEMENT ÃÄ
      ³ ES ³     ³ ESP ³     ³ --- ³     ³ 4 ³     ³ 32-BIT DISPLACEMENT ³
      ³ FS ³     ³ EBP ³     ³ EBP ³     ³   ³     À                     Ù
      ³ GS ³     ³ ESI ³     ³ ESI ³     ³ 6 ³
      À    Ù     ³ EDI ³     ³ EDI ³     À   Ù
                 À     Ù     À     Ù


2.6  Interrupts and Exceptions

The 80386 has two mechanisms for interrupting program execution:

  1.  Exceptions are synchronous events that are the responses of the CPU
      to certain conditions detected during the execution of an instruction.

  2.  Interrupts are asynchronous events typically triggered by external
      devices needing attention.

Interrupts and exceptions are alike in that both cause the processor to
temporarily suspend its present program execution in order to execute a
program of higher priority. The major distinction between these two kinds of
interrupts is their origin. An exception is always reproducible by
re-executing with the program and data that caused the exception, whereas an
interrupt is generally independent of the currently executing program.

Application programmers are not normally concerned with servicing
interrupts. More information on interrupts for systems programmers may be
found in Chapter 9. Certain exceptions, however, are of interest to
applications programmers, and many operating systems give applications
programs the opportunity to service these exceptions. However, the operating
system itself defines the interface between the applications programs and
the exception mechanism of the 80386.

Table 2-2 highlights the exceptions that may be of interest to applications
programmers.

  þ  A divide error exception results when the instruction DIV or IDIV is
     executed with a zero denominator or when the quotient is too large for
     the destination operand. (Refer to Chapter 3 for a discussion of DIV
     and IDIV.)

  þ  The debug exception may be reflected back to an applications program
     if it results from the trap flag (TF).

  þ  A breakpoint exception results when the instruction INT 3 is executed.
     This instruction is used by some debuggers to stop program execution at
     specific points.

  þ  An overflow exception results when the INTO instruction is executed
     and the OF (overflow) flag is set (after an arithmetic operation that
     set the OF flag). (Refer to Chapter 3 for a discussion of INTO).

  þ  A bounds check exception results when the BOUND instruction is
     executed and the array index it checks falls outside the bounds of the
     array. (Refer to Chapter 3 for a discussion of the BOUND instruction.)

  þ  Invalid opcodes may be used by some applications to extend the
     instruction set. In such a case, the invalid opcode exception presents
     an opportunity to emulate the opcode.

  þ  The "coprocessor not available" exception occurs if the program
     contains instructions for a coprocessor, but no coprocessor is present
     in the system.

  þ  A coprocessor error is generated when a coprocessor detects an illegal
     operation.

The instruction INT generates an interrupt whenever it is executed; the
processor treats this interrupt as an exception. The effects of this
interrupt (and the effects of all other exceptions) are determined by
exception handler routines provided by the application program or as part of
the systems software (provided by systems programmers). The INT instruction
itself is discussed in Chapter 3. Refer to Chapter 9 for a more complete
description of exceptions.


Table 2-2. 80386 Reserved Exceptions and Interrupts

Vector Number      Description

0                  Divide Error
1                  Debug Exceptions
2                  NMI Interrupt
3                  Breakpoint
4                  INTO Detected Overflow
5                  BOUND Range Exceeded
6                  Invalid Opcode
7                  Coprocessor Not Available
8                  Double Exception
9                  Coprocessor Segment Overrun
10                 Invalid Task State Segment
11                 Segment Not Present
12                 Stack Fault
13                 General Protection
14                 Page Fault
15                 (reserved)
16                 Coprocessor Error
17-32              (reserved)


Chapter 3  Applications Instruction Set

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

This chapter presents an overview of the instructions which programmers can
use to write application software for the 80386 executing in protected
virtual-address mode. The instructions are grouped by categories of related
functions.

The instructions not discussed in this chapter are those that are normally
used only by operating-system programmers. Part II describes the operation
of these instructions.

The descriptions in this chapter assume that the 80386 is operating in
protected mode with 32-bit addressing in effect; however, all instructions
discussed are also available when 16-bit addressing is in effect in
protected mode, real mode, or virtual 8086 mode. For any differences of
operation that exist in the various modes, refer to Chapter 13,
Chapter 14, or Chapter 15.

The instruction dictionary in Chapter 17 contains more detailed
descriptions of all instructions, including encoding, operation, timing,
effect on flags, and exceptions.


3.1  Data Movement Instructions

These instructions provide convenient methods for moving bytes, words, or
doublewords of data between memory and the registers of the base
architecture. They fall into the following classes:

  1.  General-purpose data movement instructions.
  2.  Stack manipulation instructions.
  3.  Type-conversion instructions.


3.1.1  General-Purpose Data Movement Instructions

MOV (Move) transfers a byte, word, or doubleword from the source operand to
the destination operand. The MOV instruction is useful for transferring data
along any of these paths
There are also variants of MOV that operate on segment registers.  These
are covered in a later section of this chapter.:

  þ  To a register from memory
  þ  To memory from a register
  þ  Between general registers
  þ  Immediate data to a register
  þ  Immediate data to a memory

The MOV instruction cannot move from memory to memory or from segment
register to segment register are not allowed. Memory-to-memory moves can be
performed, however, by the string move instruction MOVS.

XCHG (Exchange) swaps the contents of two operands. This instruction takes
the place of three MOV instructions. It does not require a temporary
location to save the contents of one operand while load the other is being
loaded. XCHG is especially useful for implementing semaphores or similar
data structures for process synchronization.

The XCHG instruction can swap two byte operands, two word operands, or two
doubleword operands. The operands for the XCHG instruction may be two
register operands, or a register operand with a memory operand. When used
with a memory operand, XCHG automatically activates the LOCK signal. (Refer
to Chapter 11 for more information on the bus lock.)


3.1.2  Stack Manipulation Instructions

PUSH (Push) decrements the stack pointer (ESP), then transfers the source
operand to the top of stack indicated by ESP (see Figure 3-1). PUSH is
often used to place parameters on the stack before calling a procedure; it
is also the basic means of storing temporary variables on the stack. The
PUSH instruction operates on memory operands, immediate operands, and
register operands (including segment registers).

PUSHA (Push All Registers) saves the contents of the eight general
registers on the stack (see Figure 3-2). This instruction simplifies
procedure calls by reducing the number of instructions required to retain
the contents of the general registers for use in a procedure. The processor
pushes the general registers on the stack in the following order: EAX, ECX,
EDX, EBX, the initial value of ESP before EAX was pushed, EBP, ESI, and
EDI. PUSHA is complemented by the POPA instruction.

POP (Pop) transfers the word or doubleword at the current top of stack
(indicated by ESP) to the destination operand, and then increments ESP to
point to the new top of stack. See Figure 3-3. POP moves information from
the stack to a general register, or to memory
There are also a variant of POP that operates on segment registers. This
is covered in a later section of this chapter..

POPA (Pop All Registers) restores the registers saved on the stack by
PUSHA, except that it ignores the saved value of ESP. See Figure 3-4.


Figure 3-1.  PUSH

       D  O        BEFORE PUSH                  AFTER PUSH
       I  F      31          0             31          0 
       R        º               º           º               º
       E  E     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       C  X     º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
       T  P     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       I  A     º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
       O  N     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       N  S     º               º           º    OPERAND    º
          I     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
        ³ O     º               º           º               º
        ³ N     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
        ³       º               º           º               º
               ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º               º
                                                         


Figure 3-2.  PUSHA

                   BEFORE PUSHA                AFTER PUSHA
                 31          0             31          0 
         D  O   º               º           º               º
         I  F   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
         R      º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
         E  E   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
         C  X   º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
         T  P   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
         I  A   º               º           º      EAX      º
         O  N   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
         N  S   º               º           º      ECX      º
            I   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
          ³ O   º               º           º      EDX      º
          ³ N   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
          ³     º               º           º      EBX      º
               ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º    OLD ESP    º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º      EBP      º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º      ESI      º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º      EDI      º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
                º               º           º               º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º               º
                                                         


3.1.3  Type Conversion Instructions

The type conversion instructions convert bytes into words, words into
doublewords, and doublewords into 64-bit items (quad-words). These
instructions are especially useful for converting signed integers, because
they automatically fill the extra bits of the larger item with the value of
the sign bit of the smaller item. This kind of conversion, illustrated by
Figure 3-5, is called sign extension.

There are two classes of type conversion instructions:

  1.  The forms CWD, CDQ, CBW, and CWDE which operate only on data in the
      EAX register.

  2.  The forms MOVSX and MOVZX, which permit one operand to be in any
      general register while permitting the other operand to be in memory or
      in a register.

CWD (Convert Word to Doubleword) and CDQ (Convert Doubleword to Quad-Word)
double the size of the source operand. CWD extends the sign of the
word in register AX throughout register DX. CDQ extends the sign of the
doubleword in EAX throughout EDX. CWD can be used to produce a doubleword
dividend from a word before a word division, and CDQ can be used to produce
a quad-word dividend from a doubleword before doubleword division.

CBW (Convert Byte to Word) extends the sign of the byte in register AL
throughout AX.

CWDE (Convert Word to Doubleword Extended) extends the sign of the word in
register AX throughout EAX.

MOVSX (Move with Sign Extension) sign-extends an 8-bit value to a 16-bit
value and a 8- or 16-bit value to 32-bit value.

MOVZX (Move with Zero Extension) extends an 8-bit value to a 16-bit value
and an 8- or 16-bit value to 32-bit value by inserting high-order zeros.


Figure 3-3.  POP

       D  O         BEFORE POP                  AFTER POP
       I  F      31          0             31          0 
       R        º               º           º               º
       E  E     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       C  X     º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
       T  P     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       I  A     º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
       O  N     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
       N  S     º    OPERAND    º           º               º
          I     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
        ³ O     º               º           º               º
        ³ N     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
        ³       º               º           º               º
               ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º               º
                                                         


Figure 3-4.  POPA

                   BEFORE POPA                  AFTER POPA
                 31          0             31          0 
         D  O   º               º           º               º
         I  F   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
         R      º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
         E  E   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
         C  X   º±±±±±±±±±±±±±±±º           º±±±±±±±±±±±±±±±º
         T  P   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
         I  A   º      EAX      º           º               º
         O  N   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
         N  S   º      ECX      º           º               º
            I   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
          ³ O   º      EDX      º           º               º
          ³ N   ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
          ³     º      EBX      º           º               º
               ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º      ESP      º           º               º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º      EPB      º           º               º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º      ESI      º           º               º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º      EDI      º           º               º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º               º
                ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹           ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                º               º           º               º
                                                         


Figure 3-5.  Sign Extension

                                    15                7               0
                                    ÉÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     BEFORE SIGN EXTENSIONÄÄÄÄÄÄÄÄĺSº N N N N N N N N N N N N N N N º
                                    ÈÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
     AFTER SIGN EXTENSIONÄÄÄÄÄÄ¿
                               ³
    31               23             15              7              0
    ÉÍËÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
    ºSºS S S S S S S S S S S S S S S S N N N N N N N N N N N N N N Nº
    ÈÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


3.2  Binary Arithmetic Instructions

The arithmetic instructions of the 80386 processor simplify the
manipulation of numeric data that is encoded in binary. Operations include
the standard add, subtract, multiply, and divide as well as increment,
decrement, compare, and change sign. Both signed and unsigned binary
integers are supported. The binary arithmetic instructions may also be used
as one step in the process of performing arithmetic on decimal integers.

Many of the arithmetic instructions operate on both signed and unsigned
integers. These instructions update the flags ZF, CF, SF, and OF in such a
manner that subsequent instructions can interpret the results of the
arithmetic as either signed or unsigned. CF contains information relevant to
unsigned integers; SF and OF contain information relevant to signed
integers. ZF is relevant to both signed and unsigned integers; ZF is set
when all bits of the result are zero.

If the integer is unsigned, CF may be tested after one of these arithmetic
operations to determine whether the operation required a carry or borrow of
a one-bit in the high-order position of the destination operand. CF is set
if a one-bit was carried out of the high-order position (addition
instructions ADD, ADC, AAA, and DAA) or if a one-bit was carried (i.e.
borrowed) into the high-order bit (subtraction instructions SUB, SBB, AAS,
DAS, CMP, and NEG).

If the integer is signed, both SF and OF should be tested. SF always has
the same value as the sign bit of the result. The most significant bit (MSB)
of a signed integer is the bit next to the signÄÄbit 6 of a byte, bit 14 of
a word, or bit 30 of a doubleword. OF is set in either of these cases:

  þ  A one-bit was carried out of the MSB into the sign bit but no one bit
     was carried out of the sign bit (addition instructions ADD, ADC, INC,
     AAA, and DAA). In other words, the result was greater than the greatest
     positive number that could be contained in the destination operand.

  þ  A one-bit was carried from the sign bit into the MSB but no one bit
     was carried into the sign bit (subtraction instructions SUB, SBB, DEC,
     AAS, DAS, CMP, and NEG). In other words, the result was smaller that
     the smallest negative number that could be contained in the destination
     operand.

These status flags are tested by executing one of the two families of
conditional instructions: Jcc (jump on condition cc) or SETcc (byte set on
condition).


3.2.1  Addition and Subtraction Instructions

ADD (Add Integers) replaces the destination operand with the sum of the
source and destination operands. Sets CF if overflow.

ADC (Add Integers with Carry) sums the operands, adds one if CF is set, and
replaces the destination operand with the result. If CF is cleared, ADC
performs the same operation as the ADD instruction. An ADD followed by
multiple ADC instructions can be used to add numbers longer than 32 bits.

INC (Increment) adds one to the destination operand. INC does not affect
CF. Use ADD with an immediate value of 1 if an increment that updates carry
(CF) is needed.

SUB (Subtract Integers) subtracts the source operand from the destination
operand and replaces the destination operand with the result. If a borrow is
required, the CF is set. The operands may be signed or unsigned bytes,
words, or doublewords.

SBB (Subtract Integers with Borrow) subtracts the source operand from the
destination operand, subtracts 1 if CF is set, and returns the result to the
destination operand. If CF is cleared, SBB performs the same operation as
SUB. SUB followed by multiple SBB instructions may be used to subtract
numbers longer than 32 bits. If CF is cleared, SBB performs the same
operation as SUB.

DEC (Decrement) subtracts 1 from the destination operand. DEC does not
update CF. Use SUB with an immediate value of 1 to perform a decrement that
affects carry.


3.2.2  Comparison and Sign Change Instruction

CMP (Compare) subtracts the source operand from the destination operand. It
updates OF, SF, ZF, AF, PF, and CF but does not alter the source and
destination operands. A subsequent Jcc or SETcc instruction can test the
appropriate flags.

NEG (Negate) subtracts a signed integer operand from zero. The effect of
NEG is to reverse the sign of the operand from positive to negative or from
negative to positive.


3.2.3  Multiplication Instructions

The 80386 has separate multiply instructions for unsigned and signed
operands. MUL operates on unsigned numbers, while IMUL operates on signed
integers as well as unsigned.

MUL (Unsigned Integer Multiply) performs an unsigned multiplication of the
source operand and the accumulator. If the source is a byte, the processor
multiplies it by the contents of AL and returns the double-length result to
AH and AL. If the source operand is a word, the processor multiplies it by
the contents of AX and returns the double-length result to DX and AX. If the
source operand is a doubleword, the processor multiplies it by the contents
of EAX and returns the 64-bit result in EDX and EAX. MUL sets CF and OF
when the upper half of the result is nonzero; otherwise, they are cleared.

IMUL (Signed Integer Multiply) performs a signed multiplication operation.
IMUL has three variations:

  1.  A one-operand form. The operand may be a byte, word, or doubleword
      located in memory or in a general register. This instruction uses EAX
      and EDX as implicit operands in the same way as the MUL instruction.

  2.  A two-operand form. One of the source operands may be in any general
      register while the other may be either in memory or in a general
      register. The product replaces the general-register operand.

  3.  A three-operand form; two are source and one is the destination
      operand. One of the source operands is an immediate value stored in
      the instruction; the second may be in memory or in any general
      register. The product may be stored in any general register. The
      immediate operand is treated as signed. If the immediate operand is a
      byte, the processor automatically sign-extends it to the size of the
      second operand before performing the multiplication.

The three forms are similar in most respects:

  þ  The length of the product is calculated to twice the length of the
     operands.

  þ  The CF and OF flags are set when significant bits are carried into the
     high-order half of the result. CF and OF are cleared when the
     high-order half of the result is the sign-extension of the low-order
     half.

However, forms 2 and 3 differ in that the product is truncated to the
length of the operands before it is stored in the destination register.
Because of this truncation, OF should be tested to ensure that no
significant bits are lost. (For ways to test OF, refer to the INTO and PUSHF
instructions.)

Forms 2 and 3 of IMUL may also be used with unsigned operands because,
whether the operands are signed or unsigned, the low-order half of the
product is the same.


3.2.4  Division Instructions

The 80386 has separate division instructions for unsigned and signed
operands. DIV operates on unsigned numbers, while IDIV operates on signed
integers as well as unsigned. In either case, an exception (interrupt zero)
occurs if the divisor is zero or if the quotient is too large for AL, AX, or
EAX.

DIV (Unsigned Integer Divide) performs an unsigned division of the
accumulator by the source operand. The dividend (the accumulator) is twice
the size of the divisor (the source operand); the quotient and remainder
have the same size as the divisor, as the following table shows.

Size of Source Operand
      (divisor)             Dividend       Quotient      Remainder

Byte                        AX             AL            AH
Word                        DX:AX          AX            DX
Doubleword                  EDX:EAX        EAX           EDX

Non-integral quotients are truncated to integers toward 0. The remainder is
always less than the divisor. For unsigned byte division, the largest
quotient is 255. For unsigned word division, the largest quotient is 65,535.
For unsigned doubleword division the largest quotient is 2^(32) -1.

IDIV (Signed Integer Divide) performs a signed division of the accumulator
by the source operand. IDIV uses the same registers as the DIV instruction.

For signed byte division, the maximum positive quotient is +127, and the
minimum negative quotient is -128. For signed word division, the maximum
positive quotient is +32,767, and the minimum negative quotient is -32,768.
For signed doubleword division the maximum positive quotient is 2^(31) -1,
the minimum negative quotient is -2^(31). Non-integral results are truncated
towards 0. The remainder always has the same sign as the dividend and is
less than the divisor in magnitude.


3.3  Decimal Arithmetic Instructions

Decimal arithmetic is performed by combining the binary arithmetic
instructions (already discussed in the prior section) with the decimal
arithmetic instructions. The decimal arithmetic instructions are used in one
of the following ways:

  þ  To adjust the results of a previous binary arithmetic operation to
     produce a valid packed or unpacked decimal result.

  þ  To adjust the inputs to a subsequent binary arithmetic operation so
     that the operation will produce a valid packed or unpacked decimal
     result.

These instructions operate only on the AL or AH registers. Most utilize the
AF flag.


3.3.1  Packed BCD Adjustment Instructions

DAA (Decimal Adjust after Addition) adjusts the result of adding two valid
packed decimal operands in AL. DAA must always follow the addition of two
pairs of packed decimal numbers (one digit in each half-byte) to obtain a
pair of valid packed decimal digits as results. The carry flag is set if
carry was needed.

DAS (Decimal Adjust after Subtraction) adjusts the result of subtracting
two valid packed decimal operands in AL. DAS must always follow the
subtraction of one pair of packed decimal numbers (one digit in each half-
byte) from another to obtain a pair of valid packed decimal digits as
results. The carry flag is set if a borrow was needed.


3.3.2  Unpacked BCD Adjustment Instructions

AAA (ASCII Adjust after Addition) changes the contents of register AL to a
valid unpacked decimal number, and zeros the top 4 bits. AAA must always
follow the addition of two unpacked decimal operands in AL. The carry flag
is set and AH is incremented if a carry is necessary.

AAS (ASCII Adjust after Subtraction) changes the contents of register AL to
a valid unpacked decimal number, and zeros the top 4 bits. AAS must always
follow the subtraction of one unpacked decimal operand from another in AL.
The carry flag is set and AH decremented if a borrow is necessary.

AAM (ASCII Adjust after Multiplication) corrects the result of a
multiplication of two valid unpacked decimal numbers. AAM must always follow
the multiplication of two decimal numbers to produce a valid decimal result.
The high order digit is left in AH, the low order digit in AL.

AAD (ASCII Adjust before Division) modifies the numerator in AH and AL to
prepare for the division of two valid unpacked decimal operands so that the
quotient produced by the division will be a valid unpacked decimal number.
AH should contain the high-order digit and AL the low-order digit. This
instruction adjusts the value and places the result in AL. AH will contain
zero.


3.4  Logical Instructions

The group of logical instructions includes:

  þ  The Boolean operation instructions
  þ  Bit test and modify instructions
  þ  Bit scan instructions
  þ  Rotate and shift instructions
  þ  Byte set on condition


3.4.1  Boolean Operation Instructions

The logical operations are AND, OR, XOR, and NOT.

NOT (Not) inverts the bits in the specified operand to form a one's
complement of the operand. The NOT instruction is a unary operation that
uses a single operand in a register or memory. NOT has no effect on the
flags.

The AND, OR, and XOR instructions perform the standard logical operations
"and", "(inclusive) or", and "exclusive or". These instructions can use the
following combinations of operands:

  þ  Two register operands

  þ  A general register operand with a memory operand

  þ  An immediate operand with either a general register operand or a
     memory operand.

AND, OR, and XOR clear OF and CF, leave AF undefined, and update SF, ZF,
and PF.


3.4.2  Bit Test and Modify Instructions

This group of instructions operates on a single bit which can be in memory
or in a general register. The location of the bit is specified as an offset
from the low-order end of the operand. The value of the offset either may be
given by an immediate byte in the instruction or may be contained in a
general register.

These instructions first assign the value of the selected bit to CF, the
carry flag. Then a new value is assigned to the selected bit, as determined
by the operation. OF, SF, ZF, AF, PF are left in an undefined state. Table
3-1 defines these instructions.


Table 3-1. Bit Test and Modify Instructions

Instruction                      Effect on CF            Effect on
                                                         Selected Bit

Bit (Bit Test)                   CF  BIT                (none)
BTS (Bit Test and Set)           CF  BIT                BIT  1
BTR (Bit Test and Reset)         CF  BIT                BIT  0
BTC (Bit Test and Complement)    CF  BIT                BIT  NOT(BIT)


3.4.3  Bit Scan Instructions

These instructions scan a word or doubleword for a one-bit and store the
index of the first set bit into a register.  The bit string being scanned
may be either in a register or in memory. The ZF flag is set if the entire
word is zero (no set bits are found); ZF is cleared if a one-bit is found.
If no set bit is found, the value of the destination register is undefined.

BSF (Bit Scan Forward) scans from low-order to high-order (starting from
bit index zero).

BSR (Bit Scan Reverse) scans from high-order to low-order (starting from
bit index 15 of a word or index 31 of a doubleword).


3.4.4  Shift and Rotate Instructions

The shift and rotate instructions reposition the bits within the specified
operand.

These instructions fall into the following classes:

  þ  Shift instructions
  þ  Double shift instructions
  þ  Rotate instructions


3.4.4.1  Shift Instructions

The bits in bytes, words, and doublewords may be shifted arithmetically or
logically. Depending on the value of a specified count, bits can be shifted
up to 31 places.

A shift instruction can specify the count in one of three ways. One form of
shift instruction implicitly specifies the count as a single shift. The
second form specifies the count as an immediate value. The third form
specifies the count as the value contained in CL. This last form allows the
shift count to be a variable that the program supplies during execution.
Only the low order 5 bits of CL are used.

CF always contains the value of the last bit shifted out of the destination
operand. In a single-bit shift, OF is set if the value of the high-order
(sign) bit was changed by the operation. Otherwise, OF is cleared. Following
a multibit shift, however, the content of OF is always undefined.

The shift instructions provide a convenient way to accomplish division or
multiplication by binary power. Note however that division of signed numbers
by shifting right is not the same kind of division performed by the IDIV
instruction.

SAL (Shift Arithmetic Left) shifts the destination byte, word, or
doubleword operand left by one or by the number of bits specified in the
count operand (an immediate value or the value contained in CL). The
processor shifts zeros in from the right (low-order) side of the operand as
bits exit from the left (high-order) side. See Figure 3-6.

SHL (Shift Logical Left) is a synonym for SAL (refer to SAL).

SHR (Shift Logical Right) shifts the destination byte, word, or doubleword
operand right by one or by the number of bits specified in the count operand
(an immediate value or the value contained in CL). The processor shifts
zeros in from the left side of the operand as bits exit from the right side.
See Figure 3-7.

SAR (Shift Arithmetic Right) shifts the destination byte, word, or
doubleword operand to the right by one or by the number of bits specified in
the count operand (an immediate value or the value contained in CL). The
processor preserves the sign of the operand by shifting in zeros on the left
(high-order) side if the value is positive or by shifting by ones if the
value is negative. See Figure 3-8.

Even though this instruction can be used to divide integers by a power of
two, the type of division is not the same as that produced by the IDIV
instruction. The quotient of IDIV is rounded toward zero, whereas the
"quotient" of SAR is rounded toward negative infinity. This difference is
apparent only for negative numbers. For example, when IDIV is used to divide
-9 by 4, the result is -2 with a remainder of -1. If SAR is used to shift
-9 right by two bits, the result is -3. The "remainder" of this kind of
division is +3; however, the SAR instruction stores only the high-order bit
of the remainder (in CF).

The code sequence in Figure 3-9 produces the same result as IDIV for any M
= 2^(N), where 0 < N < 32. This sequence takes about 12 to 18 clocks,
depending on whether the jump is taken; if ECX contains M, the corresponding
IDIV ECX instruction will take about 43 clocks.


Figure 3-6.  SAL and SHL

                     OF   CF                OPERAND

       BEFORE SHL    X    X     10001000100010001000100010001111
       OR SAL

       AFTER SHL     1    1 ÄÄ 00010001000100010001000100011110 ÄÄ 0
       OR SAL BY 1

       AFTER SHL     X    0 ÄÄ 00100010001000100011110000000000 ÄÄ 0
       OR SAL BY 10

SHL (WHICH HAS THE SYNONYM SAL) SHIFTS THE BITS IN THE REGISTER OR MEMORY
OPERAND TO THE LEFT BY THE SPECIFIED NUMBER OF BIT POSITIONS. CF RECEIVES
THE LAST BIT SHIFTED OUT OF THE LEFT OF THE OPERAND. SHL SHIFTS IN ZEROS
TO FILL THE VACATED BIT LOCATIONS. THESE INSTRUCTIONS OPERATE ON BYTE,
WORD, AND DOUBLEWORD OPERANDS.


Figure 3-7.  SHR

                                      OPERAND                  CF

       BEFORE SHR         10001000100010001000100010001111     X

       AFTER SHR    0ÄÄÄÄ01000100010001000100010001000111ÄÄÄÄ1
       BY 1

       AFTER SHR    0ÄÄÄÄ00000000001000100010001000100010ÄÄÄÄO
       BY 10

SHR SHIFTS THE BITS OF THE REGISTER OR MEMORY OPERAND TO THE RIGHT BY THE
SPECIFIED NUMBER OF BIT POSITIONS. CF RECEIVES THE LAST BIT SHIFTED OUT OF
THE RIGHT OF THE OPERAND. SHR SHIFTS IN ZEROS TO FILL THE VACATED BIT
LOCATIONS.


Figure 3-8.  SAR

                                  POSITIVE OPERAND             CF

       BEFORE SAR         01000100010001000100010001000111     X

       AFTER SAR    0ÄÄÄÄ00100010001000100010001000100011ÄÄÄÄ1
       BY 1

                                  NEGATIVE OPERAND             CF

       BEFORE SAR         11000100010001000100010001000111     X

       AFTER SAR    0ÄÄÄÄ11100010001000100010001000100011ÄÄÄÄ1
       BY 1

SAR PRESERVES THE SIGN OF THE REGISTER OR MEMORY OPERAND AS IT SHIFTS THE
OPERAND TO THE RIGHT BY THE SPECIFIED NUMBER OF BIT POSITIONS. CF RECIEVES
THE LAST BIT SHIFTED OUT OF THE RIGHT OF THE OPERAND.


Figure 3-9.  Using SAR to Simulate IDIV

    ; assuming N is in ECX, and the dividend is in EAX
    ;                                               CLOCKS
    CMP     EAX, 0      ; to set sign flag          2
    JGE     NoAdjust    ; jump if sign is zero      3 or 9
    ADD     EAX, ECX    ;                           2
    DEC     EAX         ; EAX := EAX + (N-1)        2
NoAdjust:
    SAR     EAX, CL     ;                           3
    ;                       TOTAL CLOCKS           12 or 18]


3.4.4.2  Double-Shift Instructions

These instructions provide the basic operations needed to implement
operations on long unaligned bit strings. The double shifts operate either
on word or doubleword operands, as follows:

  1.  Taking two word operands as input and producing a one-word output.

  2.  Taking two doubleword operands as input and producing a doubleword
      output.

Of the two input operands, one may either be in a general register or in
memory, while the other may only be in a general register. The results
replace the memory or register operand. The number of bits to be shifted is
specified either in the CL register or in an immediate byte of the
instruction.

Bits are shifted from the register operand into the memory or register
operand. CF is set to the value of the last bit shifted out of the
destination operand. SF, ZF, and PF are set according to the value of the
result. OF and AF are left undefined.

SHLD (Shift Left Double) shifts bits of the R/M field to the left, while
shifting high-order bits from the Reg field into the R/M field on the right
(see Figure 3-10). The result is stored back into the R/M operand. The Reg
field is not modified.

SHRD (Shift Right Double) shifts bits of the R/M field to the right, while
shifting low-order bits from the Reg field into the R/M field on the left
(see Figure 3-11). The result is stored back into the R/M operand. The Reg
field is not modified.


3.4.4.3  Rotate Instructions

Rotate instructions allow bits in bytes, words, and doublewords to be
rotated. Bits rotated out of an operand are not lost as in a shift, but are
"circled" back into the other "end" of the operand.

Rotates affect only the carry and overflow flags. CF may act as an
extension of the operand in two of the rotate instructions, allowing a bit
to be isolated and then tested by a conditional jump instruction (JC or
JNC). CF always contains the value of the last bit rotated out, even if the
instruction does not use this bit as an extension of the rotated operand.

In single-bit rotates, OF is set if the operation changes the high-order
(sign) bit of the destination operand. If the sign bit retains its original
value, OF is cleared. On multibit rotates, the value of OF is always
undefined.

ROL (Rotate Left) rotates the byte, word, or doubleword destination operand
left by one or by the number of bits specified in the count operand (an
immediate value or the value contained in CL). For each rotation specified,
the high-order bit that exits from the left of the operand returns at the
right to become the new low-order bit of the operand. See Figure 3-12.

ROR (Rotate Right) rotates the byte, word, or doubleword destination
operand right by one or by the number of bits specified in the count operand
(an immediate value or the value contained in CL). For each rotation
specified, the low-order bit that exits from the right of the operand
returns at the left to become the new high-order bit of the operand.
See Figure 3-13.

RCL (Rotate Through Carry Left) rotates bits in the byte, word, or
doubleword destination operand left by one or by the number of bits
specified in the count operand (an immediate value or the value contained in
CL).

This instruction differs from ROL in that it treats CF as a high-order
one-bit extension of the destination operand. Each high-order bit that exits
from the left side of the operand moves to CF before it returns to the
operand as the low-order bit on the next rotation cycle. See Figure 3-14.

RCR (Rotate Through Carry Right) rotates bits in the byte, word, or
doubleword destination operand right by one or by the number of bits
specified in the count operand (an immediate value or the value contained in
CL).

This instruction differs from ROR in that it treats CF as a low-order
one-bit extension of the destination operand. Each low-order bit that exits
from the right side of the operand moves to CF before it returns to the
operand as the high-order bit on the next rotation cycle. See Figure 3-15.


Figure 3-10.  Shift Left Double

                  31                   DESTINATION                 0
    ÉÍÍÍÍ»       ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
    º CF ºÄÄÄÄÄĶ                 MEMORY OF REGISTER               ºÄÄÄ¿
    ÈÍÍÍͼ       ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ    ³
             ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
             ³    31                     SOURCE                    0
             ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
             ÀÄÄĶ                      REGISTER                    º
                 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 3-11.  Shift Right Double

        31                     SOURCE                    0
       ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
       º                      REGISTER                    ÇÄÄÄ¿
       ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ   ³
   ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
   ³    31                   DESTINATION                 0
   ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»        ÉÍÍÍÍ»
   ÀÄĺ                 MEMORY OF REGISTER               ÇÄÄÄÄÄÄĺ CF º
       ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ        ÈÍÍÍͼ


Figure 3-12.  ROL

                  31                   DESTINATION                 0
    ÉÍÍÍÍ»       ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
    º CF ºÄÄÄÂÄĶ                 MEMORY OF REGISTER               ºÄÄ¿
    ÈÍÍÍͼ    ³  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ   ³
              ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ


Figure 3-13.  ROR

     ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
     ³    31                   DESTINATION                 0    ³
     ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»   ³    ÉÍÍÍÍ»
     ÀÄĺ                 MEMORY OF REGISTER               ÇÄÄÄÁÄÄĺ CF º
         ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ        ÈÍÍÍͼ


Figure 3-14.  RCL

                  31                   DESTINATION                 0
    ÉÍÍÍÍ»       ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
  ÚĶ CF ºÄÄÄÄÄĶ                 MEMORY OF REGISTER               ºÄÄ¿
  ³ ÈÍÍÍͼ       ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ   ³
  ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ


Figure 3-15.  RCR

   ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
   ³    31                   DESTINATION                 0                ³
   ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»        ÉÍÍÍÍ» ³
   ÀÄĺ                 MEMORY OF REGISTER               ÇÄÄÄÄÄÄĺ CF ÇÄÙ
       ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ        ÈÍÍÍͼ


3.4.4.4  Fast "BIT BLT" Using Double Shift Instructions

One purpose of the double shifts is to implement a bit string move, with
arbitrary misalignment of the bit strings.  This is called a "bit blt" (BIT
BLock Transfer.)  A simple example is to move a bit string from an arbitrary
offset into a doubleword-aligned byte string.  A left-to-right string is
moved 32 bits at a time if a double shift is used inside the move loop.

     MOV   ESI,ScrAddr
     MOV   EDI,DestAddr
     MOV   EBX,WordCnt
     MOV   CL,RelOffset      ; relative offset Dest-Src
     MOV   EDX,[ESI]         ; load first word of source
     ADD   ESI,4             ; bump source address
BltLoop:
     LODS                    ; new low order part
     SHLD  EDX,EAX,CL        ; EDX overwritten with aligned stuff
     XCHG  EDX,EAS           ; Swap high/low order parts
     STOS                    ; Write out next aligned chunk
     DEC   EBX
     JA    BltLoop

This loop is simple yet allows the data to be moved in 32-bit pieces for
the highest possible performance. Without a double shift, the best that can
be achieved is 16 bits per loop iteration by using a 32-bit shift and
replacing the XCHG with a ROR by 16 to swap high and low order parts of
registers. A more general loop than shown above would require some extra
masking on the first doubleword moved (before the main loop), and on the
last doubleword moved (after the main loop), but would have the same basic
32-bits per loop iteration as the code above.


3.4.4.5  Fast Bit-String Insert and Extract

The double shift instructions also enable:

  þ  Fast insertion of a bit string from a register into an arbitrary bit
     location in a larger bit string in memory without disturbing the bits
     on either side of the inserted bits.

  þ  Fast extraction of a bits string into a register from an arbitrary bit
     location in a larger bit string in memory without disturbing the bits
     on either side of the extracted bits.

The following coded examples illustrate bit insertion and extraction under
variousconditions:

  1.  Bit String Insert into Memory (when bit string is 1-25 bits long,
      i.e., spans four bytes or less):

      ; Insert a right-justified bit string from register into
      ; memory bit string.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate value
      ;    but the bit offset is held in a register.
      ;
      ; Register ESI holds the right-justified bit string
      ; to be inserted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX and ECX are also used by this
      ; "insert" operation.
      ;
      MOV   ECX,EDI      ; preserve original offset for later use
      SHR   EDI,3        ; signed divide offset by 8 (byte address)
      AND   CL,7H        ; isolate low three bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move string dword into EAX
      ROR   EAX,CL       ; right justify old bit field
      SHRD  EAX,ESI,length          ; bring in new bits
      ROL   EAX,length   ; right justify new bit field
      ROL   EAX,CL       ; bring to final position
      MOV   [EDI]strg_base,EAX      ; replace dword in memory

  2.  Bit String Insert into Memory (when bit string is 1-31 bits long, i.e.
      spans five bytes or less):

      ; Insert a right-justified bit string from register into
      ; memory bit string.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate value
      ;    but the bit offset is held in a register.
      ;
      ; Register ESI holds the right-justified bit string
      ; to be inserted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX, EBX, ECX, and EDI are also used by
      ; this "insert" operation.
      ;
      MOV   ECX,EDI     ; temp storage for offset
      SHR   EDI,5       ; signed divide offset by 32 (dword address)
      SHL   EDI,2       ; multiply by 4 (in byte address format)
      AND   CL,1FH      ; isolate low five bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move low string dword into EAX
      MOV   EDX,[EDI]strg_base+4    ; other string dword into EDX
      MOV   EBX,EAX     ; temp storage for part of string     ¿ rotate
      SHRD  EAX,EDX,CL  ; double shift by offset within dword à EDX:EAX
      SHRD  EAX,EBX,CL  ; double shift by offset within dword Ù right
      SHRD  EAX,ESI,length          ; bring in new bits
      ROL   EAX,length  ; right justify new bit field
      MOV   EBX,EAX     ; temp storage for part of string         ¿ rotate
      SHLD  EAX,EDX,CL  ; double shift back by offset within word à EDX:EAX
      SHLD  EDX,EBX,CL  ; double shift back by offset within word Ù left
      MOV   [EDI]strg_base,EAX      ; replace dword in memory
      MOV   [EDI]strg_base+4,EDX    ; replace dword in memory

  3.  Bit String Insert into Memory (when bit string is exactly 32 bits
      long, i.e., spans five or four types of memory):

      ; Insert right-justified bit string from register into
      ; memory bit string.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is 32
      ;    but the bit offset is held in a register.
      ;
      ; Register ESI holds the 32-bit string to be inserted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX, EBX, ECX, and EDI are also used by
      ; this "insert" operation.
      ;
      MOV   EDX,EDI     ; preserve original offset for later use
      SHR   EDI,5       ; signed divide offset by 32 (dword address)
      SHL   EDI,2       ; multiply by 4 (in byte address format)
      AND   CL,1FH      ; isolate low five bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move low string dword into EAX
      MOV   EDX,[EDI]strg_base+4    ; other string dword into EDX
      MOV   EBX,EAX     ; temp storage for part of string     ¿ rotate
      SHRD  EAX,EDX     ; double shift by offset within dword à EDX:EAX
      SHRD  EDX,EBX     ; double shift by offset within dword Ù right
      MOV   EAX,ESI     ; move 32-bit bit field into position
      MOV   EBX,EAX     ; temp storage for part of string         ¿ rotate
      SHLD  EAX,EDX     ; double shift back by offset within word à EDX:EAX
      SHLD  EDX,EBX     ; double shift back by offset within word Ù left
      MOV   [EDI]strg_base,EAX      ; replace dword in memory
      MOV   [EDI]strg_base,+4,EDX   ; replace dword in memory

  4.  Bit String Extract from Memory (when bit string is 1-25 bits long,
      i.e., spans four bytes or less):

      ; Extract a right-justified bit string from memory bit
      ; string into register
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate value
      ;    but the bit offset is held in a register.
      ;
      ; Register EAX holds the right-justified, zero-padded
      ; bit string that was extracted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EDI, and ECX are also used by this "extract."
      ;
      MOV  ECX,EDI      ; temp storage for offset
      SHR  EDI,3        ; signed divide offset by 8 (byte address)
      AND  CL,7H        ; isolate low three bits of offset
      MOV  EAX,[EDI]strg_base       ; move string dword into EAX
      SHR  EAX,CL       ; shift by offset within dword
      AND  EAX,mask     ; extracted bit field in EAX

  5.  Bit String Extract from Memory (when bit string is 1-32 bits long, 
      i.e., spans five bytes or less):

      ; Extract a right-justified bit string from memory bit
      ; string into register.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate
      ;    value but the bit offset is held in a register.
      ;
      ; Register EAX holds the right-justified, zero-padded
      ; bit string that was extracted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX, EBX, and ECX are also used by this "extract."
      MOV   ECX,EDI     ; temp storage for offset
      SHR   EDI,5       ; signed divide offset by 32 (dword address)
      SHL   EDI,2       ; multiply by 4 (in byte address format)
      AND   CL,1FH      ; isolate low five bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move low string dword into EAX
      MOV   EDX,[EDI]strg_base+4    ; other string dword into EDX
      SHRD  EAX,EDX,CL  ; double shift right by offset within dword
      AND   EAX,mask    ; extracted bit field in EAX


3.4.5  Byte-Set-On-Condition Instructions

This group of instructions sets a byte to zero or one depending on any of
the 16 conditions defined by the status flags. The byte may be in memory or
may be a one-byte general register. These instructions are especially useful
for implementing Boolean expressions in high-level languages such as Pascal.

SETcc (Set Byte on Condition cc) set a byte to one if condition cc is true;
sets the byte to zero otherwise. Refer to Appendix D for a definition of
the possible conditions.


3.4.6  Test Instruction

TEST (Test) performs the logical "and" of the two operands, clears OF and
CF, leaves AF undefined, and updates SF, ZF, and PF. The flags can be tested
by conditional control transfer instructions or by the byte-set-on-condition
instructions. The operands may be doublewords, words, or bytes.

The difference between TEST and AND is that TEST does not alter the
destination operand. TEST differs from BT in that TEST is useful for testing
the value of multiple bits in one operations, whereas BT tests a single bit.


3.5  Control Transfer Instructions

The 80386 provides both conditional and unconditional control transfer
instructions to direct the flow of execution. Conditional control transfers
depend on the results of operations that affect the flag register.
Unconditional control transfers are always executed.


3.5.1  Unconditional Transfer Instructions

JMP, CALL, RET, INT and IRET instructions transfer control from one code
segment location to another. These locations can be within the same code
segment (near control transfers) or in different code segments (far control
transfers). The variants of these instructions that transfer control to
other segments are discussed in a later section of this chapter. If the
model of memory organization used in a particular 80386 application does
not make segments visible to applications programmers, intersegment control
transfers will not be used.


3.5.1.1  Jump Instruction

JMP (Jump) unconditionally transfers control to the target location. JMP is
a one-way transfer of execution; it does not save a return address on the
stack.

The JMP instruction always performs the same basic function of transferring
control from the current location to a new location. Its implementation
varies depending on whether the address is specified directly within the
instruction or indirectly through a register or memory.

A direct JMP instruction includes the destination address as part of the
instruction. An indirect JMP instruction obtains the destination address
indirectly through a register or a pointer variable.

Direct near JMP. A direct JMP uses a relative displacement value contained
in the instruction. The displacement is signed and the size of the
displacement may be a byte, word, or doubleword. The processor forms an
effective address by adding this relative displacement to the address
contained in EIP. When the additions have been performed, EIP refers to the
next instruction to be executed.

Indirect near JMP. Indirect JMP instructions specify an absolute address in
one of several ways:

  1.  The program can JMP to a location specified by a general register
      (any of EAX, EDX, ECX, EBX, EBP, ESI, or EDI). The processor moves
      this 32-bit value into EIP and resumes execution.

  2.  The processor can obtain the destination address from a memory
      operand specified in the instruction.

  3.  A register can modify the address of the memory pointer to select a
      destination address.


3.5.1.2  Call Instruction

CALL (Call Procedure) activates an out-of-line procedure, saving on the
stack the address of the instruction following the CALL for later use by a
RET (Return) instruction. CALL places the current value of EIP on the stack.
The RET instruction in the called procedure uses this address to transfer
control back to the calling program.

CALL instructions, like JMP instructions have relative, direct, and
indirect versions.

Indirect CALL instructions specify an absolute address in one of these
ways:

  1.  The program can CALL a location specified by a general register (any
      of EAX, EDX, ECX, EBX, EBP, ESI, or EDI). The processor moves this
      32-bit value into EIP.

  2.  The processor can obtain the destination address from a memory
      operand specified in the instruction.


3.5.1.3  Return and Return-From-Interrupt Instruction

RET (Return From Procedure) terminates the execution of a procedure and
transfers control through a back-link on the stack to the program that
originally invoked the procedure. RET restores the value of EIP that was
saved on the stack by the previous CALL instruction.

RET instructions may optionally specify an immediate operand. By adding
this constant to the new top-of-stack pointer, RET effectively removes any
arguments that the calling program pushed on the stack before the execution
of the CALL instruction.

IRET (Return From Interrupt) returns control to an interrupted procedure.
IRET differs from RET in that it also pops the flags from the stack into the
flags register. The flags are stored on the stack by the interrupt
mechanism.


3.5.2  Conditional Transfer Instructions

The conditional transfer instructions are jumps that may or may not
transfer control, depending on the state of the CPU flags when the
instruction executes.


3.5.2.1  Conditional Jump Instructions

Table 3-2 shows the conditional transfer mnemonics and their
interpretations. The conditional jumps that are listed as pairs are actually
the same instruction. The assembler provides the alternate mnemonics for
greater clarity within a program listing.

Conditional jump instructions contain a displacement which is added to the
EIP register if the condition is true. The displacement may be a byte, a
word, or a doubleword. The displacement is signed; therefore, it can be used
to jump forward or backward.


Table 3-2. Interpretation of Conditional Transfers

Unsigned Conditional Transfers

Mnemonic         Condition Tested          "Jump If..."

JA/JNBE           (CF or ZF) = 0           above/not below nor equal
JAE/JNB           CF = 0                   above or equal/not below
JB/JNAE           CF = 1                   below/not above nor equal
JBE/JNA           (CF or ZF) = 1           below or equal/not above
JC                CF = 1                   carry
JE/JZ             ZF = 1                   equal/zero
JNC               CF = 0                   not carry
JNE/JNZ           ZF = 0                   not equal/not zero
JNP/JPO           PF = 0                   not parity/parity odd
JP/JPE            PF = 1                   parity/parity even

Signed Conditional Transfers

Mnemonic         Condition Tested          "Jump If..."
JG/JNLE          ((SF xor OF) or ZF) = 0   greater/not less nor equal
JGE/JNL          (SF xor OF) = 0           greater or equal/not less
JL/JNGE          (SF xor OF) = 1           less/not greater nor equal
JLE/JNG          ((SF xor OF) or ZF) = 1   less or equal/not greater
JNO              OF = 0                    not overflow
JNS              SF = 0                    not sign (positive, including 0)
JO               OF = 1                    overflow
JS               SF = 1                    sign (negative)


3.5.2.2  Loop Instructions

The loop instructions are conditional jumps that use a value placed in ECX
to specify the number of repetitions of a software loop. All loop
instructions automatically decrement ECX and terminate the loop when ECX=0.
Four of the five loop instructions specify a condition involving ZF that
terminates the loop before ECX reaches zero.

LOOP (Loop While ECX Not Zero) is a conditional transfer that automatically
decrements the ECX register before testing ECX for the branch condition. If
ECX is non-zero, the program branches to the target label specified in the
instruction. The LOOP instruction causes the repetition of a code section
until the operation of the LOOP instruction decrements ECX to a value of
zero. If LOOP finds ECX=0, control transfers to the instruction immediately
following the LOOP instruction. If the value of ECX is initially zero, then
the LOOP executes 2^(32) times.

LOOPE (Loop While Equal) and LOOPZ (Loop While Zero) are synonyms for the
same instruction. These instructions automatically decrement the ECX
register before testing ECX and ZF for the branch conditions. If ECX is
non-zero and ZF=1, the program branches to the target label specified in the
instruction. If LOOPE or LOOPZ finds that ECX=0 or ZF=0, control transfers
to the instruction immediately following the LOOPE or LOOPZ instruction.

LOOPNE (Loop While Not Equal) and LOOPNZ (Loop While Not Zero) are synonyms
for the same instruction. These instructions automatically decrement the ECX
register before testing ECX and ZF for the branch conditions. If ECX is
non-zero and ZF=0, the program branches to the target label specified in the
instruction. If LOOPNE or LOOPNZ finds that ECX=0 or ZF=1, control transfers
to the instruction immediately following the LOOPNE or LOOPNZ instruction.


3.5.2.3  Executing a Loop or Repeat Zero Times

JCXZ (Jump if ECX Zero) branches to the label specified in the instruction
if it finds a value of zero in ECX. JCXZ is useful in combination with the
LOOP instruction and with the string scan and compare instructions, all of
which decrement ECX. Sometimes, it is desirable to design a loop that
executes zero times if the count variable in ECX is initialized to zero.
Because the LOOP instructions (and repeat prefixes) decrement ECX before
they test it, a loop will execute 2^(32) times if the program enters the
loop with a zero value in ECX. A programmer may conveniently overcome this
problem with JCXZ, which enables the program to branch around the code
within the loop if ECX is zero when JCXZ executes. When used with repeated
string scan and compare instructions, JCXZ can determine whether the
repetitions terminated due to zero in ECX or due to satisfaction of the
scan or compare conditions.


3.5.3  Software-Generated Interrupts

The INT n, INTO, and BOUND instructions allow the programmer to specify a
transfer to an interrupt service routine from within a program.

INT n (Software Interrupt) activates the interrupt service routine that
corresponds to the number coded within the instruction. The INT instruction
may specify any interrupt type. Programmers may use this flexibility to
implement multiple types of internal interrupts or to test the operation of
interrupt service routines. (Interrupts 0-31 are reserved by Intel.) The
interrupt service routine terminates with an IRET instruction that returns
control to the instruction that follows INT.

INTO (Interrupt on Overflow) invokes interrupt 4 if OF is set. Interrupt 4
is reserved for this purpose. OF is set by several arithmetic, logical, and
string instructions.

BOUND (Detect Value Out of Range) verifies that the signed value contained
in the specified register lies within specified limits. An interrupt (INT 5)
occurs if the value contained in the register is less than the lower bound
or greater than the upper bound.

The BOUND instruction includes two operands. The first operand specifies
the register being tested. The second operand contains the effective
relative address of the two signed BOUND limit values. The BOUND instruction
assumes that the upper limit and lower limit are in adjacent memory
locations. These limit values cannot be register operands; if they are, an
invalid opcode exception occurs.

BOUND is useful for checking array bounds before using a new index value to
access an element within the array. BOUND provides a simple way to check the
value of an index register before the program overwrites information in a
location beyond the limit of the array.

The block of memory that specifies the lower and upper limits of an array
might typically reside just before the array itself. This makes the array
bounds accessible at a constant offset from the beginning of the array.
Because the address of the array will already be present in a register, this
practice avoids extra calculations to obtain the effective address of the
array bounds.

The upper and lower limit values may each be a word or a doubleword.


3.6  String and Character Translation Instructions

The instructions in this category operate on strings rather than on logical
or numeric values. Refer also to the section on I/O for information about
the string I/O instructions (also known as block I/O).

The power of 80386 string operations derives from the following features of
the architecture:

1.  A set of primitive string operations

   MOVS   ÄÄ Move String
   CMPS   ÄÄ Compare string
   SCAS   ÄÄ Scan string
   LODS   ÄÄ Load string
   STOS   ÄÄ Store string

2.  Indirect, indexed addressing, with automatic incrementing or
    decrementing of the indexes.

   Indexes:

      ESI    ÄÄ Source index register
      EDI    ÄÄ Destination index register

   Control flag:

      DF     ÄÄ Direction flag

   Control flag instructions:

      CLD    ÄÄ Clear direction flag instruction
      STD    ÄÄ Set direction flag instruction

3.  Repeat prefixes

   REP          ÄÄ Repeat while ECX not xero
   REPE/REPZ    ÄÄ Repeat while equal or zero
   REPNE/REPNZ  ÄÄ Repeat while not equal or not zero

The primitive string operations operate on one element of a string. A
string element may be a byte, a word, or a doubleword. The string elements
are addressed by the registers ESI and EDI. After every primitive operation
ESI and/or EDI are automatically updated to point to the next element of the
string. If the direction flag is zero, the index registers are incremented;
if one, they are decremented. The amount of the increment or decrement is
1, 2, or 4 depending on the size of the string element.


3.6.1  Repeat Prefixes

The repeat prefixes REP (Repeat While ECX Not Zero), REPE/REPZ (Repeat
While Equal/Zero), and REPNE/REPNZ (Repeat While Not Equal/Not Zero) specify
repeated operation of a string primitive. This form of iteration allows the
CPU to process strings much faster than would be possible with a regular
software loop.

When a primitive string operation has a repeat prefix, the operation is
executed repeatedly, each time using a different element of the string. The
repetition terminates when one of the conditions specified by the prefix is
satisfied.

At each repetition of the primitive instruction, the string operation may
be suspended temporarily in order to handle an exception or external
interrupt. After the interruption, the string operation can be restarted
again where it left off. This method of handling strings allows operations
on strings of arbitrary length, without affecting interrupt response.

All three prefixes causes the hardware to automatically repeat the
associated string primitive until ECX=0. The differences among the repeat
prefixes have to do with the second termination condition. REPE/REPZ and
REPNE/REPNZ are used exclusively with the SCAS (Scan String) and CMPS
(Compare String) primitives. When these prefixes are used, repetition of the
next instruction depends on the zero flag (ZF) as well as the ECX register.
ZF does not require initialization before execution of a repeated string
instruction, because both SCAS and CMPS set ZF according to the results of
the comparisons they make. The differences are summarized in the
accompanying table.

Prefix                      Termination         Termination
                            Condition 1         Condition 2

REP                           ECX = 0             (none)
REPE/REPZ                     ECX = 0             ZF = 0
REPNE/REPNZ                   ECX = 0             ZF = 1


3.6.2  Indexing and Direction Flag Control

The addresses of the operands of string primitives are determined by the
ESI and EDI registers. ESI points to source operands. By default, ESI refers
to a location in the segment indicated by the DS segment register. A
segment-override prefix may be used, however, to cause ESI to refer to CS,
SS, ES, FS, or GS. EDI points to destination operands in the segment
indicated by ES; no segment override is possible. The use of two different
segment registers in one instruction allows movement of strings between
different segments.

This use of ESI and DSI has led to the descriptive names source index and
destination index for the ESI and EDI registers, respectively. In all
cases other than string instructions, however, the ESI and EDI registers may
be used as general-purpose registers.

When ESI and EDI are used in string primitives, they are automatically
incremented or decremented after to operation. The direction flag determines
whether they are incremented or decremented. The instruction CLD puts zero
in DF, causing the index registers to be incremented; the instruction STD
puts one in DF, causing the index registers to be decremented. Programmers
should always put a known value in DF before using string instructions in a
procedure.


3.6.3  String Instructions

MOVS (Move String) moves the string element pointed to by ESI to the
location pointed to by EDI. MOVSB operates on byte elements, MOVSW operates
on word elements, and MOVSD operates on doublewords. The destination segment
register cannot be overridden by a segment override prefix, but the source
segment register can be overridden.

The MOVS instruction, when accompanied by the REP prefix, operates as a
memory-to-memory block transfer. To set up for this operation, the program
must initialize ECX and the register pairs ESI and EDI. ECX specifies the
number of bytes, words, or doublewords in the block.

If DF=0, the program must point ESI to the first element of the source
string and point EDI to the destination address for the first element. If
DF=1, the program must point these two registers to the last element of the
source string and to the destination address for the last element,
respectively.

CMPS (Compare Strings) subtracts the destination string element (at ES:EDI)
from the source string element (at ESI) and updates the flags AF, SF, PF, CF
and OF. If the string elements are equal, ZF=1; otherwise, ZF=0. If DF=0,
the processor increments the memory pointers (ESI and EDI) for the two
strings. CMPSB compares bytes, CMPSW compares words, and CMPSD compares
doublewords. The segment register used for the source address can be changed
with a segment override prefix while the destination segment register
cannot be overridden.

SCAS (Scan String) subtracts the destination string element at ES:EDI from
EAX, AX, or AL and updates the flags AF, SF, ZF, PF, CF and OF. If the
values are equal, ZF=1; otherwise, ZF=0. If DF=0, the processor increments
the memory pointer (EDI) for the string. SCASB scans bytes; SCASW scans
words; SCASD scans doublewords. The destination segment register (ES) cannot
be overridden.

When either the REPE or REPNE prefix modifies either the SCAS or CMPS
primitives, the processor compares the value of the current string element
with the value in EAX for doubleword elements, in AX for word elements, or
in AL for byte elements. Termination of the repeated operation depends on
the resulting state of ZF as well as on the value in ECX.

LODS (Load String) places the source string element at ESI into EAX for
doubleword strings, into AX for word strings, or into AL for byte strings.
LODS increments or decrements ESI according to DF.

STOS (Store String) places the source string element from EAX, AX, or AL
into the string at ES:DSI. STOS increments or decrements EDI according to
DF.


3.7  Instructions for Block-Structured Languages

The instructions in this section provide machine-language support for
functions normally found in high-level languages. These instructions include
ENTER and LEAVE, which simplify the programming of procedures.

ENTER (Enter Procedure) creates a stack frame that may be used to implement
the scope rules of block-structured high-level languages. A LEAVE
instruction at the end of a procedure complements an ENTER at the beginning
of the procedure to simplify stack management and to control access to
variables for nested procedures.

The ENTER instruction includes two parameters. The first parameter
specifies the number of bytes of dynamic storage to be allocated on the
stack for the routine being entered. The second parameter corresponds to the
lexical nesting level (0-31) of the routine. (Note that the lexical level
has no relationship to either the protection privilege levels or to the I/O
privilege level.)

The specified lexical level determines how many sets of stack frame
pointers the CPU copies into the new stack frame from the preceding frame.
This list of stack frame pointers is sometimes called the display. The first
word of the display is a pointer to the last stack frame. This pointer
enables a LEAVE instruction to reverse the action of the previous ENTER
instruction by effectively discarding the last stack frame.

   Example: ENTER 2048,3

   Allocates 2048 bytes of dynamic storage on the stack and sets up pointers
   to two previous stack frames in the stack frame that ENTER creates for
   this procedure.

After ENTER creates the new display for a procedure, it allocates the
dynamic storage space for that procedure by decrementing ESP by the number
of bytes specified in the first parameter. This new value of ESP serves as a
starting point for all PUSH and POP operations within that procedure.

To enable a procedure to address its display, ENTER leaves EBP pointing to
the beginning of the new stack frame. Data manipulation instructions that
specify EBP as a base register implicitly address locations within the stack
segment instead of the data segment.

The ENTER instruction can be used in two ways: nested and non-nested. If
the lexical level is 0, the non-nested form is used. Since the second
operand is 0, ENTER pushes EBP, copies ESP to EBP and then subtracts the
first operand from ESP. The nested form of ENTER occurs when the second
parameter (lexical level) is not 0.

Figure 3-16 gives the formal definition of ENTER.

The main procedure (with other procedures nested within) operates at the
highest lexical level, level 1. The first procedure it calls operates at the
next deeper lexical level, level 2. A level 2 procedure can access the
variables of the main program which are at fixed locations specified by the
compiler. In the case of level 1, ENTER allocates only the requested
dynamic storage on the stack because there is no previous display to copy.

A program operating at a higher lexical level calling a program at a lower
lexical level requires that the called procedure should have access to the
variables of the calling program. ENTER provides this access through a
display that provides addressability to the calling program's stack frame.

A procedure calling another procedure at the same lexical level implies
that they are parallel procedures and that the called procedure should not
have access to the variables of the calling procedure. In this case, ENTER
copies only that portion of the display from the calling procedure which
refers to previously nested procedures operating at higher lexical levels.
The new stack frame does not include the pointer for addressing the calling
procedure's stack frame.

ENTER treats a reentrant procedure as a procedure calling another procedure
at the same lexical level. In this case, each succeeding iteration of the
reentrant procedure can address only its own variables and the variables of
the calling procedures at higher lexical levels. A reentrant procedure can
always address its own variables; it does not require pointers to the stack
frames of previous iterations.

By copying only the stack frame pointers of procedures at higher lexical
levels, ENTER makes sure that procedures access only those variables of
higher lexical levels, not those at parallel lexical levels (see Figure
3-17). Figures 3-18 through 3-21 demonstrate the actions of the ENTER
instruction if the modules shown in Figure 3-17 were to call one another in
alphabetic order.

Block-structured high-level languages can use the lexical levels defined by
ENTER to control access to the variables of previously nested procedures.
Referring to Figure 3-17 for example, if PROCEDURE A calls PROCEDURE B
which, in turn, calls PROCEDURE C, then PROCEDURE C will have access to the
variables of MAIN and PROCEDURE A, but not PROCEDURE B because they operate
at the same lexical level. Following is the complete definition of access to
variables for Figure 3-17.

  1.  MAIN PROGRAM has variables at fixed locations.

  2.  PROCEDURE A can access only the fixed variables of MAIN.

  3.  PROCEDURE B can access only the variables of PROCEDURE A and MAIN.
      PROCEDURE B cannot access the variables of PROCEDURE C or PROCEDURE D.

  4.  PROCEDURE C can access only the variables of PROCEDURE A and MAIN.
      PROCEDURE C cannot access the variables of PROCEDURE B or PROCEDURE D.

  5.  PROCEDURE D can access the variables of PROCEDURE C, PROCEDURE A, and
      MAIN. PROCEDURE D cannot access the variables of PROCEDURE B.

ENTER at the beginning of the MAIN PROGRAM creates dynamic storage space
for MAIN but copies no pointers. The first and only word in the display
points to itself because there is no previous value for LEAVE to return to
EBP. See Figure 3-18.

After MAIN calls PROCEDURE A, ENTER creates a new display for PROCEDURE A
with the first word pointing to the previous value of EBP (BPM for LEAVE to
return to the MAIN stack frame) and the second word pointing to the current
value of EBP. Procedure A can access variables in MAIN since MAIN is at
level 1. Therefore the base for the dynamic storage for MAIN is at [EBP-2].
All dynamic variables for MAIN are at a fixed offset from this value. See
Figure 3-19.

After PROCEDURE A calls PROCEDURE B, ENTER creates a new display for
PROCEDURE B with the first word pointing to the previous value of EBP, the
second word pointing to the value of EBP for MAIN, and the third word
pointing to the value of EBP for A and the last word pointing to the current
EBP. B can access variables in A and MAIN by fetching from the display the
base addresses of the respective dynamic storage areas. See Figure 3-20.

After PROCEDURE B calls PROCEDURE C, ENTER creates a new display for
PROCEDURE C with the first word pointing to the previous value of EBP, the
second word pointing to the value of EBP for MAIN, and the third word
pointing to the EBP value for A and the third word pointing to the current
value of EBP. Because PROCEDURE B and PROCEDURE C have the same lexical
level, PROCEDURE C is not allowed access to variables in B and therefore
does not receive a pointer to the beginning of PROCEDURE B's stack frame.
See Figure 3-21.

LEAVE (Leave Procedure) reverses the action of the previous ENTER
instruction. The LEAVE instruction does not include any operands. LEAVE
copies EBP to ESP to release all stack space allocated to the procedure by
the most recent ENTER instruction. Then LEAVE pops the old value of EBP from
the stack. A subsequent RET instruction can then remove any arguments that
were pushed on the stack by the calling program for use by the called
procedure.


Figure 3-16.  Formal Definition of the ENTER Instruction

The formal definition of the ENTER instruction for all cases is given by the
following listing. LEVEL denotes the value of the second operand.

Push EBP
Set a temporary value FRAME_PTR := ESP
If LEVEL > 0 then
      Repeat (LEVEL-1) times:
          EBP :=EBP - 4
          Push the doubleword pointed to by EBP
      End repeat
      Push FRAME_PTR
End if
EBP := FRAME_PTR
ESP := ESP - first operand.


Figure 3-17.  Variable Access in Nested Procedures

      ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
      º                MAIN PROCEDURE (LEXICAL LEVEL 1)                º
      º   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»   º
      º   º              PROCEDURE A (LEXICAL LEVEL 2)             º   º
      º   º  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»  º   º
      º   º  º           PROCEDURE B (LEXICAL LEVEL 3)          º  º   º
      º   º  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ  º   º
      º   º                                                        º   º
      º   º  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»  º   º
      º   º  º           PROCEDURE C (LEXICAL LEVEL 3)          º  º   º
      º   º  º  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»  º  º   º
      º   º  º  º        PROCEDURE D (LEXICAL LEVEL 4)       º  º  º   º
      º   º  º  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ  º  º   º
      º   º  º                                                  º  º   º
      º   º  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ  º   º
      º   º                                                        º   º
      º   ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ   º
      º                                                                º
      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 3-18.  Stack Frame for MAIN at Level 1

                                      31          0 
                D  O                 º               º
                I  F              ÚÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                R                 ³  º    OLD ESP    º
                E  E     DISPLAY Ä´  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄEBP FOR
                C  X              ³  º      EBPM
EBPM = EBP VALUE FOR MAIN    º    MAIN
                T  P              ÆÍ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                I  A              ³  º               º
                O  N              ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                N  S     DYNAMIC Ä´  º               º
                   I     STORAGE  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                 ³ O              ³  º               º
                 ³ N              ÀÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
                 ³                   º               º
                                                   


Figure 3-19.  Stack Frame for Procedure A

                                      31          0 
                D  O                 º               º
                I  F                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                R                    º    OLD ESP    º
                E  E                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                C  X                 º      EBPM
EBPM = EBP VALUE FOR MAIN    º
                T  P                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                I  A                 º               º
                O  N                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                N  S                 º               º
                   I                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                 ³ O                 º               º
                 ³ N              ÚÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                 ³                ³  º      EBPM     º
                                 ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄEBP FOR A
                         DISPLAY Ä´  º      EBPM     º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º      EBPA
EBPA = EBP VALUE FOR PROCEDURE A    º
                                  ÆÍ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º               º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                         DYNAMIC Ä´  º               º
                         STORAGE  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º               º
                                  ÀÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
                                     º               º
                                                    


Figure 3-20.  Stack Frame for Procedure B at Level 3 Called from A

                                      31          0 
                D  O                 º               º
                I  F                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                R                    º    OLD ESP    º
                E  E                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                C  X                 º      EBPM
EBPM = EBP VALUE FOR MAIN    º
                T  P                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                I  A                 º               º
                O  N                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                N  S                 º               º
                   I                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                 ³ O                 º               º
                 ³ N                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                 ³                   º      EBPM     º
                                    ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º      EBPM     º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º      EBPA     º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º               º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º               º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º               º
                                  ÚÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º      EBPA     º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄEBP
                                  ³  º      EBPM     º
                         DISPLAY Ä´  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º      EBPA     º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º      EBPB
EBPB = EBP VALUE FOR PROCEDURE B    º
                                  ÆÍ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º               º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                         DYNAMIC Ä´  º               º
                         STORAGE  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º               º
                                  ÀÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
                                     º               º
                                                    


Figure 3-21.  Stack Frame for Procedure C at Level 3 Called from B

                                      31          0 
                D  O                 º               º
                I  F                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                R                    º    OLD ESP    º
                E  E                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                C  X                 º      EBPM
EBPM = EBP VALUE FOR MAIN    º
                T  P                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                I  A                 º               º
                O  N                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                N  S                 º               º
                   I                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                 ³ O                 º               º
                 ³ N                 ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                 ³                   º      EBPM     º
                                    ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º      EBPM     º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º      EBPA
EBPA = EBP VALUE FOR PROCEDURE A    º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º               º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º               º
                                     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                     º               º
                                  ÚÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º      EBPA     º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄEBP
                                  ³  º      EBPM     º
                         DISPLAY Ä´  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º      EBPA     º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º      EBPB
EBPB = EBP VALUE FOR PROCEDURE B    º
                                  ÆÍ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º               º
                                  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                         DYNAMIC Ä´  º               º
                         STORAGE  ³  ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
                                  ³  º               º
                                  ÀÄ ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄESP
                                     º               º
                                                    


3.8  Flag Control Instructions

The flag control instructions provide a method for directly changing the
state of bits in the flag register.


3.8.1  Carry and Direction Flag Control Instructions

The carry flag instructions are useful in conjunction with
rotate-with-carry instructions RCL and RCR. They can initialize the carry
flag, CF, to a known state before execution of a rotate that moves the carry
bit into one end of the rotated operand.

The direction flag control instructions are specifically included to set or
clear the direction flag, DF, which controls the left-to-right or
right-to-left direction of string processing. If DF=0, the processor
automatically increments the string index registers, ESI and EDI, after each
execution of a string primitive. If DF=1, the processor decrements these
index registers. Programmers should use one of these instructions before any
procedure that uses string instructions to insure that DF is set properly.

Flag Control Instruction                  Effect

STC (Set Carry Flag)                      CF  1
CLC (Clear Carry Flag)                    CF  0
CMC (Complement Carry Flag)               CF  NOT (CF)
CLD (Clear Direction Flag)                DF  0
STD (Set Direction Flag)                  DF  1


3.8.2  Flag Transfer Instructions

Though specific instructions exist to alter CF and DF, there is no direct
method of altering the other applications-oriented flags. The flag transfer
instructions allow a program to alter the other flag bits with the bit
manipulation instructions after transferring these flags to the stack or the
AH register.

The instructions LAHF and SAHF deal with five of the status flags, which
are used primarily by the arithmetic and logical instructions.

LAHF (Load AH from Flags) copies SF, ZF, AF, PF, and CF to AH bits 7, 6, 4,
2, and 0, respectively (see Figure 3-22). The contents of the remaining bits
(5, 3, and 1) are undefined. The flags remain unaffected.

SAHF (Store AH into Flags) transfers bits 7, 6, 4, 2, and 0 from AH into
SF, ZF, AF, PF, and CF, respectively (see Figure 3-22).

The PUSHF and POPF instructions are not only useful for storing the flags
in memory where they can be examined and modified but are also useful for
preserving the state of the flags register while executing a procedure.

PUSHF (Push Flags) decrements ESP by two and then transfers the low-order
word of the flags register to the word at the top of stack pointed to by ESP
(see Figure 3-23). The variant PUSHFD decrements ESP by four, then
transfers both words of the extended flags register to the top of the stack
pointed to by ESP (the VM and RF flags are not moved, however).

POPF (Pop Flags) transfers specific bits from the word at the top of stack
into the low-order byte of the flag register (see Figure 3-23), then
increments ESP by two. The variant POPFD transfers specific bits from the
doubleword at the top of the stack into the extended flags register (the RF
and VM flags are not changed, however), then increments ESP by four.


Figure 3-22.  LAHF and SAHF

                     7    6    5    4    3    2    1    0
                   ÉÍÍÍÍËÍÍÍÍËÍÍÍÍËÍÍÍÍËÍÍÍÍËÍÍÍÍËÍÍÍÍËÍÍÍÍ»
                   º SF º ZF º UU º AF º UU º PF º UU º CF º
                   ÈÍÍÍÍÊÍÍÍÍÊÍÍÍÍÊÍÍÍÍÊÍÍÍÍÊÍÍÍÍÊÍÍÍÍÊÍÍÍͼ

     LAHF LOADS FIVE FLAGS FROM THE FLAG REGISTER INTO REGISTER AH. SAHF
     STORES THESE SAME FIVE FLAGS FROM AH INTO THE FLAG REGISTER. THE BIT
     POSITION OF EACH FLAG IS THE SAME IN AH AS IT IS IN THE FLAG REGISTER.
     THE REMAINING BITS (MARKED UU) ARE RESERVED; DO NOT DEFINE.


3.9  Coprocessor Interface Instructions

A numerics coprocessor (e.g., the 80387 or 80287) provides an extension to
the instruction set of the base architecture. The coprocessor extends the
instruction set of the base architecture to support high-precision integer
and floating-point calculations. This extended instruction set includes
arithmetic, comparison, transcendental, and data transfer instructions. The
coprocessor also contains a set of useful constants to enhance the speed of
numeric calculations.

A program contains instructions for the coprocessor in line with the
instructions for the CPU. The system executes these instructions in the same
order as they appear in the instruction stream. The coprocessor operates
concurrently with the CPU to provide maximum throughput for numeric
calculations.

The 80386 also has features to support emulation of the numerics
coprocessor when the coprocessor is absent. The software emulation of the
coprocessor is transparent to application software but requires more time
for execution. Refer to Chapter 11 for more information on coprocessor
emulation.

ESC (Escape) is a 5-bit sequence that begins the opcodes that identify
floating point numeric instructions. The ESC pattern tells the 80386 to send
the opcode and addresses of operands to the numerics coprocessor. The
numerics coprocessor uses the escape instructions to perform
high-performance, high-precision floating point arithmetic that conforms to
the IEEE floating point standard 754.

WAIT (Wait) is an 80386 instruction that suspends program execution until
the 80386 CPU detects that the BUSY pin is inactive. This condition
indicates that the coprocessor has completed its processing task and that
the CPU may obtain the results.


Figure 3-23.  Flag Format for PUSHF and POPF

                                PUSHFD/POPFD
     ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
                                                  PUSHF/POPF
                                     ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
      31              23               15                7           0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÑÍÑÍÑÍÑÍÑÍÍÍÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍ»
     º                           ³V³R³ ³N³ID  ³O³D³I³T³S³Z³ ³A³ ³P³ ³Cº
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0³ ³ ³0³ ³    ³ ³ ³ ³ ³ ³ ³0³ ³0³ ³1³ º
     º                           ³M³F³ ³T³  PL³F³F³F³F³F³F³ ³F³ ³F³ ³Fº
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÏÍÏÍÏÍÏÍÏÍÍÍÍÏÍÏÍÏÍÏÍÏÍÏÍÏÍÏÍÏÍÏÍÏÍÏͼ

     BITS MARKED 0 AND 1 ARE RESERVED BY INTEL. DO NOT DEFINE.

   SYSTEMS FLAGS (INCLUDING THE IOPL FIELD, AND THE VM, RF, AND IF FLAGS)
   ARE PUSHED AND ARE VISIBLE TO APPLICATIONS PROGRAMS. HOWEVER, WHEN AN
   APPLICATIONS PROGRAM POPS THE FLAGS, THESE ITEMS ARE NOT CHANGED,
   REGARDLESS OF THE VALUES POPPED INTO THEM.


3.10  Segment Register Instructions

This category actually includes several distinct types of instructions.
These various types are grouped together here because, if systems designers
choose an unsegmented model of memory organization, none of these
instructions is used by applications programmers. The instructions that deal
with segment registers are:

1.  Segment-register transfer instructions.

   MOV SegReg, ...
   MOV ..., SegReg
   PUSH SegReg
   POP SegReg

2.  Control transfers to another executable segment.

   JMP far    ; direct and indirect
   CALL far
   RET far

3.  Data pointer instructions.

   LDS
   LES
   LFS
   LGS
   LSS

Note that the following interrupt-related instructions are different; all
are capable of transferring control to another segment, but the use of
segmentation is not apparent to the applications programmer.

INT n
INTO
BOUND
IRET


3.10.1  Segment-Register Transfer Instructions

The MOV, POP, and PUSH instructions also serve to load and store segment
registers. These variants operate similarly to their general-register
counterparts except that one operand can be a segment register. MOV cannot
move segment register to a segment register. Neither POP nor MOV can place a
value in the code-segment register CS; only the far control-transfer
instructions can change CS.


3.10.2  Far Control Transfer Instructions

The far control-transfer instructions transfer control to a location in
another segment by changing the content of the CS register.

Direct far JMP. Direct JMP instructions that specify a target location
outside the current code segment contain a far pointer. This pointer
consists of a selector for the new code segment and an offset within the new
segment.

Indirect far JMP. Indirect JMP instructions that specify a target location
outside the current code segment use a 48-bit variable to specify the far
pointer.

Far CALL. An intersegment CALL places both the value of EIP and CS on the
stack.

Far RET. An intersegment RET restores the values of both CS and EIP which
were saved on the stack by the previous intersegment CALL instruction.


3.10.3  Data Pointer Instructions

The data pointer instructions load a pointer (consisting of a segment
selector and an offset) to a segment register and a general register.

LDS (Load Pointer Using DS) transfers a pointer variable from the source
operand to DS and the destination register. The source operand must be a
memory operand, and the destination operand must be a general register. DS
receives the segment-selector of the pointer. The destination register
receives the offset part of the pointer, which points to a specific location
within the segment.

Example: LDS ESI, STRING_X

Loads DS with the selector identifying the segment pointed to by a
STRING_X, and loads the offset of STRING_X into ESI.  Specifying ESI as the
destination operand is a convenient way to prepare for a string operation on
a source string that is not in the current data segment.

LES (Load Pointer Using ES) operates identically to LDS except that ES
receives the segment selector rather than DS.

Example: LES EDI, DESTINATION_X

Loads ES with the selector identifying the segment pointed to by
DESTINATION_X, and loads the offset of DESTINATION_X into EDI. This
instruction provides a convenient way to select a destination for a string
operation if the desired location is not in the current extra segment.

LFS (Load Pointer Using FS) operates identically to LDS except that FS
receives the segment selector rather than DS.

LGS (Load Pointer Using GS) operates identically to LDS except that GS
receives the segment selector rather than DS.

LSS (Load Pointer Using SS) operates identically to LDS except that SS
receives the segment selector rather than DS.  This instruction is
especially important, because it allows the two registers that identify the
stack (SS:ESP) to be changed in one uninterruptible operation.  Unlike the
other instructions which load SS, interrupts are not inhibited at the end
of the LSS instruction.  The other instructions (e.g., POP SS) inhibit
interrupts to permit the following instruction to load ESP, thereby forming
an indivisible load of SS:ESP.  Since both SS and ESP can be loaded by LSS,
there is no need to inhibit interrupts.


3.11  Miscellaneous Instructions

The following instructions do not fit in any of the previous categories,
but are nonetheless useful.


3.11.1  Address Calculation Instruction

LEA (Load Effective Address) transfers the offset of the source operand
(rather than its value) to the destination operand.  The source operand must
be a memory operand, and the destination operand must be a general register.
This instruction is especially useful for initializing registers before the
execution of the string primitives (ESI, EDI) or the XLAT instruction (EBX).
The LEA can perform any indexing or scaling that may be needed.

Example: LEA EBX, EBCDIC_TABLE

Causes the processor to place the address of the starting location of the
table labeled EBCDIC_TABLE into EBX.


3.11.2  No-Operation Instruction

NOP (No Operation) occupies a byte of storage but affects nothing but the
instruction pointer, EIP.


3.11.3  Translate Instruction

XLAT (Translate) replaced a byte in the AL register with a byte from a
user-coded translation table. When XLAT is executed, AL should have the
unsigned index to the table addressed by EBX. XLAT changes the contents of
AL from table index to table entry. EBX is unchanged. The XLAT instruction
is useful for translating from one coding system to another such as from
ASCII to EBCDIC.  The translate table may be up to 256 bytes long.  The
value placed in the AL register serves as an index to the location of the
corresponding translation value.


                        PART II  SYSTEMS PROGRAMMING                       


Chapter 4  Systems Architecture

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Many of the architectural features of the 80386 are used only by systems
programmers. This chapter presents an overview of these aspects of the
architecture.

The systems-level features of the 80386 architecture include:

  Memory Management
  Protection
  Multitasking
  Input/Output
  Exceptions and Interrupts
  Initialization
  Coprocessing and Multiprocessing
  Debugging

These features are implemented by registers and instructions, all of which
are introduced in the following sections. The purpose of this chapter is not
to explain each feature in detail, but rather to place the remaining
chapters of Part II in perspective. Each mention in this chapter of a
register or instruction is either accompanied by an explanation or a
reference to a following chapter where detailed information can be obtained.


4.1  Systems Registers

The registers designed for use by systems programmers fall into these
classes:

  EFLAGS
  Memory-Management Registers
  Control Registers
  Debug Registers
  Test Registers


4.1.1  Systems Flags

The systems flags of the EFLAGS register control I/O, maskable interrupts,
debugging, task switching, and enabling of virtual 8086 execution in a
protected, multitasking environment. These flags are highlighted in Figure
4-1.

IF (Interrupt-Enable Flag, bit 9)

   Setting IF allows the CPU to recognize external (maskable) interrupt
   requests. Clearing IF disables these interrupts. IF has no effect on
   either exceptions or nonmaskable external interrupts. Refer to Chapter
   9 for more details about interrupts.

NT (Nested Task, bit 14)

   The processor uses the nested task flag to control chaining of
   interrupted and called tasks. NT influences the operation of the IRET
   instruction. Refer to Chapter 7 and Chapter 9 for more information on
   nested tasks.

RF (Resume Flag, bit 16)

   The RF flag temporarily disables debug exceptions so that an instruction
   can be restarted after a debug exception without immediately causing
   another debug exception. Refer to Chapter 12 for details.

TF (Trap Flag, bit 8)

   Setting TF puts the processor into single-step mode for debugging. In
   this mode, the CPU automatically generates an exception after each
   instruction, allowing a program to be inspected as it executes each
   instruction. Single-stepping is just one of several debugging features of
   the 80386. Refer to Chapter 12 for additional information.

VM (Virtual 8086 Mode, bit 17)

   When set, the VM flag indicates that the task is executing an 8086
   program. Refer to Chapter 14 for a detailed discussion of how the 80386
   executes 8086 tasks in a protected, multitasking environment.


Figure 4-1.  System Flags of EFLAGS Register

      31              23               15                7           0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÑÍÑÍÑØÑÍÑÍÍÍÍÑÍÑÍÑÍÑÍÑØÑÍÑÍÑÍÑÍÑÍÑÍÑÍ»
     º±±±±±±±±±±±±±±±±±±±±±±±±±±±³V³R³±³N³ID  ³O³D³I³T³S³Z³±³A³±³P³±³Cº
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0³ ³ ³0³ ³    ³±³±³ ³±³±³±³0³±³0³±³1³±º
     º±±±±±±±±±±±±±±±±±±±±±±±±±±±³M³F³±³T³  PL³F³F³F³F³F³F³±³F³±³F³±³Fº
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÏÑÏÑÏØÏÑÏÍÑÍÍÏÍÏÍÏÑÏÍÏØÏÍÏÍÏÍÏÍÏÍÏÍÏͼ
                                  ³ ³   ³  ³       ³
             VIRTUAL 8086 MODEÄÄÄÄÙ ³   ³  ³       ³
                   RESUME FLAGÄÄÄÄÄÄÙ   ³  ³       ³
              NESTED TASK FLAGÄÄÄÄÄÄÄÄÄÄÙ  ³       ³
           I/O PRIVILEGE LEVELÄÄÄÄÄÄÄÄÄÄÄÄÄÙ       ³
              INTERRUPT ENABLEÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
NOTE
      0 OR 1 INDICATES INTEL RESERVED. DO NOT DEFINE.
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ


4.1.2  Memory-Management Registers

Four registers of the 80386 locate the data structures that control
segmented memory management:

GDTR    Global Descriptor Table Register
LDTR    Local Descriptor Table Register

   These registers point to the segment descriptor tables GDT and LDT.
   Refer to Chapter 5 for an explanation of addressing via descriptor
   tables.

IDTR    Interrupt Descriptor Table Register

   This register points to a table of entry points for interrupt handlers
   (the IDT). Refer to Chapter 9 for details of the interrupt mechanism.

TR      Task Register

   This register points to the information needed by the processor to define
   the current task. Refer to Chapter 7 for a description of the
   multitasking features of the 80386.


4.1.3  Control Registers

Figure 4-2 shows the format of the 80386 control registers CR0, CR2, and
CR3. These registers are accessible to systems programmers only via variants
of the MOV instruction, which allow them to be loaded from or stored in
general registers; for example:

MOV EAX, CR0
MOV CR3, EBX

CR0 contains system control flags, which control or indicate conditions
that apply to the system as a whole, not to an individual task.

EM (Emulation, bit 2)

   EM indicates whether coprocessor functions are to be emulated. Refer to
   Chapter 11 for details.

ET (Extension Type, bit 4)

   ET indicates the type of coprocessor present in the system (80287 or
   80387). Refer to Chapter 11 and Chapter 10 for details.

MP (Math Present, bit 1)

   MP controls the function of the WAIT instruction, which is used to
   coordinate a coprocessor. Refer to Chapter 11 for details.

PE (Protection Enable, bit 0)

   Setting PE causes the processor to begin executing in protected mode.
   Resetting PE returns to real-address mode. Refer to Chapter 14 and
   Chapter 10 for more information on changing processor modes.

PG (Paging, bit 31)

   PG indicates whether the processor uses page tables to translate linear
   addresses into physical addresses. Refer to Chapter 5 for a description
   of page translation; refer to Chapter 10 for a discussion of how to set
   PG.

TS (Task Switched, bit 3)

   The processor sets TS with every task switch and tests TS when
   interpreting coprocessor instructions. Refer to Chapter 11 for details.

CR2 is used for handling page faults when PG is set. The processor stores
in CR2 the linear address that triggers the fault. Refer to Chapter 9 for a
description of page-fault handling.

CR3 is used when PG is set. CR3 enables the processor to locate the page
table directory for the current task. Refer to Chapter 5 for a description
of page tables and page translation.


Figure 4-2.  Control Registers

 31                23                15                7               0
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
º                                            º                          º
º    PAGE DIRECTORY BASE REGISTER (PDBR)     º         RESERVED         ºCR3
ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÐÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
º                                                                       º
º                       PAGE FAULT LINEAR ADDRESS                       ºCR2
ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
º                                                                       º
º                                RESERVED                               ºCR1
ÇÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÂÄÂÄÂÄÂĶ
ºP³                                                           ³E³T³E³M³Pº
ºG³                              RESERVED                     ³T³S³M³P³EºCR0
ÈÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÏÍÏÍÏÍÏÍÏͼ


4.1.4  Debug Register

The debug registers bring advanced debugging abilities to the 80386,
including data breakpoints and the ability to set instruction breakpoints
without modifying code segments. Refer to Chapter 12 for a complete
description of formats and usage.


4.1.5  Test Registers

The test registers are not a standard part of the 80386 architecture. They
are provided solely to enable confidence testing of the translation
lookaside buffer (TLB), the cache used for storing information from page
tables. Chapter 12 explains how to use these registers.


4.2  Systems Instructions

Systems instructions deal with such functions as:

  1.  Verification of pointer parameters (refer to Chapter 6):

    ARPL             ÄÄ Adjust RPL
    LAR              ÄÄ Load Access Rights
    LSL              ÄÄ Load Segment Limit
    VERR             ÄÄ Verify for Reading
    VERW             ÄÄ Verify for Writing

  2.  Addressing descriptor tables (refer to Chaper 5):

    LLDT             ÄÄ Load LDT Register
    SLDT             ÄÄ Store LDT Register
    LGDT             ÄÄ Load GDT Register
    SGDT             ÄÄ Store GDT Register

  3.  Multitasking (refer to Chapter 7):

    LTR              ÄÄ Load Task Register
    STR              ÄÄ Store Task Register

  4. Coprocessing and Multiprocessing (refer to Chapter 11):

    CLTS             ÄÄ Clear Task-Switched Flag
    ESC              ÄÄ Escape instructions
    WAIT             ÄÄ Wait until Coprocessor not Busy
    LOCK             ÄÄ Assert Bus-Lock Signal

  5.  Input and Output (refer to Chapter 8):

    IN               ÄÄ Input
    OUT              ÄÄ Output
    INS              ÄÄ Input String
    OUTS             ÄÄ Output String

  6.  Interrupt control (refer to Chapter 9):

    CLI              ÄÄ Clear Interrupt-Enable Flag
    STI              ÄÄ Set Interrupt-Enable Flag
    LIDT             ÄÄ Load IDT Register
    SIDT             ÄÄ Store IDT Register

  7.  Debugging (refer to Chapter 12):

    MOV              ÄÄ Move to and from debug registers

  8.  TLB testing (refer to Chapter 10):

    MOV              ÄÄ Move to and from test registers

  9.  System Control:

    SMSW             ÄÄ Set MSW
    LMSW             ÄÄ Load MSW
    HLT              ÄÄ Halt Processor
    MOV              ÄÄ Move to and from control registers

The instructions SMSW and LMSW are provided for compatibility with the
80286 processor.  80386 programs access the MSW in CR0 via variants of the
MOV instruction.  HLT stops the processor until receipt of an INTR or RESET
signal.

In addition to the chapters cited above, detailed information about each of
these instructions can be found in the instruction reference chapter,
Chapter 17.


Chapter 5  Memory Management

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

The 80386 transforms logical addresses (i.e., addresses as viewed by
programmers) into physical address (i.e., actual addresses in physical
memory) in two steps:

  þ  Segment translation, in which a logical address (consisting of a
     segment selector and segment offset) are converted to a linear address.

  þ  Page translation, in which a linear address is converted to a physical
     address. This step is optional, at the discretion of systems-software
     designers.

These translations are performed in a way that is not visible to
applications programmers. Figure 5-1 illustrates the two translations at a
high level of abstraction.

Figure 5-1 and the remainder of this chapter present a simplified view of
the 80386 addressing mechanism. In reality, the addressing mechanism also
includes memory protection features. For the sake of simplicity, however,
the subject of protection is taken up in another chapter, Chapter 6.


Figure 5-1.  Address Translation Overview

             15           0      31                           0
    LOGICAL ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
    ADDRESS º    SELECTOR   º   º            OFFSET            º
            ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ   ÈÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                                    
                     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
                     º     SEGMENT TRANSLATION      º
                     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                                 ÉÍÍÏÍ»       PAGING ENABLED
                                 ºPG ?ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
                                 ÈÍÍÑͼ                    ³
                   31        PAGING  DISABLED       0     ³
          LINEAR  ÉÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍ»    ³
          ADDRESS º    DIR    º   PAGE    º  OFFSET   º    ³
                  ÈÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÑÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍͼ    ³
                                                          ³
                     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»      ³
                     º       PAGE TRANSLATION       º      ³
                     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ      ³
                                    ³ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                      31                          0
            PHYSICAL ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
            ADDRESS  º                              º
                     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


5.1  Segment Translation

Figure 5-2 shows in more detail how the processor converts a logical
address into a linear address.

To perform this translation, the processor uses the following data
structures:

  þ  Descriptors
  þ  Descriptor tables
  þ  Selectors
  þ  Segment Registers


5.1.1  Descriptors

The segment descriptor provides the processor with the data it needs to map
a logical address into a linear address. Descriptors are created by
compilers, linkers, loaders, or the operating system, not by applications
programmers. Figure 5-3 illustrates the two general descriptor formats. All
types of segment descriptors take one of these formats. Segment-descriptor
fields are:

BASE: Defines the location of the segment within the 4 gigabyte linear
address space. The processor concatenates the three fragments of the base
address to form a single 32-bit value.

LIMIT: Defines the size of the segment. When the processor concatenates the
two parts of the limit field, a 20-bit value results. The processor
interprets the limit field in one of two ways, depending on the setting of
the granularity bit:

  1.  In units of one byte, to define a limit of up to 1 megabyte.

  2.  In units of 4 Kilobytes, to define a limit of up to 4 gigabytes. The
      limit is shifted left by 12 bits when loaded, and low-order one-bits
      are inserted.

Granularity bit: Specifies the units with which the LIMIT field is
interpreted. When thebit is clear, the limit is interpreted in units of one
byte; when set, the limit is interpreted in units of 4 Kilobytes.

TYPE: Distinguishes between various kinds of descriptors.

DPL (Descriptor Privilege Level): Used by the protection mechanism (refer
to Chapter 6).

Segment-Present bit: If this bit is zero, the descriptor is not valid for
use in address transformation; the processor will signal an exception when a
selector for the descriptor is loaded into a segment register. Figure 5-4
shows the format of a descriptor when the present-bit is zero. The operating
system is free to use the locations marked AVAILABLE. Operating systems that
implement segment-based virtual memory clear the present bit in either of
these cases:

  þ  When the linear space spanned by the segment is not mapped by the
     paging mechanism.

  þ  When the segment is not present in memory.

Accessed bit: The processor sets this bit when the segment is accessed;
i.e., a selector for the descriptor is loaded into a segment register or
used by a selector test instruction. Operating systems that implement
virtual memory at the segment level may, by periodically testing and
clearing this bit, monitor frequency of segment usage.

Creation and maintenance of descriptors is the responsibility of systems
software, usually requiring the cooperation of compilers, program loaders or
system builders, and therating system.


Figure 5-2.  Segment Translation

           15              0    31                                   0
  LOGICAL ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
  ADDRESS º    SELECTOR    º   º                OFFSET               º
          ÈÍÍÍÑÍÍÍÍÍÍÍÍÍÑÍͼ   ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
       ÚÄÄÄÄÄÄÙ                                   ³
       ³ DESCRIPTOR TABLE                          ³
       ³  ÉÍÍÍÍÍÍÍÍÍÍÍÍ»                           ³
       ³  º            º                           ³
       ³  º            º                           ³
       ³  º            º                           ³
       ³  º            º                           ³
       ³  ÌÍÍÍÍÍÍÍÍÍÍÍ͹                           ³
       ³  º  SEGMENT   º BASE          ÉÍÍÍ»       ³
       Àĺ DESCRIPTOR ÇÄÄÄÄÄÄÄÄÄÄÄÄÄĺ + ºÄÄÄÄÄÄÙ
          ÌÍÍÍÍÍÍÍÍÍÍÍ͹ ADDRESS       ÈÍÑͼ
          º            º                 ³
          ÈÍÍÍÍÍÍÍÍÍÍÍͼ                 ³
                                         
              LINEAR  ÉÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
              ADDRESS º    DIR     º   PAGE    º    OFFSET    º
                      ÈÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 5-3.  General Segment-Descriptor Format

         DESCRIPTORS USED FOR APPLICATIONS CODE AND DATA SEGMENTS

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÑÍÍÍÍÍÑÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                 ³ ³ ³ ³A³         ³ ³     ³ ³     ³ ³                 º
 º   BASE 31..24   ³G³X³O³V³ LIMIT   ³P³ DPL ³1³ TYPE³A³  BASE 23..16    º 4
 º                 ³ ³ ³ ³L³ 19..16  ³ ³     ³ ³     ³ ³                 º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º                                   ³                                   º
 º        SEGMENT BASE 15..0         ³       SEGMENT LIMIT 15..0         º 0
 º                                   ³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

                DESCRIPTORS USED FOR SPECIAL SYSTEM SEGMENTS

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÑÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                 ³ ³ ³ ³A³         ³ ³     ³ ³       ³                 º
 º   BASE 31..24   ³G³X³O³V³ LIMIT   ³P³ DPL ³0³  TYPE ³  BASE 23..16    º 4
 º                 ³ ³ ³ ³L³ 19..16  ³ ³     ³ ³       ³                 º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º                                   ³                                   º
 º        SEGMENT BASE 15..0         ³       SEGMENT LIMIT 15..0         º 0
 º                                   ³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

           A      - ACCESSED
           AVL    - AVAILABLE FOR USE BY SYSTEMS PROGRAMMERS
           DPL    - DESCRIPTOR PRIVILEGE LEVEL
           G      - GRANULARITY
           P      - SEGMENT PRESENT


5.1.2  Descriptor Tables

Segment descriptors are stored in either of two kinds of descriptor table:

  þ  The global descriptor table (GDT)
  þ  A local descriptor table (LDT)

A descriptor table is simply a memory array of 8-byte entries that contain
descriptors, as Figure 5-5 shows. A descriptor table is variable in length
and may contain up to 8192 (2^(13)) descriptors. The first entry of the GDT
(INDEX=0) is not used by the processor, however.

The processor locates the GDT and the current LDT in memory by means of the
GDTR and LDTR registers. These registers store the base addresses of the
tables in the linear address space and store the segment limits. The
instructions LGDT and SGDT give access to the GDTR; the instructions LLDT
and SLDT give access to the LDTR.


Figure 5-4.  Format of Not-Present Descriptor

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÑÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                                   ³ ³     ³ ³       ³                 º
 º            AVAILABLE              ³O³ DPL ³S³ TYPE  ³   AVAILABLE     º 4
 º                                   ³ ³     ³ ³       ³                 º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º                                                                       º
 º                               AVAILABLE                               º 0
 º                                                                       º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 5-5.  Descriptor Tables

      GLOBAL DESCRIPTOR TABLE                 LOCAL DESCRIPTOR TABLE
    ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»            ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
    º      ³     ³     ³      º            º      ³     ³     ³      º
    ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ            ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
    º            ³            º M          º            ³            º M
    ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ            ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ
    |                         |            |                         |
    |                         |            |                         |
    ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»            ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
    º      ³     ³     ³      º            º      ³     ³     ³      º
    ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ            ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
    º            ³            º N + 3      º            ³            º N + 3
    ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹            ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹
    º      ³     ³     ³      º            º      ³     ³     ³      º
    ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ            ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
    º            ³            º N + 2      º            ³            º N + 2
    ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹            ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹
    º      ³     ³     ³      º            º      ³     ³     ³      º
    ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ            ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
    º            ³            º N + 1      º            ³            º N + 1
    ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹            ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹
    º      ³     ³     ³      º            º      ³     ³     ³      º
    ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ            ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
    º            ³            º N          º            ³            º N
    ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ            ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ
    |                         |            |                         |
    |                         |            |                         |
    ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»            ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
    º      ³     ³     ³      º            º      ³     ³     ³      º
    ÇÄÄÄÄÄÄÁÄÄ(UNUSED)ÄÁÄÄÄÄÄĶ            ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
    º            ³            º            º            ³            º
    ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ            ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ
                                                                    
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»  ³             ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»  ³
     º         GDTR        ÇÄÄÙ             º         LDTR        ÇÄÄÙ
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ                ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


5.1.3  Selectors

The selector portion of a logical address identifies a descriptor by
specifying a descriptor table and indexing a descriptor within that table.
Selectors may be visible to applications programs as a field within a
pointer variable, but the values of selectors are usually assigned (fixed
up) by linkers or linking loaders. Figure 5-6 shows the format of a
selector.

Index: Selects one of 8192 descriptors in a descriptor table. The processor
simply multiplies this index value by 8 (the length of a descriptor), and
adds the result to the base address of the descriptor table in order to
access the appropriate segment descriptor in the table.

Table Indicator: Specifies to which descriptor table the selector refers. A
zero indicates the GDT; a one indicates the current LDT.

Requested Privilege Level: Used by the protection mechanism. (Refer to
Chapter 6.)

Because the first entry of the GDT is not used by the processor, a selector
that has an index of zero and a table indicator of zero (i.e., a selector
that points to the first entry of the GDT), can be used as a null selector.
The processor does not cause an exception when a segment register (other
than CS or SS) is loaded with a null selector. It will, however, cause an
exception when the segment register is used to access memory.  This feature
is useful for initializing unused segment registers so as to trap accidental
references.


Figure 5-6.  Format of a Selector

                         15                      4 3   0
                        ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÑÍÍÍ»
                        º                         ³T³   º
                        º           INDEX         ³ ³RPLº
                        º                         ³I³   º
                        ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÏÍÍͼ

                         TI  - TABLE INDICATOR
                         RPL - REQUESTOR'S PRIVILEGE LEVEL


Figure 5-7.  Segment Registers

             16-BIT VISIBLE
                SELECTOR                 HIDDEN DESCRIPTOR
           ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
        CS º                º                                        º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
        SS º                º                                        º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
        DS º                º                                        º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
        ES º                º                                        º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
        FS º                º                                        º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
        GS º                º                                        º
           ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


5.1.4  Segment Registers

The 80386 stores information from descriptors in segment registers, thereby
avoiding the need to consult a descriptor table every time it accesses
memory.

Every segment register has a "visible" portion and an "invisible" portion,
as Figure 5-7 illustrates. The visible portions of these segment address
registers are manipulated by programs as if they were simply 16-bit
registers. The invisible portions are manipulated by the processor.

The operations that load these registers are normal program instructions
(previously described in Chapter 3). These instructions are of two classes:

  1.  Direct load instructions; for example, MOV, POP, LDS, LSS, LGS, LFS.
      These instructions explicitly reference the segment registers.

  2.  Implied load instructions; for example, far CALL and JMP. These
      instructions implicitly reference the CS register, and load it with a
      new value.

Using these instructions, a program loads the visible part of the segment
register with a 16-bit selector. The processor automatically fetches the
base address, limit, type, and other information from a descriptor table and
loads them into the invisible part of the segment register.

Because most instructions refer to data in segments whose selectors have
already been loaded into segment registers, the processor can add the
segment-relative offset supplied by the instruction to the segment base
address with no additional overhead.


5.2  Page Translation

In the second phase of address transformation, the 80386 transforms a
linear address into a physical address. This phase of address transformation
implements the basic features needed for page-oriented virtual-memory
systems and page-level protection.

The page-translation step is optional. Page translation is in effect only
when the PG bit of CR0 is set. This bit is typically set by the operating
system during software initialization. The PG bit must be set if the
operating system is to implement multiple virtual 8086 tasks, page-oriented
protection, or page-oriented virtual memory.


5.2.1  Page Frame

A page frame is a 4K-byte unit of contiguous addresses of physical memory.
Pages begin onbyte boundaries and are fixed in size.


5.2.2  Linear Address

A linear address refers indirectly to a physical address by specifying a
page table, a page within that table, and an offset within that page. Figure
5-8 shows the format of a linear address.

Figure 5-9 shows how the processor converts the DIR, PAGE, and OFFSET
fields of a linear address into the physical address by consulting two
levels of page tables. The addressing mechanism uses the DIR field as an
index into a page directory, uses the PAGE field as an index into the page
table determined by the page directory, and uses the OFFSET field to address
a byte within the page determined by the page table.


Figure 5-8.  Format of a Linear Address

      31                 22 21                 12 11                 0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º                     º                     º                    º
     º         DIR         º        PAGE         º       OFFSET       º
     º                     º                     º                    º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 5-9.  Page Translation

                                                              PAGE FRAME
              ÉÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍ»         ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
              º    DIR    º   PAGE    º  OFFSET  º         º               º
              ÈÍÍÍÍÍÑÍÍÍÍÍÊÍÍÍÍÍÑÍÍÍÍÍÊÍÍÍÍÍÑÍÍÍͼ         º               º
                    ³           ³           ³              º               º
      ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÙ           ³           ÀÄÄÄÄÄÄÄÄÄÄÄÄĺ    PHYSICAL   º
      ³                         ³                          º    ADDRESS    º
      ³   PAGE DIRECTORY        ³      PAGE TABLE          º               º
      ³  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»      ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»      º               º
      ³  º               º      ³   º               º      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
      ³  º               º      ³   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹              
      ³  º               º      ÀÄĺ PG TBL ENTRY  ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
      ³  ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹          ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
      Àĺ   DIR ENTRY   ÇÄÄ¿       º               º
         ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ³       º               º
         º               º  ³       º               º
         ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ  ³       ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                           ³               
ÉÍÍÍÍÍÍÍ»        ³          ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
º  CR3  ÇÄÄÄÄÄÄÄÄÙ
ÈÍÍÍÍÍÍͼ


5.2.3  Page Tables

A page table is simply an array of 32-bit page specifiers. A page table is
itself a page, and therefore contains 4 Kilobytes of memory or at most 1K
32-bit entries.

Two levels of tables are used to address a page of memory. At the higher
level is a page directory. The page directory addresses up to 1K page tables
of the second level. A page table of the second level addresses up to 1K
pages. All the tables addressed by one page directory, therefore, can
address 1M pages (2^(20)). Because each page contains 4K bytes 2^(12)
bytes), the tables of one page directory can span the entire physical
address space of the 80386 (2^(20) times 2^(12) = 2^(32)).

The physical address of the current page directory is stored in the CPU
register CR3, also called the page directory base register (PDBR). Memory
management software has the option of using one page directory for all
tasks, one page directory for each task, or some combination of the two.
Refer to Chapter 10 for information on initialization of CR3. Refer to
Chapter 7 to see how CR3 can change for each task.


5.2.4  Page-Table Entries

Entries in either level of page tables have the same format. Figure 5-10
illustrates this format.


5.2.4.1  Page Frame Address

The page frame address specifies the physical starting address of a page.
Because pages are located on 4K boundaries, the low-order 12 bits are always
zero. In a page directory, the page frame address is the address of a page
table. In a second-level page table, the page frame address is the address
of the page frame that contains the desired memory operand.


5.2.4.2  Present Bit

The Present bit indicates whether a page table entry can be used in address
translation. P=1 indicates that the entry can be used.

When P=0 in either level of page tables, the entry is not valid for address
translation, and the rest of the entry is available for software use; none
of the other bits in the entry is tested by the hardware. Figure 5-11
illustrates the format of a page-table entry when P=0.

If P=0 in either level of page tables when an attempt is made to use a
page-table entry for address translation, the processor signals a page
exception. In software systems that support paged virtual memory, the
page-not-present exception handler can bring the required page into physical
memory. The instruction that caused the exception can then be reexecuted.
Refer to Chapter 9 for more information on exception handlers.

Note that there is no present bit for the page directory itself. The page
directory may be not-present while the associated task is suspended, but the
operating system must ensure that the page directory indicated by the CR3
image in the TSS is present in physical memory before the task is
dispatched. Refer to Chapter 7 for an explanation of the TSS and task
dispatching.


Figure 5-10.  Format of a Page Table Entry

       31                                  12 11                      0
      ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÑÍÍÍÑÍÑÍÑÍÍÍÑÍÑÍÑÍ»
      º                                      ³       ³   ³ ³ ³   ³U³R³ º
      º      PAGE FRAME ADDRESS 31..12       ³ AVAIL ³0 0³D³A³0 0³/³/³Pº
      º                                      ³       ³   ³ ³ ³   ³S³W³ º
      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÏÍÏÍÏÍÍÍÏÍÏÍÏͼ

                P      - PRESENT
                R/W    - READ/WRITE
                U/S    - USER/SUPERVISOR
                D      - DIRTY
                AVAIL  - AVAILABLE FOR SYSTEMS PROGRAMMER USE

                NOTE: 0 INDICATES INTEL RESERVED. DO NOT DEFINE.


Figure 5-11.  Invalid Page Table Entry

       31                                                           1 0
      ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍ»
      º                                                              ³ º
      º                            AVAILABLE                         ³0º
      º                                                              ³ º
      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏͼ


5.2.4.3  Accessed and Dirty Bits

These bits provide data about page usage in both levels of the page tables.
With the exception of the dirty bit in a page directory entry, these bits
are set by the hardware; however, the processor does not clear any of these
bits.

The processor sets the corresponding accessed bits in both levels of page
tables to one before a read or write operation to a page.

The processor sets the dirty bit in the second-level page table to one
before a write to an address covered by that page table entry. The dirty bit
in directory entries is undefined.

An operating system that supports paged virtual memory can use these bits
to determine what pages to eliminate from physical memory when the demand
for memory exceeds the physical memory available. The operating system is
responsible for testing and clearing these bits.

Refer to Chapter 11 for how the 80386 coordinates updates to the accessed
and dirty bits in multiprocessor systems.


5.2.4.4  Read/Write and User/Supervisor Bits

These bits are not used for address translation, but are used for
page-level protection, which the processor performs at the same time as
address translation. Refer to Chapter 6 where protection is discussed in
detail.


5.2.5  Page Translation Cache

For greatest efficiency in address translation, the processor stores the
most recently used page-table data in an on-chip cache. Only if the
necessary paging information is not in the cache must both levels of page
tables be referenced.

The existence of the page-translation cache is invisible to applications
programmers but not to systems programmers; operating-system programmers
must flush the cache whenever the page tables are changed. The
page-translation cache can be flushed by either of two methods:

  1.  By reloading CR3 with a MOV instruction; for example:

      MOV CR3, EAX

  2.  By performing a task switch to a TSS that has a different CR3 image
      than the current TSS. (Refer to Chapter 7 for more information on
      task switching.)


5.3  Combining Segment and Page Translation

Figure 5-12 combines Figure 5-2 and Figure 5-9 to summarize both phases
of the transformation from a logical address to a physical address when
paging is enabled. By appropriate choice of options and parameters to both
phases, memory-management software can implement several different styles of
memory management.


5.3.1  "Flat" Architecture

When the 80386 is used to execute software designed for architectures that
don't have segments, it may be expedient to effectively "turn off" the
segmentation features of the 80386. The 80386 does not have a mode that
disables segmentation, but the same effect can be achieved by initially
loading the segment registers with selectors for descriptors that encompass
the entire 32-bit linear address space. Once loaded, the segment registers
don't need to be changed. The 32-bit offsets used by 80386 instructions are
adequate to address the entire linear-address space.


5.3.2  Segments Spanning Several Pages

The architecture of the 80386 permits segments to be larger or smaller than
the size of a page (4 Kilobytes). For example, suppose a segment is used to
address and protect a large data structure that spans 132 Kilobytes. In a
software system that supports paged virtual memory, it is not necessary for
the entire structure to be in physical memory at once. The structure is
divided into 33 pages, any number of which may not be present. The
applications programmer does not need to be aware that the virtual memory
subsystem is paging the structure in this manner.


Figure 5-12.  80306 Addressing Machanism

      16                0 32                                  0
    ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ» LOGICAL
    º      SELECTOR      º                 OFFSET              º ADDRESS
    ÈÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
 ÚÄÄÄÄÄÄÄÙ                                   ³
 ³   DESCRIPTOR TABLE                         ³
 ³  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»                         ³
 ³  º               º                         ³
 ³  º               º                         ³
 ³  º               º                         ³
 ³  º               º                         ³
 ³  ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹                         ³
 ³  º   SEGMENT     º         ÉÍÍÍ»           ³
 Àĺ  DESCRIPTOR   ÇÄÄÄÄÄÄÄĺ + ºÄÄÄÄÄÄÄÄÄÄÙ
    ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹         ÈÍÑͼ
    º               º           ³
    ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ           ³
                                                              PAGE FRAME
      LINEAR  ÉÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍ»         ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
      ADDRESS º    DIR    º   PAGE    º  OFFSET  º         º               º
              ÈÍÍÍÍÍÑÍÍÍÍÍÊÍÍÍÍÍÑÍÍÍÍÍÊÍÍÍÍÍÑÍÍÍͼ         º               º
                    ³           ³           ³              º               º
      ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÙ           ³           ÀÄÄÄÄÄÄÄÄÄÄÄÄĺ    PHYSICAL   º
      ³                         ³                          º    ADDRESS    º
      ³   PAGE DIRECTORY        ³      PAGE TABLE          º               º
      ³  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»      ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»      º               º
      ³  º               º      ³   º               º      º               º
      ³  º               º      ³   º               º      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
      ³  º               º      ³   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹              
      ³  º               º      ÀÄĺ PG TBL ENTRY  ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
      ³  ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹          ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
      Àĺ   DIR ENTRY   ÇÄÄ¿       º               º
         ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ³       º               º
         º               º  ³       º               º
         ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ  ³       ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                           ³               
ÉÍÍÍÍÍÍÍ»        ³          ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
º  CR3  ÇÄÄÄÄÄÄÄÄÙ
ÈÍÍÍÍÍÍͼ


5.3.3  Pages Spanning Several Segments

On the other hand, segments may be smaller than the size of a page. For
example, consider a small data structure such as a semaphore. Because of the
protection and sharing provided by segments (refer to Chapter 6), it may be
useful to create a separate segment for each semaphore. But, because a
system may need many semaphores, it is not efficient to allocate a page for
each. Therefore, it may be useful to cluster many related segments within a
page.


5.3.4  Non-Aligned Page and Segment Boundaries

The architecture of the 80386 does not enforce any correspondence between
the boundaries of pages and segments. It is perfectly permissible for a page
to contain the end of one segment and the beginning of another. Likewise, a
segment may contain the end of one page and the beginning of another.


5.3.5  Aligned Page and Segment Boundaries

Memory-management software may be simpler, however, if it enforces some
correspondence between page and segment boundaries. For example, if segments
are allocated only in units of one page, the logic for segment and page
allocation can be combined. There is no need for logic to account for
partially used pages.


5.3.6  Page-Table per Segment

An approach to space management that provides even further simplification
of space-management software is to maintain a one-to-one correspondence
between segment descriptors and page-directory entries, as Figure 5-13
illustrates. Each descriptor has a base address in which the low-order 22
bits are zero; in other words, the base address is mapped by the first entry
of a page table. A segment may have any limit from 1 to 4 megabytes.
Depending on the limit, the segment is contained in from 1 to 1K page
frames. A task is thus limited to 1K segments (a sufficient number for many
applications), each containing up to 4 Mbytes. The descriptor, the
corresponding page-directory entry, and the corresponding page table can be
allocated and deallocated simultaneously.


Figure 5-13.  Descriptor per Page Table

                                                              PAGE FRAMES
                                                             ÉÍÍÍÍÍÍÍÍÍÍÍ»
         LDT          PAGE DIRECTORY       PAGE TABLES       º           º
    ÉÍÍÍÍÍÍÍÍÍÍ»       ÉÍÍÍÍÍÍÍÍÍÍ»        ÉÍÍÍÍÍÍÍÍÍÍ»      º           º
    º          º       º          º        º          º   ÚÄÈÍÍÍÍÍÍÍÍÍÍͼ
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹        ÌÍÍÍÍÍÍÍÍÍ͹   ³
    º          º       º          º        º   PTE    ÇÄÄÄÙ  ÉÍÍÍÍÍÍÍÍÍÍÍ»
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹        ÌÍÍÍÍÍÍÍÍÍ͹      º           º
    º          º       º          º        º   PTE    ÇÄÄÄ¿  º           º
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹        ÌÍÍÍÍÍÍÍÍÍ͹   ÀÄÈÍÍÍÍÍÍÍÍÍÍͼ
    º          º       º          º        º   PTE    ÇÄÄÄ¿
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹   ÚÄÄÄÈÍÍÍÍÍÍÍÍÍͼ   ³  ÉÍÍÍÍÍÍÍÍÍÍÍ»
    ºDESCRIPTORÇÄÄÄÄÄĺ   PDE    ÇÄÄÄÙ                   ³  º           º
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹                       ³  º           º
    ºDESCRIPTORÇÄÄÄÄÄĺ   PDE    ÇÄÄÄ¿                   ÀÄÈÍÍÍÍÍÍÍÍÍÍͼ
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹   ³    ÉÍÍÍÍÍÍÍÍÍÍ»
    º          º       º          º   ³    º          º      ÉÍÍÍÍÍÍÍÍÍÍÍ»
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹   ³    ÌÍÍÍÍÍÍÍÍÍ͹      º           º
    º          º       º          º   ³    º          º      º           º
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹   ³    ÌÍÍÍÍÍÍÍÍÍ͹   ÚÄÈÍÍÍÍÍÍÍÍÍÍͼ
    º          º       º          º   ³    º   PTE    ÇÄÄÄÙ
    ÌÍÍÍÍÍÍÍÍÍ͹       ÌÍÍÍÍÍÍÍÍÍ͹   ³    ÌÍÍÍÍÍÍÍÍÍ͹      ÉÍÍÍÍÍÍÍÍÍÍÍ»
    º          º       º          º   ³    º   PTE    ÇÄÄÄ¿  º           º
    ÈÍÍÍÍÍÍÍÍÍͼ       ÈÍÍÍÍÍÍÍÍÍͼ   ÀÄÄÄÈÍÍÍÍÍÍÍÍÍͼ   ³  º           º
        LDT           PAGE DIRECTORY       PAGE TABLES    ÀÄÈÍÍÍÍÍÍÍÍÍÍͼ
                                                              PAGE FRAMES


Chapter 6  Protection

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

6.1  Why Protection?

The purpose of the protection features of the 80386 is to help detect and
identify bugs. The 80386 supports sophisticated applications that may
consist of hundreds or thousands of program modules. In such applications,
the question is how bugs can be found and eliminated as quickly as possible
and how their damage can be tightly confined. To help debug applications
faster and make them more robust in production, the 80386 contains
mechanisms to verify memory accesses and instruction execution for
conformance to protection criteria. These mechanisms may be used or ignored,
according to system design objectives.


6.2  Overview of 80386 Protection Mechanisms

Protection in the 80386 has five aspects:

  1.  Type checking
  2.  Limit checking
  3.  Restriction of addressable domain
  4.  Restriction of procedure entry points
  5.  Restriction of instruction set

The protection hardware of the 80386 is an integral part of the memory
management hardware. Protection applies both to segment translation and to
page translation.

Each reference to memory is checked by the hardware to verify that it
satisfies the protection criteria. All these checks are made before the
memory cycle is started; any violation prevents that cycle from starting and
results in an exception. Since the checks are performed concurrently with
address formation, there is no performance penalty.

Invalid attempts to access memory result in an exception. Refer to
Chapter 9 for an explanation of the exception mechanism. The present
chapter defines the protection violations that lead to exceptions.

The concept of "privilege" is central to several aspects of protection
(numbers 3, 4, and 5 in the preceeding list). Applied to procedures,
privilege is the degree to which the procedure can be trusted not to make a
mistake that might affect other procedures or data. Applied to data,
privilege is the degree of protection that a data structure should have
from less trusted procedures.

The concept of privilege applies both to segment protection and to page
protection.


6.3  Segment-Level Protection

All five aspects of protection apply to segment translation:

  1.  Type checking
  2.  Limit checking
  3.  Restriction of addressable domain
  4.  Restriction of procedure entry points
  5.  Restriction of instruction set

The segment is the unit of protection, and segment descriptors store
protection parameters. Protection checks are performed automatically by the
CPU when the selector of a segment descriptor is loaded into a segment
register and with every segment access. Segment registers hold the
protection parameters of the currently addressable segments.


6.3.1  Descriptors Store Protection Parameters

Figure 6-1 highlights the protection-related fields of segment descriptors.

The protection parameters are placed in the descriptor by systems software
at the time a descriptor is created. In general, applications programmers do
not need to be concerned about protection parameters.

When a program loads a selector into a segment register, the processor
loads not only the base address of the segment but also protection
information. Each segment register has bits in the invisible portion for
storing base, limit, type, and privilege level; therefore, subsequent
protection checks on the same segment do not consume additional clock
cycles.


Figure 6-1.  Protection Fields of Segment Descriptors

                           DATA SEGMENT DESCRIPTOR

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º±±±±±±±±±±±±±±±±±³±³±³±³A³ LIMIT   ³±³     ³  TYPE   ³±±±±±±±±±±±±±±±±±º
 º±±±BASE 31..24±±±³G³B³0³V³ 19..16  ³P³ DPL ³         ³±±±BASE 23..16±±±º 4
 º±±±±±±±±±±±±±±±±±³±³±³±³L³         ³±³     ³1³0³E³W³A³±±±±±±±±±±±±±±±±±º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                                   º
 º±±±±±±±±SEGMENT BASE 15..0±±±±±±±±±³        SEGMENT LIMIT 15..0        º 0
 º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

                        EXECUTABLE SEGMENT DESCRIPTOR

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º±±±±±±±±±±±±±±±±±³±³±³±³A³ LIMIT   ³±³     ³  TYPE   ³±±±±±±±±±±±±±±±±±º
 º±±±BASE 31..24±±±³G³D³0³V³ 19..16  ³P³ DPL ³         ³±±±BASE 23..16±±±º 4
 º±±±±±±±±±±±±±±±±±³±³±³±³L³         ³±³     ³1³0³C³R³A³±±±±±±±±±±±±±±±±±º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                                   º
 º±±±±±±±±SEGMENT BASE 15..0±±±±±±±±±³        SEGMENT LIMIT 15..0        º 0
 º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

                         SYSTEM SEGMENT DESCRIPTOR

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÑÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º±±±±±±±±±±±±±±±±±³±³±³±³A³ LIMIT   ³±³     ³ ³       ³±±±±±±±±±±±±±±±±±º
 º±±±BASE 31..24±±±³G³X³0³V³ 19..16  ³P³ DPL ³0³ TYPE  ³±±±BASE 23..16±±±º 4
 º±±±±±±±±±±±±±±±±±³±³±³±³L³         ³±³     ³ ³       ³±±±±±±±±±±±±±±±±±º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                                   º
 º±±±±±±±±SEGMENT BASE 15..0±±±±±±±±±³       SEGMENT LIMIT 15..0         º 0
 º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


        A   - ACCESSED                              E   - EXPAND-DOWN
        AVL - AVAILABLE FOR PROGRAMMERS USE         G   - GRANULARITY
        B   - BIG                                   P   - SEGMENT PRESENT
        C   - CONFORMING                            R   - READABLE
        D   - DEFAULT                               W   - WRITABLE
        DPL - DESCRIPTOR PRIVILEGE LEVEL


6.3.1.1  Type Checking

The TYPE field of a descriptor has two functions:

  1.  It distinguishes among different descriptor formats.
  2.  It specifies the intended usage of a segment.

Besides the descriptors for data and executable segments commonly used by
applications programs, the 80386 has descriptors for special segments used
by the operating system and for gates. Table 6-1 lists all the types defined
for system segments and gates. Note that not all descriptors define
segments; gate descriptors have a different purpose that is discussed later
in this chapter.

The type fields of data and executable segment descriptors include bits
which further define the purpose of the segment (refer to Figure 6-1):

  þ  The writable bit in a data-segment descriptor specifies whether
     instructions can write into the segment.

  þ  The readable bit in an executable-segment descriptor specifies
     whether instructions are allowed to read from the segment (for example,
     to access constants that are stored with instructions). A readable,
     executable segment may be read in two ways:

  1.  Via the CS register, by using a CS override prefix.

  2.  By loading a selector of the descriptor into a data-segment register
      (DS, ES, FS,or GS).

Type checking can be used to detect programming errors that would attempt
to use segments in ways not intended by the programmer. The processor
examines type information on two kinds of occasions:

  1.  When a selector of a descriptor is loaded into a segment register.
      Certain segment registers can contain only certain descriptor types;
      for example:

  þ  The CS register can be loaded only with a selector of an executable
     segment.

  þ  Selectors of executable segments that are not readable cannot be
     loaded into data-segment registers.

  þ  Only selectors of writable data segments can be loaded into SS.

  2.  When an instruction refers (implicitly or explicitly) to a segment
      register. Certain segments can be used by instructions only in certain
      predefined ways; for example:

  þ  No instruction may write into an executable segment.

  þ  No instruction may write into a data segment if the writable bit is
     not set.

  þ  No instruction may read an executable segment unless the readable bit
     is set.


Table 6-1. System and Gate Descriptor Types

Code      Type of Segment or Gate

  0       -reserved
  1       Available 286 TSS
  2       LDT
  3       Busy 286 TSS
  4       Call Gate
  5       Task Gate
  6       286 Interrupt Gate
  7       286 Trap Gate
  8       -reserved
  9       Available 386 TSS
  A       -reserved
  B       Busy 386 TSS
  C       386 Call Gate
  D       -reserved
  E       386 Interrupt Gate
  F       386 Trap Gate


6.3.1.2  Limit Checking

The limit field of a segment descriptor is used by the processor to prevent
programs from addressing outside the segment. The processor's interpretation
of the limit depends on the setting of the G (granularity) bit. For data
segments, the processor's interpretation of the limit depends also on the
E-bit (expansion-direction bit) and the B-bit (big bit) (refer to Table
6-2).

When G=0, the actual limit is the value of the 20-bit limit field as it
appears in the descriptor. In this case, the limit may range from 0 to
0FFFFFH (2^(20) - 1 or 1 megabyte). When G=1, the processor appends 12
low-order one-bits to the value in the limit field. In this case the actual
limit may range from 0FFFH (2^(12) - 1 or 4 kilobytes) to 0FFFFFFFFH(2^(32)
- 1 or 4 gigabytes).

For all types of segments except expand-down data segments, the value of
the limit is one less than the size (expressed in bytes) of the segment. The
processor causes a general-protection exception in any of these cases:

  þ  Attempt to access a memory byte at an address > limit.
  þ  Attempt to access a memory word at an address òlimit.
  þ  Attempt to access a memory doubleword at an address ò(limit-2).

For expand-down data segments, the limit has the same function but is
interpreted differently. In these cases the range of valid addresses is from
limit + 1 to either 64K or 2^(32) - 1 (4 Gbytes) depending on the B-bit. An
expand-down segment has maximum size when the limit is zero.

The expand-down feature makes it possible to expand the size of a stack by
copying it to a larger segment without needing also to update intrastack
pointers.

The limit field of descriptors for descriptor tables is used by the
processor to prevent programs from selecting a table entry outside the
descriptor table. The limit of a descriptor table identifies the last valid
byte of the last descriptor in the table. Since each descriptor is eight
bytes long, the limit value is N * 8 - 1 for a table that can contain up to
N descriptors.

Limit checking catches programming errors such as runaway subscripts and
invalid pointer calculations. Such errors are detected when they occur, so
that identification of the cause is easier. Without limit checking, such
errors could corrupt other modules; the existence of such errors would not
be discovered until later, when the corrupted module behaves incorrectly,
and when identification of the cause is difficult.


Table 6-2. Useful Combinations of E, G, and B Bits


Case:                    1         2         3         4

Expansion Direction      U         U         D         D
G-bit                    0         1         0         1
B-bit                    X         X         0         1

Lower bound is:
     0                   X         X
     LIMIT+1                                 X
shl(LIMIT,12,1)+1                                      X

Upper bound is:
     LIMIT               X
     shl(LIMIT,12,1)               X
     64K-1                                   X
     4G-1                                              X

Max seg size is:
     64K                 X
     64K-1                         X
     4G-4K                                   X
     4G                                                X

Min seg size is:
     0                   X         X
     4K                                      X         X

shl (X, 12, 1) = shift X left by 12 bits inserting one-bits on the right


6.3.1.3  Privilege Levels

The concept of privilege is implemented by assigning a value from zero to
three to key objects recognized by the processor. This value is called the
privilege level. The value zero represents the greatest privilege, the
value three represents the least privilege. The following
processor-recognized objects contain privilege levels:

  þ  Descriptors contain a field called the descriptor privilege level
     (DPL).

  þ  Selectors contain a field called the requestor's privilege level
     (RPL). The RPL is intended to represent the privilege level of
     the procedure that originates a selector.

  þ  An internal processor register records the current privilege level
     (CPL). Normally the CPL is equal to the DPL of the segment that
     the processor is currently executing. CPL changes as control is
     transferred to segments with differing DPLs.

The processor automatically evaluates the right of a procedure to access
another segment by comparing the CPL to one or more other privilege levels.
The evaluation is performed at the time the selector of a descriptor is
loaded into a segment register. The criteria used for evaluating access to
data differs from that for evaluating transfers of control to executable
segments; therefore, the two types of access are considered separately in
the following sections.

Figure 6-2 shows how these levels of privilege can be interpreted as rings
of protection. The center is for the segments containing the most critical
software, usually the kernel of the operating system. Outer rings are for
the segments of less critical software.

It is not necessary to use all four privilege levels. Existing software
that was designed to use only one or two levels of privilege can simply
ignore the other levels offered by the 80386. A one-level system should use
privilege level zero; a two-level system should use privilege levels zero
and three.


Figure 6-2.  Levels of Privilege

                                     TASK C
              ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
              ³ ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ» ³
              ³ º                 APPLICATIONS                  º ³
              ³ º     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»     º ³
              ³ º     º        CUSTOM EXTENSIONS          º     º ³
              ³ º     º     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»     º     º ³
              ³ º     º     º    SYSTEM SERVICES    º     º     º ³
              ³ º     º     º     ÉÍÍÍÍÍÍÍÍÍÍÍ»     º     º     º ³
              ³ º     º     º     º  KERNAL   º     º     º     º ³
              ÆÍÇÄÄÄÄÄ×ÄÄÄÄÄ×ÄÄÄÄÄ×ÄÄÄÄÄÂÄÄÄÄÄ×ÄÄÄÄÄ×ÄÄÄÄÄ×ÄÄÄÄĶ͵
              ³ º     º     º     º     ³LEVELºLEVELºLEVELºLEVELº ³
              ³ º     º     º     º     ³  0  º  1  º  2  º  3  º ³
              ³ º     º     º     ÈÍÍÍÍÍØÍÍÍÍͼ     º     º     º ³
              ³ º     º     º           ³           º     º     º ³
              ³ º     º     ÈÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍͼ     º     º ³
              ³ º     º                 ³                 º     º ³
              ³ º     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ     º ³
              ³ º                       ³                       º ³
        TASK B´ ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ ÃTASK A
              ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ


6.3.2  Restricting Access to Data

To address operands in memory, an 80386 program must load the selector of a
data segment into a data-segment register (DS, ES, FS, GS, SS). The
processor automatically evaluates access to a data segment by comparing
privilege levels. The evaluation is performed at the time a selector for the
descriptor of the target segment is loaded into the data-segment register.
As Figure 6-3 shows, three different privilege levels enter into this type
of privilege check:

  1.  The CPL (current privilege level).

  2.  The RPL (requestor's privilege level) of the selector used to specify
      the target segment.

  3.  The DPL of the descriptor of the target segment.

Instructions may load a data-segment register (and subsequently use the
target segment) only if the DPL of the target segment is numerically greater
than or equal to the maximum of the CPL and the selector's RPL. In other
words, a procedure can only access data that is at the same or less
privileged level.

The addressable domain of a task varies as CPL changes. When CPL is zero,
data segments at all privilege levels are accessible; when CPL is one, only
data segments at privilege levels one through three are accessible; when CPL
is three, only data segments at privilege level three are accessible. This
property of the 80386 can be used, for example, to prevent applications
procedures from reading or changing tables of the operating system.


Figure 6-3.  Privilege Check for Data Access

          16-BIT VISIBLE
             SELECTOR            INVISIBLE DESCRIPTOR
        ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍËÍÍÍÍÍÍÍÍÍÍÍ»
     CS º               º                   ºCPLº           º
        ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÑÍÊÍÍÍÍÍÍÍÍÍÍͼ
                                              ³
    TARGET SEGMENT SELECTOR                   ³        ÉÍÍÍÍÍÍÍÍÍÍÍ»
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍËÍÍÍ»              ÀÄÄÄÄÄÄĺ PRIVILEGE º
 º         INDEX         º ºRPLÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĺ CHECK     º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÊÍÍͼ              ÚÄÄÄÄÄÄĺ BY CPU    º
                                              ³        ÈÍÍÍÍÍÍÍÍÍÍͼ
     DATA SEGMENT DESCRIPTOR              ÚÄÄÄÙ
                                          ³
  31                23                15  ³             7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÏÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                 ³ ³ ³ ³A³ LIMIT   ³ ³     ³  TYPE   ³                 º
 º   BASE 31..24   ³G³B³0³V³         ³P³ DPL ³         ³   BASE 23..16   º 4
 º                 ³ ³ ³ ³L³  19..16 ³ ³     ³1³0³E³W³A³                 º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º                                   ³                                   º
 º        SEGMENT BASE 15..0         ³        SEGMENT LIMIT 15..0        º 0
 º                                   ³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


    CPL - CURRENT PRIVILEGE LEVEL
    RPL - REQUESTOR'S PRIVILEGE LEVEL
    DPL - DESCRIPTOR PRIVILEGE LEVEL


6.3.2.1  Accessing Data in Code Segments

Less common than the use of data segments is the use of code segments to
store data. Code segments may legitimately hold constants; it is not
possible to write to a segment described as a code segment. The following
methods of accessing data in code segments are possible:

  1.  Load a data-segment register with a selector of a nonconforming,
      readable, executable segment.

  2.  Load a data-segment register with a selector of a conforming,
      readable, executable segment.

  3.  Use a CS override prefix to read a readable, executable segment whose
      selector is already loaded in the CS register.

The same rules as for access to data segments apply to case 1. Case 2 is
always valid because the privilege level of a segment whose conforming bit
is set is effectively the same as CPL regardless of its DPL. Case 3 always
valid because the DPL of the code segment in CS is, by definition, equal to
CPL.


6.3.3  Restricting Control Transfers

With the 80386, control transfers are accomplished by the instructions JMP,
CALL, RET, INT, and IRET, as well as by the exception and interrupt
mechanisms. Exceptions and interrupts are special cases that Chapter 9
covers. This chapter discusses only JMP, CALL, and RET instructions.

The "near" forms of JMP, CALL, and RET transfer within the current code
segment, and therefore are subject only to limit checking. The processor
ensures that the destination of the JMP, CALL, or RET instruction does not
exceed the limit of the current executable segment. This limit is cached in
the CS register; therefore, protection checks for near transfers require no
extra clock cycles.

The operands of the "far" forms of JMP and CALL refer to other segments;
therefore, the processor performs privilege checking. There are two ways a
JMP or CALL can refer to another segment:

  1.  The operand selects the descriptor of another executable segment.

  2.  The operand selects a call gate descriptor. This gated form of
      transfer is discussed in a later section on call gates.

As Figure 6-4 shows, two different privilege levels enter into a privilege
check for a control transfer that does not use a call gate:

  1.  The CPL (current privilege level).
  2.  The DPL of the descriptor of the target segment.

Normally the CPL is equal to the DPL of the segment that the processor is
currently executing. CPL may, however, be greater than DPL if the conforming
bit is set in the descriptor of the current executable segment. The
processor keeps a record of the CPL cached in the CS register; this value
can be different from the DPL in the descriptor of the code segment.

The processor permits a JMP or CALL directly to another segment only if one
of the following privilege rules is satisfied:

  þ  DPL of the target is equal to CPL.

  þ  The conforming bit of the target code-segment descriptor is set, and
     the DPL of the target is less than or equal to CPL.

An executable segment whose descriptor has the conforming bit set is called
a conforming segment. The conforming-segment mechanism permits sharing of
procedures that may be called from various privilege levels but should
execute at the privilege level of the calling procedure. Examples of such
procedures include math libraries and some exception handlers. When control
is transferred to a conforming segment, the CPL does not change. This is
the only case when CPL may be unequal to the DPL of the current executable
segment.

Most code segments are not conforming. The basic rules of privilege above
mean that, for nonconforming segments, control can be transferred without a
gate only to executable segments at the same level of privilege. There is a
need, however, to transfer control to (numerically) smaller privilege
levels; this need is met by the CALL instruction when used with call-gate
descriptors, which are explained in the next section. The JMP instruction
may never transfer control to a nonconforming segment whose DPL does not
equal CPL.


Figure 6-4.  Privilege Check for Control Transfer without Gate

          16-BIT VISIBLE
             SELECTOR               INVISIBLE PART
        ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍËÍÍÍÍÍÍÍÍÍÍÍ»
        º               º                   ºCPLº           º CS
        ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÑÍÊÍÍÍÍÍÍÍÍÍÍͼ
                                              ³
                                              ³        ÉÍÍÍÍÍÍÍÍÍÍÍ»
                                              ÀÄÄÄÄÄÄĺ PRIVILEGE º
                                          ÚÄÄÄÄÄÄÄÄÄÄĺ CHECK     º
                                          ³       ÚÄÄĺ BY CPU    º
     CODE-SEGMENT DESCRIPTOR              ³       ³    ÈÍÍÍÍÍÍÍÍÍÍͼ
                                          ³       ³
  31                23                15  ³       ³     7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÏÍÍÑÍÍÍÍØÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                 ³ ³ ³ ³A³ LIMIT   ³ ³     ³    ³    ³                 º
 º   BASE 31..24   ³G³D³0³V³         ³P³ DPL ³    ³    ³   BASE 23..16   º 4
 º                 ³ ³ ³ ³L³  19..16 ³ ³     ³1³1³C³R³A³                 º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º                                   ³                                   º
 º        SEGMENT BASE 15..0         ³        SEGMENT LIMIT 15..0        º 0
 º                                   ³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

    CPL - CURRENT PRIVILEGE LEVEL
    DPL - DESCRIPTOR PRIVILEGE LEVEL
    C   - CONFORMING BIT


6.3.4  Gate Descriptors Guard Procedure Entry Points

To provide protection for control transfers among executable segments
at different privilege levels, the 80386 uses gate descriptors. There are
four kinds of gate descriptors:

  þ  Call gates
  þ  Trap gates
  þ  Interrupt gates
  þ  Task gates

This chapter is concerned only with call gates. Task gates are used for
task switching, and therefore are discussed in Chapter 7. Chapter 9
explains how trap gates and interrupt gates are used by exceptions and
interrupts. Figure 6-5 illustrates the format of a call gate. A call gate
descriptor may reside in the GDT or in an LDT, but not in the IDT.

A call gate has two primary functions:

  1.  To define an entry point of a procedure.
  2.  To specify the privilege level of the entry point.

Call gate descriptors are used by call and jump instructions in the same
manner as code segment descriptors. When the hardware recognizes that the
destination selector refers to a gate descriptor, the operation of the
instruction is expanded as determined by the contents of the call gate.

The selector and offset fields of a gate form a pointer to the entry point
of a procedure. A call gate guarantees that all transitions to another
segment go to a valid entry point, rather than possibly into the middle of a
procedure (or worse, into the middle of an instruction). The far pointer
operand of the control transfer instruction does not point to the segment
and offset of the target instruction; rather, the selector part of the
pointer selects a gate, and the offset is not used. Figure 6-6 illustrates
this style of addressing.

As Figure 6-7 shows, four different privilege levels are used to check the
validity of a control transfer via a call gate:

  1.  The CPL (current privilege level).

  2.  The RPL (requestor's privilege level) of the selector used to specify
      the call gate.

  3.  The DPL of the gate descriptor.

  4.  The DPL of the descriptor of the target executable segment.

The DPL field of the gate descriptor determines what privilege levels can
use the gate. One code segment can have several procedures that are intended
for use by different privilege levels. For example, an operating system may
have some services that are intended to be used by applications, whereas
others may be intended only for use by other systems software.

Gates can be used for control transfers to numerically smaller privilege
levels or to the same privilege level (though they are not necessary for
transfers to the same level). Only CALL instructions can use gates to
transfer to smaller privilege levels. A gate may be used by a JMP
instruction only to transfer to an executable segment with the same
privilege level or to a conforming segment.

For a JMP instruction to a nonconforming segment, both of the following
privilege rules must be satisfied; otherwise, a general protection exception
results.

   MAX (CPL,RPL) ó gate DPL
   target segment DPL = CPL

For a CALL instruction (or for a JMP instruction to a conforming segment),
both of the following privilege rules must be satisfied; otherwise, a
general protection exception results.

   MAX (CPL,RPL) ó gate DPL
   target segment DPL ó CPL


Figure 6-5.  Format of 80386 Call Gate

   31                23                15               7              0
  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍ»
  º                                   ³ ³     ³  TYPE   ³     ³  DWORD  º
  º           OFFSET 31..16           ³P³ DPL ³         ³0 0 0³         º 4
  º                                   ³ ³     ³0 1 1 0 0³     ³  COUNT  º
  ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄĶ
  º                                   ³                                 º
  º              SELECTOR             ³           OFFSET 15..0          º 0
  º                                   ³                                 º
  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 6-6.  Indirect Transfer via Call Gate

         OPCODE                 OFFSET                   SELECTOR
       ÉÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍËÍËÍÍÍ»
       º  CALL   º            (NOT USED)              º INDEX º ºRPLº
       ÈÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÑÍÍÍÊÍÊÍÍͼ
                                                          ³
                      DESCRIPTOR TABLE                    ³
                 ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»              ³
                 º      ³     ³     ³      º              ³
                 ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ              ³
                 º            ³            º              ³
                 ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ              ³
                                                        ³
                                                        ³
                                                        ³
                 ÉÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»              ³
         GATE    º   OFFSET   ³ DPL ³COUNT ºÄÄÄÄÄÄÄÄÄÄÄÄÄÙ    EXECUTABLE
      DESCRIPTOR ÇÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ                     SEGMENT
          ÚÄÄÄÄÄĶ  SELECTOR  ³   OFFSET   ÇÄÄÄÄÄ¿          ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
          ³      ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹     ³          º              º
          ³      º      ³     ³     ³      º     ³          º              º
          ³      ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ     ³          º              º
          ³      º            ³            º     ³          º              º
          ³      ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹     ³          º              º
          ³      º      ³     ³     ³      º     ÀÄÄÄÄÄÄÄÄĺ  PROCEDURE   º
          ³      ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ                º              º
          ³      º            ³            º                º              º
                ÌÍÍÍÍÍÍÑÍÍÍÍÍØÍÍÍÍÍÑÍÍÍÍÍ͹                º              º
      EXECUTABLE º BASE ³     ³ DPL ³ BASE º                º              º
       SEGMENT   ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ     ÚÄÄÄÄÄÄÄÄÄÈÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
      DESCRIPTOR º    BASE    ³            ÇÄÄÄÄÄÙ
                 ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ
                                          
                                          
                                          
                 ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
                 º      ³     ³     ³      º
                 ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
                 º            ³            º
                 ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 6-7.  Privilege Check via Call Gate

               16-BIT VISIBLE
                  SELECTOR            INVISIBLE DESCRIPTOR
             ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍËÍÍÍÍÍÍÍÍÍÍÍ»
          CS º               º                   ºCPLº           º
             ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÑÍÊÍÍÍÍÍÍÍÍÍÍͼ
                                                   ³
              TARGET SELECTOR                      ³        ÉÍÍÍÍÍÍÍÍÍÍÍ»
      ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍËÍÍÍ»              ÀÄÄÄÄÄÄĺ PRIVILEGE º
      º         INDEX         º ºRPLÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĺ CHECK     º
      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÊÍÍͼ   ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĺ BY        º
                                        ³               ÚÄĺ CPU       º
                                 ÚÄÄÄÄÄÄÙ               ³   ÈÍÍÍÍÍÍÍÍÍÍͼ
                                 ³                      ³
         GATE DESCRIPTOR                               ³
    ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍ»    ³
    º        OFFSET         º   DPL    º   COUNT   º    ³
    ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍ͹    ³
    º       SELECTOR        º        OFFSET        º    ³
    ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ    ³
                                                        ³
                                                        ³
                          ÉÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÏÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍ»
               EXECUTABLE º   BASE    º   LIMIT   º    DPL   º   BASE    º
                SEGMENT   ÌÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍ͹
               DESCRIPTOR º          BASE         º        LIMIT         º
                          ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

            CPL     - CURRENT PRIVILEGE LEVEL
            RPL     - REQUESTOR'S PRIVILEGE LEVEL
            DPL     - DESCRIPTOR PRIVILEGE LEVEL


6.3.4.1  Stack Switching

If the destination code segment of the call gate is at a different
privilege level than the CPL, an interlevel transfer is being requested.

To maintain system integrity, each privilege level has a separate stack.
These stacks assure sufficient stack space to process calls from less
privileged levels. Without them, a trusted procedure would not work
correctly if the calling procedure did not provide sufficient space on the
caller's stack.

The processor locates these stacks via the task state segment (see Figure
6-8). Each task has a separate TSS, thereby permitting tasks to have
separate stacks. Systems software is responsible for creating TSSs and
placing correct stack pointers in them. The initial stack pointers in the
TSS are strictly read-only values. The processor never changes them during
the course of execution.

When a call gate is used to change privilege levels, a new stack is
selected by loading a pointer value from the Task State Segment (TSS). The
processor uses the DPL of the target code segment (the new CPL) to index the
initial stack pointer for PL 0, PL 1, or PL 2.

The DPL of the new stack data segment must equal the new CPL; if it does
not, a stack exception occurs. It is the responsibility of systems software
to create stacks and stack-segment descriptors for all privilege levels that
are used. Each stack must contain enough space to hold the old SS:ESP, the
return address, and all parameters and local variables that may be required
to process a call.

As with intralevel calls, parameters for the subroutine are placed on the
stack. To make privilege transitions transparent to the called procedure,
the processor copies the parameters to the new stack. The count field of a
call gate tells the processor how many doublewords (up to 31) to copy from
the caller's stack to the new stack. If the count is zero, no parameters are
copied.

The processor performs the following stack-related steps in executing an
interlevel CALL.

  1.  The new stack is checked to assure that it is large enough to hold
      the parameters and linkages; if it is not, a stack fault occurs with
      an error code of 0.

  2.  The old value of the stack registers SS:ESP is pushed onto the new
      stack as two doublewords.

  3.  The parameters are copied.

  4.  A pointer to the instruction after the CALL instruction (the former
      value of CS:EIP) is pushed onto the new stack. The final value of
      SS:ESP points to this return pointer on the new stack.

Figure 6-9 illustrates the stack contents after a successful interlevel
call.

The TSS does not have a stack pointer for a privilege level 3 stack,
because privilege level 3 cannot be called by any procedure at any other
privilege level.

Procedures that may be called from another privilege level and that require
more than the 31 doublewords for parameters must use the saved SS:ESP link
to access all parameters beyond the last doubleword copied.

A call via a call gate does not check the values of the words copied onto
the new stack. The called procedure should check each parameter for
validity. A later section discusses how the ARPL, VERR, VERW, LSL, and LAR
instructions can be used to check pointer values.


Figure 6-8.  Initial Stack Pointers of TSS

                 31       23       15       7      0
                ÉÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍ»64
                                                   
                                                   
                                                   
                º                                   º
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍ͹
                º              EFLAGS               º24
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍ͹
                º     INSTRUCTION POINTER (EIP)     º20
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍ͹
                º            CR3 (PDBR)             º1C
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍØÍÍÍÍÍËÍ͹  Ä¿
                º00000000 00000000º       SS2    º10º18 ³
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍØÍÍÍÍÍÊÍ͹   ³
                º               ESP2                º14 ³
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍØÍÍÍÍÍËÍ͹   ³
                º00000000 00000000º       SS1    º01º10 ³  INITIAL
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍØÍÍÍÍÍÊÍ͹   ÃÄ STACK
                º               ESP1                º0C ³  POINTERS
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍØÍÍÍÍÍËÍ͹   ³
                º00000000 00000000º       SS0    º00º8  ³
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍØÍÍÍÍÍÊÍ͹   ³
                º               ESP0                º4  ³
                ÌÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍ͹  ÄÙ
                º00000000 00000000º  TSS BACK LINK  º0
                ÈÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍͼ


Figure 6-9.  Stack Contents after an Interlevel Call

                                            31            0     SS:ESP
                                           ÉÍÍÍÍÍÍÍØÍÍÍÍÍÍÍ»ÄÄFROM TSS
                 31            0           º±±±±±±±³OLD SS º
                ÉÍÍÍÍÍÍÍØÍÍÍÍÍÍÍ»          ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       D  O     º               º          º    OLD ESP    º
       I  F     º               º          ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       R        º               º          º    PARM 3     º
       E  E     º               º          ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       C  X     º               º          º    PARM 2     º
       T  P     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹          ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       I  A     º    PARM 3     º          º    PARM 1     º
       O  N     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹          ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹
       N  S     º    PARM 2     º          º±±±±±±±³OLD CS º     NEW
          I     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹  OLD     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹    SS:ESP
        ³ O     º    PARM 1     º  SS:ESP  º    OLD EIP    º      ³
        ³ N     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄÄÙ     ÌÍÍÍÍÍÍÍØÍÍÍÍÍÍ͹ÄÄÄÄÄÙ
        ³       º               º          º               º
               º               º          º               º
                ÈÍÍÍÍÍÍÍØÍÍÍÍÍÍͼ          ÈÍÍÍÍÍÍÍØÍÍÍÍÍÍͼ
                    OLD STACK                  NEW STACK


6.3.4.2  Returning from a Procedure

The "near" forms of the RET instruction transfer control within the current
code segment and therefore are subject only to limit checking. The offset of
the instruction following the corresponding CALL, is popped from the stack.
The processor ensures that this offset does not exceed the limit of the
current executable segment.

The "far" form of the RET instruction pops the return pointer that was
pushed onto the stack by a prior far CALL instruction. Under normal
conditions, the return pointer is valid, because of its relation to the
prior CALL or INT. Nevertheless, the processor performs privilege checking
because of the possibility that the current procedure altered the pointer or
failed to properly maintain the stack. The RPL of the CS selector popped
off the stack by the return instruction identifies the privilege level of
the calling procedure.

An intersegment return instruction can change privilege levels, but only
toward procedures of lesser privilege. When the RET instruction encounters a
saved CS value whose RPL is numerically greater than the CPL, an interlevel
return occurs. Such a return follows these steps:

  1.  The checks shown in Table 6-3 are made, and CS:EIP and SS:ESP are
      loaded with their former values that were saved on the stack.

  2.  The old SS:ESP (from the top of the current stack) value is adjusted
      by the number of bytes indicated in the RET instruction. The resulting
      ESP value is not compared to the limit of the stack segment. If ESP is
      beyond the limit, that fact is not recognized until the next stack
      operation. (The SS:ESP value of the returning procedure is not
      preserved; normally, this value is the same as that contained in the
      TSS.)

  3.  The contents of the DS, ES, FS, and GS segment registers are checked.
      If any of these registers refer to segments whose DPL is greater than
      the new CPL (excluding conforming code segments), the segment register
      is loaded with the null selector (INDEX = 0, TI = 0). The RET
      instruction itself does not signal exceptions in these cases;
      however, any subsequent memory reference that attempts to use a
      segment register that contains the null selector will cause a general
      protection exception. This prevents less privileged code from
      accessing more privileged segments using selectors left in the
      segment registers by the more privileged procedure.


6.3.5  Some Instructions are Reserved for Operating System

Instructions that have the power to affect the protection mechanism or to
influence general system performance can only be executed by trusted
procedures. The 80386 has two classes of such instructions:

  1.  Privileged instructions ÄÄ those used for system control.

  2.  Sensitive instructions ÄÄ those used for I/O and I/O related
      activities.


Table 6-3. Interlevel Return Checks


Type of Check                              Exception
SF  Stack Fault
GP  General Protection Exception
NP  Segment-Not-Present Exception  Error Code

ESP is within current SS segment           SF          0
ESP + 7 is within current SS segment       SF          0
RPL of return CS is greater than CPL       GP          Return CS
Return CS selector is not null             GP          Return CS
Return CS segment is within descriptor
table limit                                GP          Return CS
Return CS descriptor is a code segment     GP          Return CS
Return CS segment is present               NP          Return CS
DPL of return nonconforming code
segment = RPL of return CS, or DPL of
return conforming code segment ó RPL
of return CS                               GP          Return CS
ESP + N + 15 is within SS segment
N   Immediate Operand of RET N Instruction         SF          Return SS
SS selector at ESP + N + 12 is not null    GP          Return SS
SS selector at ESP + N + 12 is within
descriptor table limit                     GP          Return SS
SS descriptor is writable data segment     GP          Return SS
SS segment is present                      SF          Return SS
Saved SS segment DPL = RPL of saved
CS                                         GP          Return SS
Saved SS selector RPL = Saved SS
segment DPL                                GP          Return SS


6.3.5.1  Privileged Instructions

The instructions that affect system data structures can only be executed
when CPL is zero. If the CPU encounters one of these instructions when CPL
is greater than zero, it signals a general protection exception. These
instructions include:

   CLTS                 ÄÄ Clear TaskÄSwitched Flag
   HLT                  ÄÄ Halt Processor
   LGDT                 ÄÄ Load GDL Register
   LIDT                 ÄÄ Load IDT Register
   LLDT                 ÄÄ Load LDT Register
   LMSW                 ÄÄ Load Machine Status Word
   LTR                  ÄÄ Load Task Register
   MOV to/from CRn      ÄÄ Move to Control Register n
   MOV to /from DRn     ÄÄ Move to Debug Register n
   MOV to/from TRn      ÄÄ Move to Test Register n


6.3.5.2  Sensitive Instructions

Instructions that deal with I/O need to be restricted but also need to be
executed by procedures executing at privilege levels other than zero. The
mechanisms for restriction of I/O operations are covered in detail in
Chapter 8, "Input/Output".


6.3.6  Instructions for Pointer Validation

Pointer validation is an important part of locating programming errors.
Pointer validation is necessary for maintaining isolation between the
privilege levels. Pointer validation consists of the following steps:

  1.  Check if the supplier of the pointer is entitled to access the
      segment.

  2.  Check if the segment type is appropriate to its intended use.

  3.  Check if the pointer violates the segment limit.

Although the 80386 processor automatically performs checks 2 and 3 during
instruction execution, software must assist in performing the first check.
The unprivileged instruction ARPL is provided for this purpose. Software can
also explicitly perform steps 2 and 3 to check for potential violations
(rather than waiting for an exception). The unprivileged instructions LAR,
LSL, VERR, and VERW are provided for this purpose.

LAR (Load Access Rights) is used to verify that a pointer refers to a
segment of the proper privilege level and type. LAR has one operandÄÄa
selector for a descriptor whose access rights are to be examined. The
descriptor must be visible at the privilege level which is the maximum of
the CPL and the selector's RPL. If the descriptor is visible, LAR obtains a
masked form of the second doubleword of the descriptor, masks this value
with 00FxFF00H, stores the result into the specified 32-bit destination
register, and sets the zero flag. (The x indicates that the corresponding
four bits of the stored value are undefined.) Once loaded, the access-rights
bits can be tested. All valid descriptor types can be tested by the LAR
instruction. If the RPL or CPL is greater than DPL, or if the selector is
outside the table limit, no access-rights value is returned, and the zero
flag is cleared. Conforming code segments may be accessed from any privilege
level.

LSL (Load Segment Limit) allows software to test the limit of a descriptor.
If the descriptor denoted by the given selector (in memory or a register) is
visible at the CPL, LSL loads the specified 32-bit register with a 32-bit,
byte granular, unscrambled limit that is calculated from fragmented limit
fields and the G-bit of that descriptor. This can only be done for segments
(data, code, task state, and local descriptor tables); gate descriptors are
inaccessible. (Table 6-4 lists in detail which types are valid and which
are not.) Interpreting the limit is a function of the segment type. For
example, downward expandable data segments treat the limit differently than
code segments do. For both LAR and LSL, the zero flag (ZF) is set if the
loading was performed; otherwise, the ZF is cleared.


Table 6-4. Valid Descriptor Types for LSL

    Type   Descriptor Type             Valid?
    Code

    0      (invalid)                   NO 
    1      Available 286 TSS           YES
    2      LDT                         YES
    3      Busy 286 TSS                YES
    4      286 Call Gate               NO 
    5      Task Gate                   NO 
    6      286 Trap Gate               NO 
    7      286 Interrupt Gate          NO 
    8      (invalid)                   NO 
    9      Available 386 TSS           YES
    A      (invalid)                   NO 
    B      Busy 386 TSS                YES
    C      386 Call Gate               NO 
    D      (invalid)                   NO 
    E      386 Trap Gate               NO 
    F      386 Interrupt Gate          NO 


6.3.6.1  Descriptor Validation

The 80386 has two instructions, VERR and VERW, which determine whether a
selector points to a segment that can be read or written at the current
privilege level. Neither instruction causes a protection fault if the result
is negative.

VERR (Verify for Reading) verifies a segment for reading and loads ZF with
1 if that segment is readable from the current privilege level. VERR checks
that:

  þ  The selector points to a descriptor within the bounds of the GDT or
     LDT.

  þ  It denotes a code or data segment descriptor.

  þ  The segment is readable and of appropriate privilege level.

The privilege check for data segments and nonconforming code segments is
that the DPL must be numerically greater than or equal to both the CPL and
the selector's RPL. Conforming segments are not checked for privilege level.

VERW (Verify for Writing) provides the same capability as VERR for
verifying writability. Like the VERR instruction, VERW loads ZF if the
result of the writability check is positive. The instruction checks that the
descriptor is within bounds, is a segment descriptor, is writable, and that
its DPL is numerically greater or equal to both the CPL and the selector's
RPL. Code segments are never writable, conforming or not.


6.3.6.2  Pointer Integrity and RPL

The Requestor's Privilege Level (RPL) feature can prevent inappropriate use
of pointers that could corrupt the operation of more privileged code or data
from a less privileged level.

A common example is a file system procedure, FREAD (file_id, n_bytes,
buffer_ptr). This hypothetical procedure reads data from a file into a
buffer, overwriting whatever is there. Normally, FREAD would be available at
the user level, supplying only pointers to the file system procedures and
data located and operating at a privileged level. Normally, such a procedure
prevents user-level procedures from directly changing the file tables.
However, in the absence of a standard protocol for checking pointer
validity, a user-level procedure could supply a pointer into the file tables
in place of its buffer pointer, causing the FREAD procedure to corrupt them
unwittingly.

Use of RPL can avoid such problems. The RPL field allows a privilege
attribute to be assigned to a selector. This privilege attribute would
normally indicate the privilege level of the code which generated the
selector. The 80386 processor automatically checks the RPL of any selector
loaded into a segment register to determine whether the RPL allows access.

To take advantage of the processor's checking of RPL, the called procedure
need only ensure that all selectors passed to it have an RPL at least as
high (numerically) as the original caller's CPL. This action guarantees that
selectors are not more trusted than their supplier. If one of the selectors
is used to access a segment that the caller would not be able to access
directly, i.e., the RPL is numerically greater than the DPL, then a
protection fault will result when that selector is loaded into a segment
register.

ARPL (Adjust Requestor's Privilege Level) adjusts the RPL field of a
selector to become the larger of its original value and the value of the RPL
field in a specified register. The latter is normally loaded from the image
of the caller's CS register which is on the stack. If the adjustment changes
the selector's RPL, ZF (the zero flag) is set; otherwise, ZF is cleared.


6.4  Page-Level Protection

Two kinds of protection are related to pages:

  1.  Restriction of addressable domain.
  2.  Type checking.


6.4.1  Page-Table Entries Hold Protection Parameters

Figure 6-10 highlights the fields of PDEs and PTEs that control access to
pages.


Figure 6-10.  Protection Fields of Page Table Entries

      31                                  12 11          7           0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÑÍÍÍÑÍÑÍÑÍÍÍÑÍÑÍÑÍ»
     º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³±±±±±±±³±±±³±³±³±±±³U³R³±º
     º±±±±±±PAGE FRAME ADDRESS 31..12±±±±±±±³±AVAIL±³0±0³D³A³0±0³/³/³Pº
     º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³±±±±±±±³±±±³±³±³±±±³S³W³±º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÏÍÍÍÏÍÏÍÏÍÍÍÏÍÏÍÏͼ
      R/W     - READ/WRITE
      U/S     - USER/SUPERVISOR


6.4.1.1  Restricting Addressable Domain

The concept of privilege for pages is implemented by assigning each page to
one of two levels:

  1.  Supervisor level (U/S=0) ÄÄ for the operating system and other systems
      software and related data.

  2.  User level (U/S=1) ÄÄ for applications procedures and data.

The current level (U or S) is related to CPL.  If CPL is 0, 1, or 2, the
processor is executing at supervisor level. If CPL is 3, the processor is
executing at user level.

When the processor is executing at supervisor level, all pages are
addressable, but, when the processor is executing at user level, only pages
that belong to the user level are addressable.


6.4.1.2  Type Checking

At the level of page addressing, two types are defined:

  1.  Read-only access (R/W=0)
  2.  Read/write access (R/W=1)

When the processor is executing at supervisor level, all pages are both
readable and writable. When the processor is executing at user level, only
pages that belong to user level and are marked for read/write access are
writable; pages that belong to supervisor level are neither readable nor
writable from user level.


6.4.2  Combining Protection of Both Levels of Page Tables

For any one page, the protection attributes of its page directory entry may
differ from those of its page table entry. The 80386 computes the effective
protection attributes for a page by examining the protection attributes in
both the directory and the page table.  Table 6-5 shows the effective
protection provided by the possible combinations of protection attributes.


6.4.3  Overrides to Page Protection

Certain accesses are checked as if they are privilege-level 0 references,
even if CPL = 3:

  þ  LDT, GDT, TSS, IDT references.
  þ  Access to inner stack during ring-crossing CALL/INT.


6.5  Combining Page and Segment Protection

When paging is enabled, the 80386 first evaluates segment protection, then
evaluates page protection. If the processor detects a protection violation
at either the segment or the page level, the requested operation cannot
proceed; a protection exception occurs instead.

For example, it is possible to define a large data segment which has some
subunits that are read-only and other subunits that are read-write.  In this
case, the page directory (or page table) entries for the read-only subunits
would have the U/S and R/W bits set to x0, indicating no write rights for
all the pages described by that directory entry (or for individual pages).
This technique might be used, for example, in a UNIX-like system to define
a large data segment, part of which is read only (for shared data or ROMmed
constants). This enables UNIX-like systems to define a "flat" data space as
one large segment, use "flat" pointers to address within this "flat" space,
yet be able to protect shared data, shared files mapped into the virtual
space, and supervisor areas.


Table 6-5. Combining Directory and Page Protection

Page Directory Entry     Page Table Entry      Combined Protection
  U/S          R/W         U/S      R/W          U/S         R/W

  S-0          R-0         S-0      R-0           S           x
  S-0          R-0         S-0      W-1           S           x
  S-0          R-0         U-1      R-0           S           x
  S-0          R-0         U-1      W-1           S           x
  S-0          W-1         S-0      R-0           S           x
  S-0          W-1         S-0      W-1           S           x
  S-0          W-1         U-1      R-0           S           x
  S-0          W-1         U-1      W-1           S           x
  U-1          R-0         S-0      R-0           S           x
  U-1          R-0         S-0      W-1           S           x
  U-1          R-0         U-1      R-0           U           R
  U-1          R-0         U-1      W-1           U           R
  U-1          W-1         S-0      R-0           S           x
  U-1          W-1         S-0      W-1           S           x
  U-1          W-1         U-1      R-0           U           R
  U-1          W-1         U-1      W-1           U           W

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
NOTE
  S ÄÄ Supervisor
  R ÄÄ Read only
  U ÄÄ User
  W ÄÄ Read and Write
  x indicates that when the combined U/S attribute is S, the R/W attribute
  is not checked.
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ


Chapter 7  Multitasking

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

To provide efficient, protected multitasking, the 80386 employs several
special data structures. It does not, however, use special instructions to
control multitasking; instead, it interprets ordinary control-transfer
instructions differently when they refer to the special data structures. The
registers and data structures that support multitasking are:

  þ  Task state segment
  þ  Task state segment descriptor
  þ  Task register
  þ  Task gate descriptor

With these structures the 80386 can rapidly switch execution from one task
to another, saving the context of the original task so that the task can be
restarted later. In addition to the simple task switch, the 80386 offers two
other task-management features:

  1.  Interrupts and exceptions can cause task switches (if needed in the
      system design). The processor not only switches automatically to the
      task that handles the interrupt or exception, but it automatically
      switches back to the interrupted task when the interrupt or exception
      has been serviced. Interrupt tasks may interrupt lower-priority
      interrupt tasks to any depth.

  2.  With each switch to another task, the 80386 can also switch to
      another LDT and to another page directory. Thus each task can have a
      different logical-to-linear mapping and a different linear-to-physical
      mapping. This is yet another protection feature, because tasks can be
      isolated and prevented from interfering with one another.


7.1  Task State Segment

All the information the processor needs in order to manage a task is stored
in a special type of segment, a task state segment (TSS). Figure 7-1 shows
the format of a TSS for executing 80386 tasks. (Another format is used for
executing 80286 tasks; refer to Chapter 13.)

The fields of a TSS belong to two classes:

  1.  A dynamic set that the processor updates with each switch from the
      task. This set includes the fields that store:

      þ  The general registers (EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI).

      þ  The segment registers (ES, CS, SS, DS, FS, GS).

      þ  The flags register (EFLAGS).

      þ  The instruction pointer (EIP).

      þ  The selector of the TSS of the previously executing task (updated
         only when a return is expected).

  2.  A static set that the processor reads but does not change. This set
      includes the fields that store:

      þ  The selector of the task's LDT.

      þ  The register (PDBR) that contains the base address of the task's
         page directory (read only when paging is enabled).

      þ  Pointers to the stacks for privilege levels 0-2.

      þ  The T-bit (debug trap bit) which causes the processor to raise a
         debug exception when a task switch occurs. (Refer to Chapter 12
         for more information on debugging.)

      þ  The I/O map base (refer to Chapter 8 for more information on the
         use of the I/O map).

Task state segments may reside anywhere in the linear space. The only case
that requires caution is when the TSS spans a page boundary and the
higher-addressed page is not present. In this case, the processor raises an
exception if it encounters the not-present page while reading the TSS during
a task switch. Such an exception can be avoided by either of two strategies:

  1.  By allocating the TSS so that it does not cross a page boundary.

  2.  By ensuring that both pages are either both present or both 
      not-present at the time of a task switch. If both pages are 
      not-present, then the page-fault handler must make both pages present 
      before restarting the instruction that caused the task switch.


Figure 7-1.  80386 32-Bit Task State Segment

      31              23              15              7             0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍËÍ»
     º          I/O MAP BASE         º 0 0 0 0 0 0 0   0 0 0 0 0 0 ºTº64
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÐĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              LDT              º60
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              GS               º5C
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              FS               º58
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              DS               º54
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              SS               º50
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              CS               º4C
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              ES               º48
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                              EDI                              º44
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                              ESI                              º40
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                              EBP                              º3C
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                              ESP                              º38
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                              EBX                              º34
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                              EDX                              º30
     ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
     º                              ECX                              º2C
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                              EAX                              º28
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                            EFLAGS                             º24
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                    INSTRUCTION POINTER (EIP)                  º20
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                          CR3  (PDPR)                          º1C
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              SS2              º18
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                             ESP2                              º14
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              SS1              º10
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                             ESP1                              º0C
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º              SS0              º8
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                             ESP0                              º4
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0º   BACK LINK TO PREVIOUS TSS   º0
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
NOTE
      0 MEANS INTEL RESERVED. DO NOT DEFINE.
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ


7.2  TSS Descriptor

The task state segment, like all other segments, is defined by a
descriptor. Figure 7-2 shows the format of a TSS descriptor.

The B-bit in the type field indicates whether the task is busy. A type code
of 9 indicates a non-busy task; a type code of 11 indicates a busy task.
Tasks are not reentrant. The B-bit allows the processor to detect an attempt
to switch to a task that is already busy.

The BASE, LIMIT, and DPL fields and the G-bit and P-bit have functions
similar to their counterparts in data-segment descriptors. The LIMIT field,
however, must have a value equal to or greater than 103. An attempt to
switch to a task whose TSS descriptor has a limit less that 103 causes an
exception. A larger limit is permissible, and a larger limit is required if
an I/O permission map is present. A larger limit may also be convenient for
systems software if additional data is stored in the same segment as the
TSS.

A procedure that has access to a TSS descriptor can cause a task switch. In
most systems the DPL fields of TSS descriptors should be set to zero, so
that only trusted software has the right to perform task switching.

Having access to a TSS-descriptor does not give a procedure the right to
read or modify a TSS. Reading and modification can be accomplished only with
another descriptor that redefines the TSS as a data segment. An attempt to
load a TSS descriptor into any of the segment registers (CS, SS, DS, ES, FS,
GS) causes an exception.

TSS descriptors may reside only in the GDT. An attempt to identify a TSS
with a selector that has TI=1 (indicating the current LDT) results in an
exception.


Figure 7-2.  TSS Descriptor for 32-bit TSS

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÑÍÑÍÑÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                 ³ ³ ³ ³A³ LIMIT   ³ ³     ³  TYPE   ³                 º
 º   BASE 31..24   ³G³0³0³V³         ³P³ DPL ³         ³   BASE 23..16   º 4
 º                 ³ ³ ³ ³L³  19..16 ³ ³     ³0³1³0³B³1³                 º
 ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º                                   ³                                   º
 º             BASE 15..0            ³             LIMIT 15..0           º 0
 º                                   ³                                   º
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


7.3  Task Register

The task register (TR) identifies the currently executing task by pointing
to the TSS. Figure 7-3 shows the path by which the processor accesses the
current TSS.

The task register has both a "visible" portion (i.e., can be read and
changed by instructions) and an "invisible" portion (maintained by the
processor to correspond to the visible portion; cannot be read by any
instruction). The selector in the visible portion selects a TSS descriptor
in the GDT. The processor uses the invisible portion to cache the base and
limit values from the TSS descriptor. Holding the base and limit in a
register makes execution of the task more efficient, because the processor
does not need to repeatedly fetch these values from memory when it
references the TSS of the current task.

The instructions LTR and STR are used to modify and read the visible
portion of the task register. Both instructions take one operand, a 16-bit
selector located in memory or in a general register.

LTR (Load task register) loads the visible portion of the task register
with the selector operand, which must select a TSS descriptor in the GDT.
LTR also loads the invisible portion with information from the TSS
descriptor selected by the operand. LTR is a privileged instruction; it may
be executed only when CPL is zero. LTR is generally used during system
initialization to give an initial value to the task register; thereafter,
the contents of TR are changed by task switch operations.

STR (Store task register) stores the visible portion of the task register
in a general register or memory word. STR is not privileged.


Figure 7-3.  Task Register 

                          ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
                          º                         º
                          º                         º
                          º       TASK STATE        º
                          º        SEGMENT          ºÄÄÄÄÄÄÄÄÄ¿
                          º                         º          ³
                          º                         º          ³
                          ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ          ³
           16-BIT VISIBLE                                     ³
              REGISTER                ³   HIDDEN REGISTER      ³
       ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍ»
    TR º      SELECTOR      º      (BASE)        º       (LIMT)       º
       ÈÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                 ³                                         
                 ³                    ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿   ³
                 ³          GLOBAL DESCRIPTOR TABLE     ³   ³
                 ³        ÕÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͸   ³   ³
                 ³        |     TSS DESCRIPTOR      |   ³   ³
                 ³        ÉÍÍÍÍÍÍËÍÍÍÍÍËÍÍÍÍÍËÍÍÍÍÍÍ»   ³   ³
                 ³        º      º     º     º      ÇÄÄÄÙ   ³
                 ³        ÌÍÍÍÍÍÍÊÍÍÍÍÍÎÍÍÍÍÍÊÍÍÍÍÍ͹       ³
                 ÀÄÄÄÄÄÄĺ            º            ÇÄÄÄÄÄÄÄÙ
                          ÈÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍͼ
                          |                         |
                          ÔÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ;


7.4  Task Gate Descriptor

A task gate descriptor provides an indirect, protected reference to a TSS.
Figure 7-4 illustrates the format of a task gate.

The SELECTOR field of a task gate must refer to a TSS descriptor. The value
of the RPL in this selector is not used by the processor.

The DPL field of a task gate controls the right to use the descriptor to
cause a task switch. A procedure may not select a task gate descriptor
unless the maximum of the selector's RPL and the CPL of the procedure is
numerically less than or equal to the DPL of the descriptor. This constraint
prevents untrusted procedures from causing a task switch. (Note that when a
task gate is used, the DPL of the target TSS descriptor is not used for
privilege checking.)

A procedure that has access to a task gate has the power to cause a task
switch, just as a procedure that has access to a TSS descriptor. The 80386
has task gates in addition to TSS descriptors to satisfy three needs:

  1.  The need for a task to have a single busy bit. Because the busy-bit
      is stored in the TSS descriptor, each task should have only one such
      descriptor. There may, however, be several task gates that select the
      single TSS descriptor.

  2.  The need to provide selective access to tasks. Task gates fulfill
      this need, because they can reside in LDTs and can have a DPL that is
      different from the TSS descriptor's DPL. A procedure that does not
      have sufficient privilege to use the TSS descriptor in the GDT (which
      usually has a DPL of 0) can still switch to another task if it has
      access to a task gate for that task in its LDT. With task gates,
      systems software can limit the right to cause task switches to
      specific tasks.

  3.  The need for an interrupt or exception to cause a task switch. Task
      gates may also reside in the IDT, making it possible for interrupts
      and exceptions to cause task switching. When interrupt or exception
      vectors to an IDT entry that contains a task gate, the 80386 switches
      to the indicated task. Thus, all tasks in the system can benefit from
      the protection afforded by isolation from interrupt tasks.

Figure 7-5 illustrates how both a task gate in an LDT and a task gate in
the IDT can identify the same task.


Figure 7-4.  Task Gate Descriptor

   31                23               15                7             0
  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÑÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
  º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³ ³     ³         ³±±±±±±±±±±±±±±±±±º
  º±±±±±±±±±±±±(NOT USED)±±±±±±±±±±±±³P³ DPL ³0 0 1 0 1³±±±(NOT USED)±±±±º 4
  º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³ ³     ³         ³±±±±±±±±±±±±±±±±±º
  ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
  º                                  ³±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±º
  º              SELECTOR            ³±±±±±±±±±±±±(NOT USED)±±±±±±±±±±±±±º 0
  º                                  ³±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±º
  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 7-5.  Task Gate Indirectly Identifies Task

         LOCAL DESCRIPTOR TABLE              INTERRUPT DESCRIPTOR TABLE
       ÕÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͸           ÕÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͸
       |                         |           |                         |
       |        TASK GATE        |           |        TASK GATE        |
       ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»           ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
       º      ³     ³     ³      º           º      ³     ³     ³      º
       ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ           ÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
    ÚÄĶ            ³            º        ÚÄĶ            ³            º
    ³  ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ        ³  ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ
    ³  |                         |        ³  |                         |
    ³  |                         |        ³  |                         |
    ³  ÔÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ;        ³  ÔÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ;
    ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿  ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                     ³  ³    GLOBAL DESCRIPTOR TABLE
                     ³  ³  ÕÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͸
                     ³  ³  |                         |
                     ³  ³  |     TASK DESCRIPTOR     |
                     ³  ³  ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
                     ³  ³  º      ³     ³     ³      º
                     ³  ÀÄÇÄÄÄÄÄÄÁÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄĶ
                     ÀÄÄÄĺ            ³            ÇÄÄ¿
                           ÈÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍͼ  ³
                           |                         |  ³
                           |                         |  ³
                           ÔÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ;  ³
                                                        ³
                           ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»  ³
                           º                         º  ³
                           º                         º  ³
                           º                         º  ³
                           º       TASK STATE        º  ³
                           º         SEGMENT         º  ³
                           º                         º  ³
                           º                         º  ³
                           º                         º  ³
                           ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼÄÙ


7.5  Task Switching

The 80386 switches execution to another task in any of four cases:

  1.  The current task executes a JMP or CALL that refers to a TSS
      descriptor.

  2.  The current task executes a JMP or CALL that refers to a task gate.

  3.  An interrupt or exception vectors to a task gate in the IDT.

  4.  The current task executes an IRET when the NT flag is set.

JMP, CALL, IRET, interrupts, and exceptions are all ordinary mechanisms of
the 80386 that can be used in circumstances that do not require a task
switch. Either the type of descriptor referenced or the NT (nested task) bit
in the flag word distinguishes between the standard mechanism and the
variant that causes a task switch.

To cause a task switch, a JMP or CALL instruction can refer either to a TSS
descriptor or to a task gate. The effect is the same in either case: the
80386 switches to the indicated task.

An exception or interrupt causes a task switch when it vectors to a task
gate in the IDT. If it vectors to an interrupt or trap gate in the IDT, a
task switch does not occur. Refer to Chapter 9 for more information on the
interrupt mechanism.

Whether invoked as a task or as a procedure of the interrupted task, an
interrupt handler always returns control to the interrupted procedure in the
interrupted task. If the NT flag is set, however, the handler is an
interrupt task, and the IRET switches back to the interrupted task.

A task switching operation involves these steps:

  1.  Checking that the current task is allowed to switch to the designated
      task. Data-access privilege rules apply in the case of JMP or CALL
      instructions. The DPL of the TSS descriptor or task gate must be less
      than or equal to the maximum of CPL and the RPL of the gate selector.
      Exceptions, interrupts, and IRETs are permitted to switch tasks
      regardless of the DPL of the target task gate or TSS descriptor.

  2.  Checking that the TSS descriptor of the new task is marked present
      and has a valid limit. Any errors up to this point occur in the
      context of the outgoing task. Errors are restartable and can be
      handled in a way that is transparent to applications procedures.

  3.  Saving the state of the current task. The processor finds the base
      address of the current TSS cached in the task register. It copies the
      registers into the current TSS (EAX, ECX, EDX, EBX, ESP, EBP, ESI,
      EDI, ES, CS, SS, DS, FS, GS, and the flag register). The EIP field of
      the TSS points to the instruction after the one that caused the task
      switch.

  4.  Loading the task register with the selector of the incoming task's
      TSS descriptor, marking the incoming task's TSS descriptor as busy,
      and setting the TS (task switched) bit of the MSW. The selector is
      either the operand of a control transfer instruction or is taken from
      a task gate.

  5.  Loading the incoming task's state from its TSS and resuming
      execution. The registers loaded are the LDT register; the flag
      register; the general registers EIP, EAX, ECX, EDX, EBX, ESP, EBP,
      ESI, EDI; the segment registers ES, CS, SS, DS, FS, and GS; and PDBR.
      Any errors detected in this step occur in the context of the incoming
      task. To an exception handler, it appears that the first instruction
      of the new task has not yet executed.

Note that the state of the outgoing task is always saved when a task switch
occurs. If execution of that task is resumed, it starts after the
instruction that caused the task switch. The registers are restored to the
values they held when the task stopped executing.

Every task switch sets the TS (task switched) bit in the MSW (machine
status word). The TS flag is useful to systems software when a coprocessor
(such as a numerics coprocessor) is present. The TS bit signals that the
context of the coprocessor may not correspond to the current 80386 task.
Chapter 11 discusses the TS bit and coprocessors in more detail.

Exception handlers that field task-switch exceptions in the incoming task
(exceptions due to tests 4 thru 16 of Table 7-1) should be cautious about
taking any action that might load the selector that caused the exception.
Such an action will probably cause another exception, unless the exception
handler first examines the selector and fixes any potential problem.

The privilege level at which execution resumes in the incoming task is
neither restricted nor affected by the privilege level at which the outgoing
task was executing. Because the tasks are isolated by their separate address
spaces and TSSs and because privilege rules can be used to prevent improper
access to a TSS, no privilege rules are needed to constrain the relation
between the CPLs of the tasks. The new task begins executing at the
privilege level indicated by the RPL of the CS selector value that is loaded
from the TSS.


Table 7-1. Checks Made during a Task Switch


Test     Test Description                   Exception
NP = Segment-not-present exception, GP = General protection fault, TS =
Invalid TSS, SF = Stack fault   Error Code Selects

  1      Incoming TSS descriptor is         NP           Incoming TSS
         present
  2      Incoming TSS descriptor is         GP           Incoming TSS
         marked not-busy
  3      Limit of incoming TSS is           TS           Incoming TSS
         greater than or equal to 103

             ÄÄ All register and selector values are loaded ÄÄ

  4      LDT selector of incoming           TS           Incoming TSS
         task is valid
  5      LDT of incoming task is            TS           Incoming TSS
         present
  6      CS selector is valid
Validity tests of a selector check that the selector is in the proper
table (eg., the LDT selector refers to the GDT), lies within the bounds of
the table, and refers to the proper type of descriptor (e.g., the LDT
selector refers to an LDT descriptor).              TS           Code segment
  7      Code segment is present            NP           Code segment
  8      Code segment DPL matches           TS           Code segment
         CS RPL
  9      Stack segment is valid
Validity tests of a selector check that the selector is in the proper
table (eg., the LDT selector refers to the GDT), lies within the bounds of
the table, and refers to the proper type of descriptor (e.g., the LDT
selector refers to an LDT descriptor).            GP           Stack segment
 10      Stack segment is present           SF           Stack segment
 11      Stack segment DPL = CPL            SF           Stack segment
 12      Stack-selector RPL = CPL           GP           Stack segment
 13      DS, ES, FS, GS selectors are       GP           Segment
         valid
Validity tests of a selector check that the selector is in the proper
table (eg., the LDT selector refers to the GDT), lies within the bounds of
the table, and refers to the proper type of descriptor (e.g., the LDT
selector refers to an LDT descriptor).


 14      DS, ES, FS, GS segments            GP           Segment
         are readable
 15      DS, ES, FS, GS segments            NP           Segment
         are present
 16      DS, ES, FS, GS segment DPL         GP           Segment
         ò CPL (unless these are
         conforming segments)


7.6  Task Linking

The back-link field of the TSS and the NT (nested task) bit of the flag
word together allow the 80386 to automatically return to a task that CALLed
another task or was interrupted by another task. When a CALL instruction, an
interrupt instruction, an external interrupt, or an exception causes a
switch to a new task, the 80386 automatically fills the back-link of the new
TSS with the selector of the outgoing task's TSS and, at the same time,
sets the NT bit in the new task's flag register. The NT flag indicates
whether the back-link field is valid. The new task releases control by
executing an IRET instruction. When interpreting an IRET, the 80386 examines
the NT flag. If NT is set, the 80386 switches back to the task selected by
the back-link field. Table 7-2 summarizes the uses of these fields.


Table 7-2. Effect of Task Switch on BUSY, NT, and Back-Link

Affected Field      Effect of JMP      Effect of            Effect of
                    Instruction        CALL Instruction     IRET Instruction

Busy bit of         Set, must be       Set, must be 0       Unchanged,
incoming task       0 before           before               must be set

Busy bit of         Cleared            Unchanged            Cleared
outgoing task                          (already set)

NT bit of           Cleared            Set                  Unchanged
incoming task

NT bit of           Unchanged          Unchanged            Cleared
outgoing task

Back-link of        Unchanged          Set to outgoing      Unchanged
incoming task                          TSS selector

Back-link of        Unchanged          Unchanged            Unchanged
outgoing task


7.6.1  Busy Bit Prevents Loops

The B-bit (busy bit) of the TSS descriptor ensures the integrity of the
back-link. A chain of back-links may grow to any length as interrupt tasks
interrupt other interrupt tasks or as called tasks call other tasks. The
busy bit ensures that the CPU can detect any attempt to create a loop. A
loop would indicate an attempt to reenter a task that is already busy;
however, the TSS is not a reentrable resource.

The processor uses the busy bit as follows:

  1.  When switching to a task, the processor automatically sets the busy
      bit of the new task.

  2.  When switching from a task, the processor automatically clears the
      busy bit of the old task if that task is not to be placed on the
      back-link chain (i.e., the instruction causing the task switch is JMP
      or IRET). If the task is placed on the back-link chain, its busy bit
      remains set.

  3.  When switching to a task, the processor signals an exception if the
      busy bit of the new task is already set.

By these actions, the processor prevents a task from switching to itself or
to any task that is on a back-link chain, thereby preventing invalid reentry
into a task.

The busy bit is effective even in multiprocessor configurations, because
the processor automatically asserts a bus lock when it sets or clears the
busy bit. This action ensures that two processors do not invoke the same
task at the same time. (Refer to Chapter 11 for more on multiprocessing.)


7.6.2  Modifying Task Linkages

Any modification of the linkage order of tasks should be accomplished only
by software that can be trusted to correctly update the back-link and the
busy-bit. Such changes may be needed to resume an interrupted task before
the task that interrupted it. Trusted software that removes a task from the
back-link chain must follow one of the following policies:

  1.  First change the back-link field in the TSS of the interrupting task,
      then clear the busy-bit in the TSS descriptor of the task removed from
      the list.

  2.  Ensure that no interrupts occur between updating the back-link chain
      and the busy bit.


7.7  Task Address Space

The LDT selector and PDBR fields of the TSS give software systems designers
flexibility in utilization of segment and page mapping features of the
80386. By appropriate choice of the segment and page mappings for each task,
tasks may share address spaces, may have address spaces that are largely
distinct from one another, or may have any degree of sharing between these
two extremes.

The ability for tasks to have distinct address spaces is an important
aspect of 80386 protection. A module in one task cannot interfere with a
module in another task if the modules do not have access to the same address
spaces. The flexible memory management features of the 80386 allow systems
designers to assign areas of shared address space to those modules of
different tasks that are designed to cooperate with each other.


7.7.1  Task Linear-to-Physical Space Mapping

The choices for arranging the linear-to-physical mappings of tasks fall
into two general classes:

  1.  One linear-to-physical mapping shared among all tasks.

      When paging is not enabled, this is the only possibility. Without page
      tables, all linear addresses map to the same physical addresses.

      When paging is enabled, this style of linear-to-physical mapping 
      results from using one page directory for all tasks. The linear space
      utilized may exceed the physical space available if the operating 
      system also implements page-level virtual memory.

  2.  Several partially overlapping linear-to-physical mappings.

      This style is implemented by using a different page directory for each
      task. Because the PDBR (page directory base register) is loaded from 
      the TSS with each task switch, each task may have a different page 
      directory.

In theory, the linear address spaces of different tasks may map to
completely distinct physical addresses. If the entries of different page
directories point to different page tables and the page tables point to
different pages of physical memory, then the tasks do not share any physical
addresses.

In practice, some portion of the linear address spaces of all tasks must
map to the same physical addresses. The task state segments must lie in a
common space so that the mapping of TSS addresses does not change while the
processor is reading and updating the TSSs during a task switch. The linear
space mapped by the GDT should also be mapped to a common physical space;
otherwise, the purpose of the GDT is defeated. Figure 7-6 shows how the
linear spaces of two tasks can overlap in the physical space by sharing
page tables.


7.7.2  Task Logical Address Space

By itself, a common linear-to-physical space mapping does not enable
sharing of data among tasks. To share data, tasks must also have a common
logical-to-linear space mapping; i.e., they must also have access to
descriptors that point into a shared linear address space. There are three
ways to create common logical-to-physical address-space mappings:

  1.  Via the GDT. All tasks have access to the descriptors in the GDT. If
      those descriptors point into a linear-address space that is mapped to
      a common physical-address space for all tasks, then the tasks can
      share data and instructions.

  2.  By sharing LDTs. Two or more tasks can use the same LDT if the LDT
      selectors in their TSSs select the same LDT segment. Those
      LDT-resident descriptors that point into a linear space that is mapped
      to a common physical space permit the tasks to share physical memory.
      This method of sharing is more selective than sharing by the GDT; the
      sharing can be limited to specific tasks. Other tasks in the system
      may have different LDTs that do not give them access to the shared
      areas.

  3.  By descriptor aliases in LDTs. It is possible for certain descriptors
      of different LDTs to point to the same linear address space. If that
      linear address space is mapped to the same physical space by the page
      mapping of the tasks involved, these descriptors permit the tasks to
      share the common space. Such descriptors are commonly called
      "aliases". This method of sharing is even more selective than the
      prior two; other descriptors in the LDTs may point to distinct linear
      addresses or to linear addresses that are not shared.


Figure 7-6. Partially-Overlapping Linear Spaces

         TSSs                                              PAGE FRAMES
                                                          ÉÍÍÍÍÍÍÍÍÍÍ»
      TASK A TSS    PAGE DIRECTORIES     PAGE TABLES      º  TASK A  º
     ÉÍÍÍÍÍÍÍÍÍÍ»     ÉÍÍÍÍÍÍÍÍÍÍÍ»     ÉÍÍÍÍÍÍÍÍÍÍÍ»  Úĺ   PAGE   º
     º          º     º           º     º           º  ³  ÈÍÍÍÍÍÍÍÍÍͼ
     º          º     ÌÍÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹  ³  ÉÍÍÍÍÍÍÍÍÍÍ»
     º          º     º           º     º    PTE    ÇÄÄÙ  º  TASK A  º
     º          º     ÌÍÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹  Úĺ   PAGE   º
     º          º     º           º     º    PTE    ÇÄÄÙ  ÈÍÍÍÍÍÍÍÍÍͼ
     ÌÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹     ÉÍÍÍÍÍÍÍÍÍÍ»
     º   PDBR   ÇÄÄÄĺ    PDE    ÇÄÄÄĺ    PTE    ÇÄÄ¿  º  TASK A  º
     ÌÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹     ÈÍÍÍÍÍÍÍÍÍÍͼ  Àĺ   PAGE   º
     º          º     º    PDE    ÇÄÄ¿    SHARED PT       ÈÍÍÍÍÍÍÍÍÍͼ
     ÈÍÍÍÍÍÍÍÍÍͼ     ÈÍÍÍÍÍÍÍÍÍÍͼ  ³  ÉÍÍÍÍÍÍÍÍÍÍÍ»     ÉÍÍÍÍÍÍÍÍÍÍ»
                                     ³  º           º     º  SHARED  º
                                     ³  ÌÍÍÍÍÍÍÍÍÍÍ͹  Úĺ   PAGE   º
                                     ³  º           º  ³  ÈÍÍÍÍÍÍÍÍÍͼ
                                     ³  ÌÍÍÍÍÍÍÍÍÍÍ͹  ³  ÉÍÍÍÍÍÍÍÍÍÍ»
                                     ³  º    PTE    ÇÄÄÙ  º  SHARED  º
                                     ³  ÌÍÍÍÍÍÍÍÍÍÍ͹  Úĺ   PAGE   º
                                     Ãĺ    PTE    ÇÄÄÙ  ÈÍÍÍÍÍÍÍÍÍͼ
      TASK B TSS                     ³  ÈÍÍÍÍÍÍÍÍÍÍͼ     ÉÍÍÍÍÍÍÍÍÍÍ»
     ÉÍÍÍÍÍÍÍÍÍÍ»     ÉÍÍÍÍÍÍÍÍÍÍÍ»  ³                    º  TASK B  º
     º          º     º           º  ³                ÚÄĺ   PAGE   º
     º          º     ÌÍÍÍÍÍÍÍÍÍÍ͹  ³  ÉÍÍÍÍÍÍÍÍÍÍÍ» ³   ÈÍÍÍÍÍÍÍÍÍͼ
     º          º     º           º  ³  º           º ³   ÉÍÍÍÍÍÍÍÍÍÍ»
     º          º     ÌÍÍÍÍÍÍÍÍÍÍ͹  ³  ÌÍÍÍÍÍÍÍÍÍÍ͹ ³   º  TASK B  º
     º          º     º           º  ³  º           º ³ Úº   PAGE   º
     ÌÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹  ³  ÌÍÍÍÍÍÍÍÍÍÍ͹ ³ ³ ÈÍÍÍÍÍÍÍÍÍͼ
     º   PDBR   ÇÄÄÄĺ    PDE    ÇÄÄÙ  º    PTE    ÇÄÙ ³  PAGE FRAMES
     ÌÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹     ÌÍÍÍÍÍÍÍÍÍÍ͹   ³
     º          º     º    PDE    ÇÄÄÄĺ    PTE    ÇÄÄÄÙ
     ÈÍÍÍÍÍÍÍÍÍͼ     ÈÍÍÍÍÍÍÍÍÍÍͼ     ÈÍÍÍÍÍÍÍÍÍÍͼ
         TSSs       PAGE DIRECTORIES     PAGE TABLES


Chapter 8  Input/Output

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

This chapter presents the I/O features of the 80386 from the following
perspectives:

  þ  Methods of addressing I/O ports

  þ  Instructions that cause I/O operations

  þ  Protection as it applies to the use of I/O instructions and I/O port
     addresses.


8.1  I/O Addressing

The 80386 allows input/output to be performed in either of two ways:

  þ  By means of a separate I/O address space (using specific I/O
     instructions)

  þ  By means of memory-mapped I/O (using general-purpose operand
     manipulationinstructions).


8.1.1  I/O Address Space

The 80386 provides a separate I/O address space, distinct from physical
memory, that can be used to address the input/output ports that are used for
external 16 devices. The I/O address space consists of 2^(16) (64K)
individually addressable 8-bit ports; any two consecutive 8-bit ports can be
treated as a 16-bit port; and four consecutive 8-bit ports can be treated
as a 32-bit port. Thus, the I/O address space can accommodate up to 64K
8-bit ports, up to 32K 16-bit ports, or up to 16K 32-bit ports.

The program can specify the address of the port in two ways. Using an
immediate byte constant, the program can specify:

  þ  256 8-bit ports numbered 0 through 255.
  þ  128 16-bit ports numbered 0, 2, 4, . . . , 252, 254.
  þ  64 32-bit ports numbered 0, 4, 8, . . . , 248, 252.

Using a value in DX, the program can specify:

  þ  8-bit ports numbered 0 through 65535
  þ  16-bit ports numbered 0, 2, 4, . . . , 65532, 65534
  þ  32-bit ports numbered 0, 4, 8, . . . , 65528, 65532

The 80386 can transfer 32, 16, or 8 bits at a time to a device located in
the I/O space. Like doublewords in memory, 32-bit ports should be aligned at
addresses evenly divisible by four so that the 32 bits can be transferred in
a single bus access. Like words in memory, 16-bit ports should be aligned at
even-numbered addresses so that the 16 bits can be transferred in a single
bus access. An 8-bit port may be located at either an even or odd address.

The instructions IN and OUT move data between a register and a port in the
I/O address space. The instructions INS and OUTS move strings of data
between the memory address space and ports in the I/O address space.


8.1.2  Memory-Mapped I/O

I/O devices also may be placed in the 80386 memory address space. As long
as the devices respond like memory components, they are indistinguishable to
the processor.

Memory-mapped I/O provides additional programming flexibility. Any
instruction that references memory may be used to access an I/O port located
in the memory space. For example, the MOV instruction can transfer data
between any register and a port; and the AND, OR, and TEST instructions may
be used to manipulate bits in the internal registers of a device (see Figure
8-1). Memory-mapped I/O performed via the full instruction set maintains
the full complement of addressing modes for selecting the desired I/O
device (e.g., direct address, indirect address, base register, index
register, scaling).

Memory-mapped I/O, like any other memory reference, is subject to access
protection and control when executing in protected mode. Refer to Chapter 6
for a discussion of memory protection.


8.2  I/O Instructions

The I/O instructions of the 80386 provide access to the processor's I/O
ports for the transfer of data to and from peripheral devices. These
instructions have as one operand the address of a port in the I/O address
space. There are two classes of I/O instruction:

  1.  Those that transfer a single item (byte, word, or doubleword) located
      in a register.

  2.  Those that transfer strings of items (strings of bytes, words, or
      doublewords) located in memory. These are known as "string I/O
      instructions" or "block I/O instructions".


8.2.1  Register I/O Instructions

The I/O instructions IN and OUT are provided to move data between I/O ports
and the EAX (32-bit I/O), the AX (16-bit I/O), or AL (8-bit I/O) general
registers. IN and OUT instructions address I/O ports either directly, with
the address of one of up to 256 port addresses coded in the instruction, or
indirectly via the DX register to one of up to 64K port addresses.

IN (Input from Port) transfers a byte, word, or doubleword from an input
port to AL, AX, or EAX. If a program specifies AL with the IN instruction,
the processor transfers 8 bits from the selected port to AL. If a program
specifies AX with the IN instruction, the processor transfers 16 bits from
the port to AX. If a program specifies EAX with the IN instruction, the
processor transfers 32 bits from the port to EAX.

OUT (Output to Port) transfers a byte, word, or doubleword to an output
port from AL, AX, or EAX. The program can specify the number of the port
using the same methods as the IN instruction.


Figure 8-1.  Memory-Mapped I/O

                MEMORY
             ADDRESS SPACE                        I/O DEVICE 1
           ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»                  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
           º               º                  º INTERNAL REGISTER º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ Ä Ä Ä Ä Ä Ä Ä Ä ÄºÄÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ» º
           º               º                  º º               º º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ Ä Ä Ä Ä Ä Ä Ä Ä ÄºÄÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ º
           º               º                  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
           º               º
           º               º
           º               º
           º               º                      I/O DEVICE 2
           º               º                  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
           º               º                  º INTERNAL REGISTER º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ Ä Ä Ä Ä Ä Ä Ä Ä ÄºÄÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ» º
           º               º                  º º               º º
           ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ Ä Ä Ä Ä Ä Ä Ä Ä ÄºÄÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ º
           º               º                  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
           ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


8.2.2  Block I/O Instructions

The block (or string) I/O instructions INS and OUTS move blocks of data
between I/O ports and memory space. Block I/O instructions use the DX
register to specify the address of a port in the I/O address space. INS and
OUTS use DX to specify:

  þ  8-bit ports numbered 0 through 65535
  þ  16-bit ports numbered 0, 2, 4, . . . , 65532, 65534
  þ  32-bit ports numbered 0, 4, 8, . . . , 65528, 65532

Block I/O instructions use either SI or DI to designate the source or
destination memory address. For each transfer, SI or DI are automatically
either incremented or decremented as specified by the direction bit in the
flags register.

INS and OUTS, when used with repeat prefixes, cause block input or output
operations. REP, the repeat prefix, modifies INS and OUTS to provide a means
of transferring blocks of data between an I/O port and memory. These block
I/O instructions are string primitives (refer also to Chapter 3 for more on
string primitives). They simplify programming and increase the speed of data
transfer by eliminating the need to use a separate LOOP instruction or an
intermediate register to hold the data.

The string I/O primitives can operate on byte strings, word strings, or
doubleword strings. After each transfer, the memory address in ESI or EDI is
updated by 1 for byte operands, by 2 for word operands, or by 4 for
doubleword operands. The value in the direction flag (DF) determines whether
the processor automatically increments ESI or EDI (DF=0) or whether it
automatically decrements these registers (DF=1).

INS (Input String from Port) transfers a byte or a word string element from
an input port to memory. The mnemonics INSB, INSW, and INSD are variants
that explicitly specify the size of the operand. If a program specifies
INSB, the processor transfers 8 bits from the selected port to the memory
location indicated by ES:EDI. If a program specifies INSW, the processor
transfers 16 bits from the port to the memory location indicated by ES:EDI.
If a program specifies INSD, the processor transfers 32 bits from the port
to the memory location indicated by ES:EDI. The destination segment register
choice (ES) cannot be changed for the INS instruction. Combined with the REP
prefix, INS moves a block of information from an input port to a series of
consecutive memory locations.

OUTS (Output String to Port) transfers a byte, word, or doubleword string
element to an output port from memory. The mnemonics OUTSB, OUTSW, and OUTSD
are variants that explicitly specify the size of the operand. If a program
specifies OUTSB, the processor transfers 8 bits from the memory location
indicated by ES:EDI to the the selected port. If a program specifies OUTSW,
the processor transfers 16 bits from the memory location indicated by ES:EDI
to the the selected port. If a program specifies OUTSD, the processor
transfers 32 bits from the memory location indicated by ES:EDI to the the
selected port. Combined with the REP prefix, OUTS moves a block of
information from a series of consecutive memory locations indicated by
DS:ESI to an output port.


8.3  Protection and I/O

Two mechanisms provide protection for I/O functions:

  1.  The IOPL field in the EFLAGS register defines the right to use
      I/O-related instructions.

  2.  The I/O permission bit map of a 80386 TSS segment defines the right
      to use ports in the I/O address space.

These mechanisms operate only in protected mode, including virtual 8086
mode; they do not operate in real mode. In real mode, there is no protection
of the I/O space; any procedure can execute I/O instructions, and any I/O
port can be addressed by the I/O instructions.


8.3.1  I/O Privilege Level

Instructions that deal with I/O need to be restricted but also need to be
executed by procedures executing at privilege levels other than zero. For
this reason, the processor uses two bits of the flags register to store the
I/O privilege level (IOPL). The IOPL defines the privilege level
needed to execute I/O-related instructions.

The following instructions can be executed only if CPL ó IOPL:

IN      ÄÄ Input
INS     ÄÄ Input String
OUT     ÄÄ Output
OUTS    ÄÄ Output String
CLI     ÄÄ Clear Interrupt-Enable Flag
STI     ÄÄ Set Interrupt-Enable

These instructions are called "sensitive" instructions, because they are
sensitive to IOPL.

To use sensitive instructions, a procedure must execute at a privilege
level at least as privileged as that specified by the IOPL (CPL ó IOPL). Any
attempt by a less privileged procedure to use a sensitive instruction
results in a general protection exception.

Because each task has its own unique copy of the flags register, each task
can have a different IOPL. A task whose primary function is to perform I/O
(a device driver) can benefit from having an IOPL of three, thereby
permitting all procedures of the task to performI/O. Other tasks typically
have IOPL set to zero or one, reserving the right to perform I/O
instructions for the most privileged procedures.

A task can change IOPL only with the POPF instruction; however, such
changes are privileged. No procedure may alter IOPL (the I/O privilege level
in the flag register) unless the procedure is executing at privilege level
0. An attempt by a less privileged procedure to alter IOPL does not result
in an exception; IOPL simply remains unaltered.

The POPF instruction may be used in addition to CLI and STI to alter the
interrupt-enable flag (IF); however, changes to IF by POPF are
IOPL-sensitive. A procedure may alter IF with a POPF instruction only when
executing at a level that is at least as privileged as IOPL. An attempt by a
less privileged procedure to alter IF in this manner does not result in an
exception; IF simply remains unaltered.


8.3.2  I/O Permission Bit Map

The I/O instructions that directly refer to addresses in the processor's
I/O space are IN, INS, OUT, OUTS. The 80386 has the ability to selectively
trap references to specific I/O addresses. The structure that enables
selective trapping is the I/O Permission Bit Map in the TSS segment (see
Figure 8-2). The I/O permission map is a bit vector. The size of the map
and its location in the TSS segment are variable. The processor locates the
I/O permission map by means of the I/O map base field in the fixed portion
of the TSS. The I/O map base field is 16 bits wide and contains the offset
of the beginning of the I/O permission map. The upper limit of the I/O
permission map is the same as the limit of the TSS segment.

In protected mode, when it encounters an I/O instruction (IN, INS, OUT, or
OUTS), the processor first checks whether CPL ó IOPL. If this condition is
true, the I/O operation may proceed. If not true, the processor checks the
I/O permission map. (In virtual 8086 mode, the processor consults the map
without regard for IOPL. Refer to Chapter 15.)

Each bit in the map corresponds to an I/O port byte address; for example,
the bit for port 41 is found at I/O map base + 5, bit offset 1. The
processor tests all the bits that correspond to the I/O addresses spanned by
an I/O operation; for example, a doubleword operation tests four bits
corresponding to four adjacent byte addresses. If any tested bit is set,
the processor signals a general protection exception. If all the tested bits
are zero, the I/O operation may proceed.

It is not necessary for the I/O permission map to represent all the I/O
addresses. I/O addresses not spanned by the map are treated as if they had
one bits in the map. For example, if TSS limit is equal to I/O map base +
31, the first 256 I/O ports are mapped; I/O operations on any port greater
than 255 cause an exception.

If I/O map base is greater than or equal to TSS limit, the TSS segment has
no I/O permission map, and all I/O instructions in the 80386 program cause
exceptions when CPL > IOPL.

Because the I/O permission map is in the TSS segment, different tasks can
have different maps. Thus, the operating system can allocate ports to a task
by changing the I/O permission map in the task's TSS.


Figure 8-2.  I/O Address Bit Map

                                    TSS SEGMEMT

                        31       23       15       7      0
                       ÉÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍ»
              LIMITÄÄĺ                                   º
                       º Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä º
                                                          
                             I/O PERMISSION BIT MAP       
                                                          
                       º Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä º
                 ÚÄÄÄĺ                                   º
                 ³     ÇÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄĶ
                 ³                                        
                 ³                                        
                 ³                                        
                 ³     ÇÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄĶ
                 ÀÄÄÄÄĶ  I/O MAP BASE   ³uuuuuuuu uuuuuuuTº64
                       ÇÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄĶ
                       º00000000 00000000³       LOT       º60
                       ÇÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄĶ
                       º00000000 00000000³       GS        º5C
                       ÇÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄĶ
                       º                                   º58
                                                          
                                                          
                                                          
                       º                                   º4
                       ÇÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄĶ
                       º00000000 00000000³  TSS BACK LINK  º0
                       ÈÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍͼ


Chapter 9  Exceptions and Interrupts

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Interrupts and exceptions are special kinds of control transfer; they work
somewhat like unprogrammed CALLs. They alter the normal program flow to
handle external events or to report errors or exceptional conditions. The
difference between interrupts and exceptions is that interrupts are used to
handle asynchronous events external to the processor, but exceptions handle
conditions detected by the processor itself in the course of executing
instructions.

There are two sources for external interrupts and two sources for
exceptions:

  1.  Interrupts

      þ  Maskable interrupts, which are signalled via the INTR pin.

      þ  Nonmaskable interrupts, which are signalled via the NMI
         (Non-Maskable Interrupt) pin.

  2.  Exceptions

      þ  Processor detected. These are further classified as faults, traps,
         and aborts.

      þ  Programmed. The instructions INTO, INT 3, INT n, and BOUND can
         trigger exceptions. These instructions are often called "software
         interrupts", but the processor handles them as exceptions.

This chapter explains the features that the 80386 offers for controlling
and responding to interrupts when it is executing in protected mode.


9.1  Identifying Interrupts

The processor associates an identifying number with each different type of
interrupt or exception.

The NMI and the exceptions recognized by the processor are assigned
predetermined identifiers in the range 0 through 31. Not all of these
numbers are currently used by the 80386; unassigned identifiers in this
range are reserved by Intel for possible future expansion.

The identifiers of the maskable interrupts are determined by external
interrupt controllers (such as Intel's 8259A Programmable Interrupt
Controller) and communicated to the processor during the processor's
interrupt-acknowledge sequence. The numbers assigned by an 8259A PIC can be
specified by software. Any numbers in the range 32 through 255 can be used.
Table 9-1 shows the assignment of interrupt and exception identifiers.

Exceptions are classified as faults, traps, or aborts depending on the way
they are reported and whether restart of the instruction that caused the
exception is supported.

Faults  Faults are exceptions that are reported "before" the
        instruction causingthe exception. Faults are either detected before
        the instruction begins to execute, or during execution of the
        instruction. If detected during the instruction, the fault is
        reported with the machine restored to a state that permits the
        instruction to be restarted.

Traps   A trap is an exception that is reported at the instruction
        boundary immediately after the instruction in which the
        exception was detected.

Aborts  An abort is an exception that permits neither precise location
        of the instruction causing the exception nor restart of the program
        that caused the exception. Aborts are used to report severe errors,
        such as hardware errors and inconsistent or illegal values in system
        tables.


Table 9-1. Interrupt and Exception ID Assignments

Identifier   Description

0            Divide error
1            Debug exceptions
2            Nonmaskable interrupt
3            Breakpoint (one-byte INT 3 instruction)
4            Overflow (INTO instruction)
5            Bounds check (BOUND instruction)
6            Invalid opcode
7            Coprocessor not available
8            Double fault
9            (reserved)
10           Invalid TSS
11           Segment not present
12           Stack exception
13           General protection
14           Page fault
15           (reserved)
16           Coprecessor error
17-31        (reserved)
32-255       Available for external interrupts via INTR pin


9.2  Enabling and Disabling Interrupts

The processor services interrupts and exceptions only between the end of
one instruction and the beginning of the next. When the repeat prefix is
used to repeat a string instruction, interrupts and exceptions may occur
between repetitions. Thus, operations on long strings do not delay interrupt
response.

Certain conditions and flag settings cause the processor to inhibit certain
interrupts and exceptions at instruction boundaries.


9.2.1  NMI Masks Further NMIs

While an NMI handler is executing, the processor ignores further interrupt
signals at the NMI pin until the next IRET instruction is executed.


9.2.2  IF Masks INTR

The IF (interrupt-enable flag) controls the acceptance of external
interrupts signalled via the INTR pin. When IF=0, INTR interrupts are
inhibited; when IF=1, INTR interrupts are enabled. As with the other flag
bits, the processor clears IF in response to a RESET signal. The
instructions CLI and STI alter the setting of IF.

CLI (Clear Interrupt-Enable Flag) and STI (Set Interrupt-Enable Flag)
explicitly alter IF (bit 9 in the flag register). These instructions may be
executed only if CPL ó IOPL. A protection exception occurs if they are
executed when CPL > IOPL.

The IF is also affected implicitly by the following operations:

  þ  The instruction PUSHF stores all flags, including IF, in the stack
     where they can be examined.

  þ  Task switches and the instructions POPF and IRET load the flags
     register; therefore, they can be used to modify IF.

  þ  Interrupts through interrupt gates automatically reset IF, disabling
     interrupts. (Interrupt gates are explained later in this chapter.)


9.2.3  RF Masks Debug Faults

The RF bit in EFLAGS controls the recognition of debug faults. This permits
debug faults to be raised for a given instruction at most once, no matter
how many times the instruction is restarted. (Refer to Chapter 12 for more
information on debugging.)


9.2.4  MOV or POP to SS Masks Some Interrupts and Exceptions

Software that needs to change stack segments often uses a pair of
instructions; for example:

  MOV SS, AX
  MOV ESP, StackTop

If an interrupt or exception is processed after SS has been changed but
before ESP has received the corresponding change, the two parts of the stack
pointer SS:ESP are inconsistent for the duration of the interrupt handler or
exception handler.

To prevent this situation, the 80386, after both a MOV to SS and a POP to
SS instruction, inhibits NMI, INTR, debug exceptions, and single-step traps
at the instruction boundary following the instruction that changes SS. Some
exceptions may still occur; namely, page fault and general protection fault.
Always use the 80386 LSS instruction, and the problem will not occur.


9.3  Priority Among Simultaneous Interrupts and Exceptions

If more than one interrupt or exception is pending at an instruction
boundary, the processor services one of them at a time. The priority among
classes of interrupt and exception sources is shown in Table 9-2. The
processor first services a pending interrupt or exception from the class
that has the highest priority, transferring control to the first
instruction of the interrupt handler. Lower priority exceptions are
discarded; lower priority interrupts are held pending. Discarded exceptions
will be rediscovered when the interrupt handler returns control to the point
of interruption.


9.4  Interrupt Descriptor Table

The interrupt descriptor table (IDT) associates each interrupt or exception
identifier with a descriptor for the instructions that service the
associated event. Like the GDT and LDTs, the IDT is an array of 8-byte
descriptors. Unlike the GDT and LDTs, the first entry of the IDT may contain
a descriptor. To form an index into the IDT, the processor multiplies the
interrupt or exception identifier by eight. Because there are only 256
identifiers, the IDT need not contain more than 256 descriptors. It can
contain fewer than 256 entries; entries are required only for interrupt
identifiers that are actually used.

The IDT may reside anywhere in physical memory. As Figure 9-1 shows, the
processor locates the IDT by means of the IDT register (IDTR). The
instructions LIDT and SIDT operate on the IDTR. Both instructions have one
explicit operand: the address in memory of a 6-byte area. Figure 9-2 shows
the format of this area.

LIDT (Load IDT register) loads the IDT register with the linear base
address and limit values contained in the memory operand.  This instruction
can be executed only when the CPL is zero. It is normally used by the
initialization logic of an operating system when creating an IDT.  An
operating system may also use it to change from one IDT to another.

SIDT (Store IDT register) copies the base and limit value stored in IDTR
to a memory location. This instruction can be executed at any privilege
level.


Table 9-2. Priority Among Simultaneous Interrupts and Exceptions

Priority   Class of Interrupt or Exception

HIGHEST    Faults except debug faults
           Trap instructions INTO, INT n, INT 3
           Debug traps for this instruction
           Debug faults for next instruction
           NMI interrupt
LOWEST     INTR interrupt


Figure 9-1.  IDT Register and Table

                                              INTERRUPT DESCRIPTOR TABLE
                                              ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
                                        ÚÄÄÄĺ      ³     ³     ³      º
                                        ³     ÇÄ GATE FOR INTERRUPT #N Ķ
                                        ³     º      ³     ³     ³      º
                                        ³     ÈÍÍÍÍÍÍÏÍÍÍÍÍÏÍÍÍÍÍÏÍÍÍÍÍͼ
                                        ³                              
                                        ³                              
                                        ³                              
                                        ³     ÉÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍ»
                                        ³     º      ³     ³     ³      º
                                        ³     ÇÄ GATE FOR INTERRUPT #2 Ķ
                                        ³     º      ³     ³     ³      º
                                        ³     ÌÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍ͹
            IDT REGISTER                ³     º      ³     ³     ³      º
                                        ³     ÇÄ GATE FOR INTERRUPT #1 Ķ
                    15            0     ³     º      ³     ³     ³      º
                   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»    ³     ÌÍÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍ͹
                   º   IDT LIMIT   ÇÄÄÄÄÙ     º      ³     ³     ³      º
  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹          ÇÄ GATE FOR INTERRUPT #0 Ķ
  º            IDT BASE            ÇÄÄÄÄÄÄÄÄĺ      ³     ³     ³      º
  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ          ÈÍÍÍÍÍÍÏÍÍÍÍÍÏÍÍÍÍÍÏÍÍÍÍÍͼ
   31                             0


Figure 9-2.  Pseudo-Descriptor Format for LIDT and SIDT

  31                23                15                7               0
 ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 º                                 BASE                                  º2
 ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                                     º               LIMIT               º0
                                     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


9.5  IDT Descriptors

The IDT may contain any of three kinds of descriptor:

   þ  Task gates
   þ  Interrupt gates
   þ  Trap gates

Figure 9-3 illustrates the format of task gates and 80386 interrupt gates
and trap gates. (The task gate in an IDT is the same as the task gate
already discussed in Chapter 7.)


Figure 9-3.  80306 IDT Gate Descriptors

                                80386 TASK GATE
   31                23                15                7                0
  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÑÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
  º±±±±±±±±±±±±±(NOT USED)±±±±±±±±±±±±³ P ³DPL³0 0 1 0 1³±±±(NOT USED)±±±±º4
  ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
  º             SELECTOR              ³±±±±±±±±±±±±±(NOT USED)±±±±±±±±±±±±º0
  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

                                80386 INTERRUPT GATE
   31                23                15                7                0
  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÑÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍ»
  º           OFFSET 31..16           ³ P ³DPL³0 1 1 1 0³0 0 0³(NOT USED) º4
  ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄĶ
  º             SELECTOR              ³           OFFSET 15..0            º0
  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

                                80386 TRAP GATE
   31                23                15                7                0
  ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÑÍÍÍÑÍÍÍÍÍÍÍÍÍØÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍ»
  º          OFFSET 31..16            ³ P ³DPL³0 1 1 1 1³0 0 0³(NOT USED) º4
  ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄĶ
  º             SELECTOR              ³           OFFSET 15..0            º0
  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


9.6  Interrupt Tasks and Interrupt Procedures

Just as a CALL instruction can call either a procedure or a task, so an
interrupt or exception can "call" an interrupt handler that is either a
procedure or a task. When responding to an interrupt or exception, the
processor uses the interrupt or exception identifier to index a descriptor
in the IDT. If the processor indexes to an interrupt gate or trap gate, it
invokes the handler in a manner similar to a CALL to a call gate. If the
processor finds a task gate, it causes a task switch in a manner similar to
a CALL to a task gate.


9.6.1  Interrupt Procedures

An interrupt gate or trap gate points indirectly to a procedure which will
execute in the context of the currently executing task as illustrated by
Figure 9-4. The selector of the gate points to an executable-segment
descriptor in either the GDT or the current LDT. The offset field of the
gate points to the beginning of the interrupt or exception handling
procedure.

The 80386 invokes an interrupt or exception handling procedure in much the
same manner as it CALLs a procedure; the differences are explained in the
following sections.


Figure 9-4.  Interrupt Vectoring for Procedures

                  IDT                                    EXECUTABLE SEGMENT
           ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»                             ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
           º               º                       OFFSETº               º
           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĺ ENTRY POINT   º
           º               º  ³      LDT OR GDT          º               º
           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ³   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»      º               º
           º               º  ³   º               º      º               º
INTERRUPT  ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ³   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹      º               º
   IDÄÄÄÄĺ TRAP GATE OR  ÇÄÄÙ   º               º      º               º
           ºINTERRUPT GATE ÇÄÄ¿   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹      º               º
           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ³   º               º      º               º
           º               º  ³   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹      º               º
           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹  ÀÄĺ   SEGMENT     ÇÄ¿    º               º
           º               º      º  DESCRIPTOR   º ³    º               º
           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹      ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ ³    º               º
           º               º      º               º ³    º               º
           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹      ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ ³    º               º
           º               º      º               º ³BASEº               º
           ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ      ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ ÀÄÄÄÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                                  º               º
                                  º               º
                                  º               º
                                  ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


9.6.1.1  Stack of Interrupt Procedure

Just as with a control transfer due to a CALL instruction, a control
transfer to an interrupt or exception handling procedure uses the stack to
store the information needed for returning to the original procedure. As
Figure 9-5 shows, an interrupt pushes the EFLAGS register onto the stack
before the pointer to the interrupted instruction.

Certain types of exceptions also cause an error code to be pushed on the
stack. An exception handler can use the error code to help diagnose the
exception.


9.6.1.2  Returning from an Interrupt Procedure

An interrupt procedure also differs from a normal procedure in the method
of leaving the procedure. The IRET instruction is used to exit from an
interrupt procedure. IRET is similar to RET except that IRET increments EIP
by an extra four bytes (because of the flags on the stack) and moves the
saved flags into the EFLAGS register. The IOPL field of EFLAGS is changed
only if the CPL is zero. The IF flag is changed only if CPL ó IOPL.


Figure 9-5.  Stack Layout after Exception of Interrupt 

                           WITHOUT PRIVILEGE TRANSITION

      D  O      31          0                     31          0
      I  F    ÌÍÍÍÍÍÍÍËÍÍÍÍÍÍ͹                 ÌÍÍÍÍÍÍÍËÍÍÍÍÍÍ͹
      R       º±±±±±±±º±±±±±±±º    OLD          º±±±±±±±º±±±±±±±º    OLD
      E  E    ÌÍÍÍÍÍÍÍÎÍÍÍÍÍÍ͹   SS:ESP        ÌÍÍÍÍÍÍÍÎÍÍÍÍÍÍ͹   SS:ESP
      C  X    º±±±±±±±º±±±±±±±º     ³           º±±±±±±±º±±±±±±±º     ³
      T  P    ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹ÄÄÄÄÙ           ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹ÄÄÄÄÙ
      I  A    º  OLD EFLAGS   º                 º  OLD EFLAGS   º
      O  N    ÌÍÍÍÍÍÍÍËÍÍÍÍÍÍ͹                 ÌÍÍÍÍÍÍÍËÍÍÍÍÍÍ͹
      N  S    º±±±±±±±ºOLD CS º    NEW          º±±±±±±±ºOLD CS º
         I    ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹   SS:ESP        ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹
       ³ O    º    OLD EIP    º     ³           º    OLD EIP    º    NEW
       ³ N    ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ÄÄÄÄÙ           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹   SS:ESP
       ³      º               º                 º  ERROR CODE   º     ³
                                             ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ÄÄÄÄÙ
                                              º               º
                             
              WITHOUT ERROR CODE                 WITH ERROR CODE

                             WITH PRIVILEGE TRANSITION

      D  O     31            0                     31          0
      I  F    ÉÍÍÍÍÍÍÍËÍÍÍÍÍÍÍ»ÄÄÄÄ¿           ÉÍÍÍÍÍÍÍËÍÍÍÍÍÍÍ»ÄÄÄÄ¿
      R       º±±±±±±±ºOLD SS º     ³           º±±±±±±±ºOLD SS º     ³
      E  E    ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹   SS:ESP        ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹   SS:ESP
      C  X    º    OLD ESP    º  FROM TSS       º    OLD ESP    º  FROM TSS
      T  P    ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹                 ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
      I  A    º  OLD EFLAGS   º                 º  OLD EFLAGS   º
      O  N    ÌÍÍÍÍÍÍÍËÍÍÍÍÍÍ͹                 ÌÍÍÍÍÍÍÍËÍÍÍÍÍÍ͹
      N  S    º±±±±±±±ºOLD CS º    NEW          º±±±±±±±ºOLD CS º
         I    ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹   SS:EIP        ÌÍÍÍÍÍÍÍÊÍÍÍÍÍÍ͹
       ³ O    º    OLD EIP    º     ³           º    OLD EIP    º    NEW
       ³ N    ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ÄÄÄÄÙ           ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹   SS:ESP
       ³      º               º                 º  ERROR CODE   º     ³
                                             ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹ÄÄÄÄÙ
                                              º               º
                             
              WITHOUT ERROR CODE                 WITH ERROR CODE


9.6.1.3  Flags Usage by Interrupt Procedure

Interrupts that vector through either interrupt gates or trap gates cause
TF (the trap flag) to be reset after the current value of TF is saved on the
stack as part of EFLAGS. By this action the processor prevents debugging
activity that uses single-stepping from affecting interrupt response. A
subsequent IRET instruction restores TF to the value in the EFLAGS image on
the stack.

The difference between an interrupt gate and a trap gate is in the effect
on IF (the interrupt-enable flag). An interrupt that vectors through an
interrupt gate resets IF, thereby preventing other interrupts from
interfering with the current interrupt handler. A subsequent IRET
instruction restores IF to the value in the EFLAGS image on the stack. An
interrupt through a trap gate does not change IF.


9.6.1.4  Protection in Interrupt Procedures

The privilege rule that governs interrupt procedures is similar to that for
procedure calls: the CPU does not permit an interrupt to transfer control to
a procedure in a segment of lesser privilege (numerically greater privilege
level) than the current privilege level. An attempt to violate this rule
results in a general protection exception.

Because occurrence of interrupts is not generally predictable, this
privilege rule effectively imposes restrictions on the privilege levels at
which interrupt and exception handling procedures can execute. Either of the
following strategies can be employed to ensure that the privilege rule is
never violated.

  þ  Place the handler in a conforming segment. This strategy suits the
     handlers for certain exceptions (divide error, for example). Such a
     handler must use only the data available to it from the stack. If it
     needed data from a data segment, the data segment would have to have
     privilege level three, thereby making it unprotected.

  þ  Place the handler procedure in a privilege level zero segment.


9.6.2  Interrupt Tasks

A task gate in the IDT points indirectly to a task, as Figure 9-6
illustrates. The selector of the gate points to a TSS descriptor in the GDT.

When an interrupt or exception vectors to a task gate in the IDT, a task
switch results. Handling an interrupt with a separate task offers two
advantages:

  þ  The entire context is saved automatically.

  þ  The interrupt handler can be isolated from other tasks by giving it a
     separate address space, either via its LDT or via its page directory.

The actions that the processor takes to perform a task switch are discussed
in Chapter 7. The interrupt task returns to the interrupted task by
executing an IRET instruction.

If the task switch is caused by an exception that has an error code, the
processor automatically pushes the error code onto the stack that
corresponds to the privilege level of the first instruction to be executed
in the interrupt task.

When interrupt tasks are used in an operating system for the 80386, there
are actually two schedulers: the software scheduler (part of the operating
system) and the hardware scheduler (part of the processor's interrupt
mechanism). The design of the software scheduler should account for the fact
that the hardware scheduler may dispatch an interrupt task whenever
interrupts are enabled.


Figure 9-6.  Interrupt Vectoring for Tasks

            IDT                       GDT
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»        ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º                º        º                º              TSS
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ        ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ       ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º                º        º                º       º                º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ        ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ       º                º
     º                º        º                º       º                º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ        ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ       º                º
 ÚÄĺ   TASK GATE    ÇÄÄÄ¿    º                º       º                º
 ³   ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ   ³    ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ       º                º
 ³   º                º   ÀÄÄĺ TSS DESCRIPTOR ÇÄÄÄ¿   º                º
 ³   ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ        ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ   ³   º                º
 ³   º                º        º                º   ³   º                º
 ³   ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ        ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ   ÀÄÄÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
 ³   º                º        º                º
 ³   ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ        ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 ³   º                º        º                º
 ³   ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ        ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
 ³
 ÀÄINTERRUPT ID


9.7  Error Code

With exceptions that relate to a specific segment, the processor pushes an
error code onto the stack of the exception handler (whether procedure or
task). The error code has the format shown in Figure 9-7. The format of the
error code resembles that of a selector; however, instead of an RPL field,
the error code contains two one-bit items:

  1.  The processor sets the EXT bit if an event external to the program
      caused the exception.

  2.  The processor sets the I-bit (IDT-bit) if the index portion of the
      error code refers to a gate descriptor in the IDT.

If the I-bit is not set, the TI bit indicates whether the error code refers
to the GDT (value 0) or to the LDT (value 1). The remaining 14 bits are the
upper 14 bits of the segment selector involved. In some cases the error code
on the stack is null, i.e., all bits in the low-order word are zero.


Figure 9-7.  Error Code Format

       31              15                                         2 1 0
      ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÑÍÑÍÑÍ»
      º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                         ³T³ ³Eº
      º±±±±±±±±±±±UNDEFINED±±±±±±±±±±±±³     SELECTOR INDEX      ³ ³I³ º
      º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³                         ³I³ ³Xº
      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÏÍÏÍÏͼ


9.8  Exception Conditions

The following sections describe each of the possible exception conditions
in detail. Each description classifies the exception as a fault, trap, or
abort. This classification provides information needed by systems
programmers for restarting the procedure in which the exception occurred:

Faults   The CS and EIP values saved when a fault is reported point to the
         instruction causing the fault.

Traps    The CS and EIP values stored when the trap is reported point to the
         instruction dynamically after the instruction causing the trap. If
         a trap is detected during an instruction that alters program flow,
         the reported values of CS and EIP reflect the alteration of program
         flow. For example, if a trap is detected in a JMP instruction, the
         CS and EIP values pushed onto the stack point to the target of the
         JMP, not to the instruction after the JMP.

Aborts   An abort is an exception that permits neither precise location of
         the instruction causing the exception nor restart of the program
         that caused the exception. Aborts are used to report severe errors,
         such as hardware errors and inconsistent or illegal values in
         system tables.


9.8.1  Interrupt 0 ÄÄ Divide Error

The divide-error fault occurs during a DIV or an IDIV instruction when the
divisor is zero.


9.8.2  Interrupt 1 ÄÄ Debug Exceptions

The processor triggers this interrupt for any of a number of conditions;
whether the exception is a fault or a trap depends on the condition:

   þ  Instruction address breakpoint fault.
   þ  Data address breakpoint trap.
   þ  General detect fault.
   þ  Single-step trap.
   þ  Task-switch breakpoint trap.

The processor does not push an error code for this exception. An exception
handler can examine the debug registers to determine which condition caused
the exception. Refer to Chapter 12 for more detailed information about
debugging and the debug registers.


9.8.3  Interrupt 3 ÄÄ Breakpoint

The INT 3 instruction causes this trap. The INT 3 instruction is one byte
long, which makes it easy to replace an opcode in an executable segment with
the breakpoint opcode. The operating system or a debugging subsystem can use
a data-segment alias for an executable segment to place an INT 3 anywhere it
is convenient to arrest normal execution so that some sort of special
processing can be performed. Debuggers typically use breakpoints as a way of
displaying registers, variables, etc., at crucial points in a task.

The saved CS:EIP value points to the byte following the breakpoint. If a
debugger replaces a planted breakpoint with a valid opcode, it must subtract
one from the saved EIP value before returning. Refer also to Chapter 12 for
more information on debugging.


9.8.4  Interrupt 4 ÄÄ Overflow

This trap occurs when the processor encounters an INTO instruction and the
OF (overflow) flag is set. Since signed arithmetic and unsigned arithmetic
both use the same arithmetic instructions, the processor cannot determine
which is intended and therefore does not cause overflow exceptions
automatically. Instead it merely sets OF when the results, if interpreted as
signed numbers, would be out of range. When doing arithmetic on signed
operands, careful programmers and compilers either test OF directly or use
the INTO instruction.


9.8.5  Interrupt 5 ÄÄ Bounds Check

This fault occurs when the processor, while executing a BOUND instruction,
finds that the operand exceeds the specified limits. A program can use the
BOUND instruction to check a signed array index against signed limits
defined in a block of memory.


9.8.6  Interrupt 6 ÄÄ Invalid Opcode

This fault occurs when an invalid opcode is detected by the execution unit.
(The exception is not detected until an attempt is made to execute the
invalid opcode; i.e., prefetching an invalid opcode does not cause this
exception.) No error code is pushed on the stack. The exception can be
handled within the same task.

This exception also occurs when the type of operand is invalid for the
given opcode. Examples include an intersegment JMP referencing a register
operand, or an LES instruction with a register source operand.


9.8.7  Interrupt 7 ÄÄ Coprocessor Not Available

This exception occurs in either of two conditions:

  þ  The processor encounters an ESC (escape) instruction, and the EM
     (emulate) bit ofCR0 (control register zero) is set.

  þ  The processor encounters either the WAIT instruction or an ESC
     instruction, and both the MP (monitor coprocessor) and TS (task
     switched) bits of CR0 are set.

Refer to Chapter 11 for information about the coprocessor interface.


9.8.8  Interrupt 8 ÄÄ Double Fault

Normally, when the processor detects an exception while trying to invoke
the handler for a prior exception, the two exceptions can be handled
serially. If, however, the processor cannot handle them serially, it signals
the double-fault exception instead. To determine when two faults are to be
signalled as a double fault, the 80386 divides the exceptions into three
classes: benign exceptions, contributory exceptions, and page faults. Table
9-3 shows this classification.

Table 9-4 shows which combinations of exceptions cause a double fault and
which do not.

The processor always pushes an error code onto the stack of the
double-fault handler; however, the error code is always zero. The faulting
instruction may not be restarted. If any other exception occurs while
attempting to invoke the double-fault handler, the processor shuts down.


Table 9-3. Double-Fault Detection Classes

Class           ID          Description

                 1          Debug exceptions
                 2          NMI
                 3          Breakpoint
Benign           4          Overflow
Exceptions       5          Bounds check
                 6          Invalid opcode
                 7          Coprocessor not available
                16          Coprocessor error

                 0          Divide error
                 9          Coprocessor Segment Overrun
Contributory    10          Invalid TSS
Exceptions      11          Segment not present
                12          Stack exception
                13          General protection

Page Faults     14          Page fault


Table 9-4. Double-Fault Definition

                                   SECOND EXCEPTION

                           Benign       Contributory    Page
                           Exception    Exception       Fault


           Benign          OK           OK              OK
           Exception

FIRST      Contributory    OK           DOUBLE          OK
EXCEPTION  Exception

           Page
           Fault           OK           DOUBLE          DOUBLE


9.8.9  Interrupt 9 ÄÄ Coprocessor Segment Overrun

This exception is raised in protected mode if the 80386 detects a page or
segment violation while transferring the middle portion of a coprocessor
operand to the NPX. This exception is avoidable. Refer to Chapter 11 for
more information about the coprocessor interface.


9.8.10  Interrupt 10 ÄÄ Invalid TSS

Interrupt 10 occurs if during a task switch the new TSS is invalid. A TSS
is considered invalid in the cases shown in Table 9-5. An error code is
pushed onto the stack to help identify the cause of the fault. The EXT bit
indicates whether the exception was caused by a condition outside the
control of the program; e.g., an external interrupt via a task gate
triggered a switch to an invalid TSS.

This fault can occur either in the context of the original task or in the
context of the new task. Until the processor has completely verified the
presence of the new TSS, the exception occurs in the context of the original
task. Once the existence of the new TSS is verified, the task switch is
considered complete; i.e., TR is updated and, if the switch is due to a
CALL or interrupt, the backlink of the new TSS is set to the old TSS. Any
errors discovered by the processor after this point are handled in the
context of the new task.

To insure a proper TSS to process it, the handler for exception 10 must be
a task invoked via a task gate.


Table 9-5. Conditions That Invalidate the TSS

Error Code              Condition

TSS id + EXT            The limit in the TSS descriptor is less than 103
LTD id + EXT            Invalid LDT selector or LDT not present
SS id + EXT             Stack segment selector is outside table limit
SS id + EXT             Stack segment is not a writable segment
SS id + EXT             Stack segment DPL does not match new CPL
SS id + EXT             Stack segment selector RPL < >  CPL
CS id + EXT             Code segment selector is outside table limit
CS id + EXT             Code segment selector does not refer to code
                        segment
CS id + EXT             DPL of non-conforming code segment < > new CPL
CS id + EXT             DPL of conforming code segment > new CPL
DS/ES/FS/GS id + EXT    DS, ES, FS, or GS segment selector is outside
                        table limits
DS/ES/FS/GS id + EXT    DS, ES, FS, or GS is not readable segment


9.8.11  Interrupt 11 ÄÄ Segment Not Present

Exception 11 occurs when the processor detects that the present bit of a
descriptor is zero. The processor can trigger this fault in any of these
cases:

  þ  While attempting to load the CS, DS, ES, FS, or GS registers; loading
     the SS register, however, causes a stack fault.

  þ  While attempting loading the LDT register with an LLDT instruction;
     loading the LDT register during a task switch operation, however,
     causes the "invalid TSS" exception.

  þ  While attempting to use a gate descriptor that is marked not-present.

This fault is restartable. If the exception handler makes the segment
present and returns, the interrupted program will resume execution.

If a not-present exception occurs during a task switch, not all the steps
of the task switch are complete. During a task switch, the processor first
loads all the segment registers, then checks their contents for validity. If
a not-present exception is discovered, the remaining segment registers have
not been checked and therefore may not be usable for referencing memory. The
not-present handler should not rely on being able to use the values found
in CS, SS, DS, ES, FS, and GS without causing another exception. The
exception handler should check all segment registers before trying to resume
the new task; otherwise, general protection faults may result later under
conditions that make diagnosis more difficult. There are three ways to
handle this case:

  1.  Handle the not-present fault with a task. The task switch back to the
      interrupted task will cause the processor to check the registers as it
      loads them from the TSS.

  2.  PUSH and POP all segment registers. Each POP causes the processor to
      check the new contents of the segment register.

  3.  Scrutinize the contents of each segment-register image in the TSS,
      simulating the test that the processor makes when it loads a segment
      register.

This exception pushes an error code onto the stack. The EXT bit of the
error code is set if an event external to the program caused an interrupt
that subsequently referenced a not-present segment. The I-bit is set if the
error code refers to an IDT entry, e.g., an INT instruction referencing a
not-present gate.

An operating system typically uses the "segment not present" exception to
implement virtual memory at the segment level. A not-present indication in a
gate descriptor, however, usually does not indicate that a segment is not
present (because gates do not necessarily correspond to segments).
Not-present gates may be used by an operating system to trigger exceptions
of special significance to the operating system.


9.8.12  Interrupt 12 ÄÄ Stack Exception

A stack fault occurs in either of two general conditions:

  þ  As a result of a limit violation in any operation that refers to the
     SS register. This includes stack-oriented instructions such as POP,
     PUSH, ENTER, and LEAVE, as well as other memory references that
     implicitly use SS (for example, MOV AX, [BP+6]). ENTER causes this
     exception when the stack is too small for the indicated local-variable
     space.

  þ  When attempting to load the SS register with a descriptor that is
     marked not-present but is otherwise valid. This can occur in a task
     switch, an interlevel CALL, an interlevel return, an LSS instruction,
     or a MOV or POP instruction to SS.

When the processor detects a stack exception, it pushes an error code onto
the stack of the exception handler. If the exception is due to a not-present
stack segment or to overflow of the new stack during an interlevel CALL, the
error code contains a selector to the segment in question (the exception
handler can test the present bit in the descriptor to determine which
exception occurred); otherwise the error code is zero.

An instruction that causes this fault is restartable in all cases. The
return pointer pushed onto the exception handler's stack points to the
instruction that needs to be restarted. This instruction is usually the one
that caused the exception; however, in the case of a stack exception due to
loading of a not-present stack-segment descriptor during a task switch, the
indicated instruction is the first instruction of the new task.

When a stack fault occurs during a task switch, the segment registers may
not be usable for referencing memory. During a task switch, the selector
values are loaded before the descriptors are checked. If a stack fault is
discovered, the remaining segment registers have not been checked and
therefore may not be usable for referencing memory. The stack fault handler
should not rely on being able to use the values found in CS, SS, DS, ES,
FS, and GS without causing another exception. The exception handler should
check all segment registers before trying to resume the new task; otherwise,
general protection faults may result later under conditions that make
diagnosis more difficult.


9.8.13  Interrupt 13 ÄÄ General Protection Exception

All protection violations that do not cause another exception cause a
general protection exception. This includes (but is not limited to):

  1.  Exceeding segment limit when using CS, DS, ES, FS, or GS

  2.  Exceeding segment limit when referencing a descriptor table

  3.  Transferring control to a segment that is not executable

  4.  Writing into a read-only data segment or into a code segment

  5.  Reading from an execute-only segment

  6.  Loading the SS register with a read-only descriptor (unless the 
      selector comes from the TSS during a task switch, in which case a TSS 
      exception occurs

  7.  Loading SS, DS, ES, FS, or GS with the descriptor of a system segment

  8.  Loading DS, ES, FS, or GS with the descriptor of an executable 
      segment that is not also readable

  9.  Loading SS with the descriptor of an executable segment

  10. Accessing memory via DS, ES, FS, or GS when the segment register
      contains a null selector

  11. Switching to a busy task

  12. Violating privilege rules

  13. Loading CR0 with PG=1 and PE=0.

  14. Interrupt or exception via trap or interrupt gate from V86 mode to
      privilege level other than zero.

  15. Exceeding the instruction length limit of 15 bytes (this can occur
      only if redundant prefixes are placed before an instruction)

The general protection exception is a fault. In response to a general
protection exception, the processor pushes an error code onto the exception
handler's stack. If loading a descriptor causes the exception, the error
code contains a selector to the descriptor; otherwise, the error code is
null. The source of the selector in an error code may be any of the
following:

  1.  An operand of the instruction.
  2.  A selector from a gate that is the operand of the instruction.
  3.  A selector from a TSS involved in a task switch.


9.8.14  Interrupt 14 ÄÄ Page Fault

This exception occurs when paging is enabled (PG=1) and the processor
detects one of the following conditions while translating a linear address
to a physical address:

  þ  The page-directory or page-table entry needed for the address
     translation has zero in its present bit.

  þ  The current procedure does not have sufficient privilege to access the
     indicated page.

The processor makes available to the page fault handler two items of
information that aid in diagnosing the exception and recovering from it:

  þ  An error code on the stack. The error code for a page fault has a
     format different from that for other exceptions (see Figure 9-8). The
     error code tells the exception handler three things:

     1.  Whether the exception was due to a not present page or to an access
         rights violation.

     2.  Whether the processor was executing at user or supervisor level at
         the time of the exception.

     3.  Whether the memory access that caused the exception was a read or
         write.

  þ  CR2 (control register two). The processor stores in CR2 the linear
     address used in the access that caused the exception (see Figure 9-9).
     The exception handler can use this address to locate the corresponding
     page directory and page table entries. If another page fault can occur
     during execution of the page fault handler, the handler should push CR2
     onto the stack.


Figure 9-8.  Page-Fault Error Code Format

 ÉÍÍÍÍÍÑÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
 ºField³Value³                         Description                        º
 ÇÄÄÄÄÄÅÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
 º U/S ³  0  ³ The access causing the fault originated when the processor º
 º     ³     ³ was executing in supervisor mode.                          º
 º     ³     ³                                                            º
 º     ³  1  ³ The access causing the fault originated when the processor º
 º     ³     ³ was executing in user mode.                                º
 º     ³     ³                                                            º
 º W/R ³  0  ³ The access causing the fault was a read.                   º
 º     ³     ³                                                            º
 º     ³  1  ³ The access causing the fault was a write.                  º
 º     ³     ³                                                            º
 º P   ³  0  ³ The fault was caused by a not-present page.                º
 º     ³     ³                                                            º
 º     ³  1  ³ The fault was caused by a page-level protection violation. º
 ÈÍÍÍÍÍÏÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

       31                               15             7        3 2 1 0
      ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÑÍÑÍ»
      º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³U³W³ º
      º±±±±±±±±±±±±±±±±±±±±±±±±±±UNDEFINED±±±±±±±±±±±±±±±±±±±±±±±³/³/³Pº
      º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³S³R³ º
      ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÏÍÏͼ


9.8.14.1  Page Fault During Task Switch

The processor may access any of four segments during a task switch:

  1.  Writes the state of the original task in the TSS of that task.

  2.  Reads the GDT to locate the TSS descriptor of the new task.

  3.  Reads the TSS of the new task to check the types of segment
      descriptors from the TSS.

  4.  May read the LDT of the new task in order to verify the segment
      registers stored in the new TSS.

A page fault can result from accessing any of these segments. In the latter
two cases the exception occurs in the context of the new task. The
instruction pointer refers to the next instruction of the new task, not to
the instruction that caused the task switch. If the design of the operating
system permits page faults to occur during task-switches, the page-fault
handler should be invoked via a task gate.


Figure 9-9.  CR2 Format

      31               23               15               7              0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º                                                                   º
     º                      PAGE FAULT LINEAR ADDRESS                    º
     º                                                                   º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


9.8.14.2  Page Fault with Inconsistent Stack Pointer

Special care should be taken to ensure that a page fault does not cause the
processor to use an invalid stack pointer (SS:ESP). Software written for
earlier processors in the 8086 family often uses a pair of instructions to
change to a new stack; for example:

MOV SS, AX
MOV SP, StackTop

With the 80386, because the second instruction accesses memory, it is
possible to get a page fault after SS has been changed but before SP has
received the corresponding change. At this point, the two parts of the stack
pointer SS:SP (or, for 32-bit programs, SS:ESP) are inconsistent.

The processor does not use the inconsistent stack pointer if the handling
of the page fault causes a stack switch to a well defined stack (i.e., the
handler is a task or a more privileged procedure). However, if the page
fault handler is invoked by a trap or interrupt gate and the page fault
occurs at the same privilege level as the page fault handler, the processor
will attempt to use the stack indicated by the current (invalid) stack
pointer.

In systems that implement paging and that handle page faults within the
faulting task (with trap or interrupt gates), software that executes at the
same privilege level as the page fault handler should initialize a new stack
by using the new LSS instruction rather than an instruction pair shown
above. When the page fault handler executes at privilege level zero (the
normal case), the scope of the problem is limited to privilege-level zero
code, typically the kernel of the operating system.


9.8.15  Interrupt 16 ÄÄ Coprocessor Error

The 80386 reports this exception when it detects a signal from the 80287 or
80387 on the 80386's ERROR# input pin. The 80386 tests this pin only at the
beginning of certain ESC instructions and when it encounters a WAIT
instruction while the EM bit of the MSW is zero (no emulation). Refer to
Chapter 11 for more information on the coprocessor interface.


9.9  Exception Summary


Table 9-6 summarizes the exceptions recognized by the 386.

Table 9-6. Exception Summary


Description               Interrupt   Return Address  Exception     Function
That Can Generate
                          Number      Points to       Type          the
Exception
                                      Faulting
                                      Instruction

Divide error               0          YES             FAULT         DIV, IDIV
Debug exceptions           1          
Some debug exceptions are traps and some are faults.  The exception
handler can determine which has occurred by examining DR6.  (Refer to
Chapter 12.)               
Some debug exceptions are traps and some are faults.  The exception
handler can determine which has occurred by examining DR6.  (Refer to
Chapter 12.)             Any instruction
Breakpoint                 3          NO              TRAP          One-byte
INT 3
Overflow                   4          NO              TRAP          INTO
Bounds check               5          YES             FAULT         BOUND
Invalid opcode             6          YES             FAULT         Any
illegal instruction
Coprocessor not available  7          YES             FAULT         ESC, WAIT
Double fault               8          YES             ABORT         Any
instruction that can
                                                                    generate
an exception
Coprocessor Segment
Overrun                    9          NO              ABORT         Any
operand of an ESC
                                                                   
instruction that wraps around
                                                                    the end of
a segment.
Invalid TSS               10          YES             FAULT
An invalid-TSS fault is not restartable if it occurs during the
processing of an external interrupt.        JMP, CALL, IRET, any interrupt
Segment not present       11          YES             FAULT         Any
segment-register modifier
Stack exception           12          YES             FAULT         Any memory
reference thru SS
General Protection        13          YES             FAULT/ABORT
All GP faults are restartable. If the fault occurs while attempting to
vector to the handler for an external interrupt, the interrupted program is
restartable, but the interrupt may be lost.  Any memory reference or code
                                                                    fetch
Page fault                14          YES             FAULT         Any memory
reference or code
                                                                    fetch
Coprocessor error         16          YES             FAULT
Coprocessor errors are reported as a fault on the first ESC or WAIT
instruction executed after the ESC instruction that caused the error.       
ESC, WAIT
Two-byte SW Interrupt     0-255       NO              TRAP          INT n


9.10  Error Code Summary

Table 9-7 summarizes the error information that is available with each
exception.


Table 9-7. Error-Code Summary

Description                       Interrupt     Error Code
                                  Number

Divide error                       0            No
Debug exceptions                   1            No
Breakpoint                         3            No
Overflow                           4            No
Bounds check                       5            No
Invalid opcode                     6            No
Coprocessor not available          7            No
System error                       8            Yes (always 0)
Coprocessor Segment Overrun        9            No
Invalid TSS                       10            Yes
Segment not present               11            Yes
Stack exception                   12            Yes
General protection fault          13            Yes
Page fault                        14            Yes
Coprocessor error                 16            No
Two-byte SW interrupt             0-255         No


Chapter 10  Initialization

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

After a signal on the RESET pin, certain registers of the 80386 are set to
predefined values. These values are adequate to enable execution of a
bootstrap program, but additional initialization must be performed by
software before all the features of the processor can be utilized.


10.1  Processor State After Reset

The contents of EAX depend upon the results of the power-up self test. The
self-test may be requested externally by assertion of BUSY# at the end of
RESET. The EAX register holds zero if the 80386 passed the test. A nonzero
value in EAX after self-test indicates that the particular 80386 unit is
faulty. If the self-test is not requested, the contents of EAX after RESET
is undefined.

DX holds a component identifier and revision number after RESET as Figure
10-1 illustrates. DH contains 3, which indicates an 80386 component. DL
contains a unique identifier of the revision level.

Control register zero (CR0) contains the values shown in Figure 10-2. The
ET bit of CR0 is set if an 80387 is present in the configuration (according
to the state of the ERROR# pin after RESET). If ET is reset, the
configuration either contains an 80287 or does not contain a coprocessor. A
software test is required to distinguish between these latter two
possibilities.

The remaining registers and flags are set as follows:

   EFLAGS             =00000002H
   IP                 =0000FFF0H
   CS selector        =000H
   DS selector        =0000H
   ES selector        =0000H
   SS selector        =0000H
   FS selector        =0000H
   GS selector        =0000H
   IDTR:
              base    =0
              limit   =03FFH

All registers not mentioned above are undefined.

These settings imply that the processor begins in real-address mode with
interrupts disabled.


Figure 10-1.  Contents of EDX after RESET

                                 EDX REGISTER

      31               23               15               7              0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
     º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³       DH       ³      DL        º
     º±±±±±±±±±±±±UNDEFINED±±±±±±±±±±±±³   DEVICE ID    ³  STEPPING ID   º
     º±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±³       3        ³   (UNIQUE)     º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 10-2.  Initial Contents of CR0

                               CONTROL REGISTER ZERO

   31                23                15                  7     4 3   1  0
  ÉÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÑÍÑÍÑÍÑÍÑÍ»
  ºP³                                                           ³E³T³E³M³Pº
  º ³                            UNDEFINED                      ³ ³ ³ ³ ³ º
  ºG³                                                           ³T³S³M³P³Eº
  ÈÑÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÏÑÏÑÏÑÏÑÏѼ
   ³                                                             ³ ³ ³ ³ ³
   ÀÄÄÄÄÄÄÄÄÄÄÄÄÄ0 - PAGING DISABLED                             ³ ³ ³ ³ ³
                 * - INDICATES PRESENCE OF 80387ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³ ³ ³
                 0 - NO TASK SWITCHÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³ ³
                 0 - DO NOT MONITOR COPROCESSORÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³
                 0 - COPROCESSOR NOT PRESENTÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³
                 0 - PROTECTION NOT ENABLED (REAL ADDRESS MODE)ÄÄÄÄÄÄÄÄÄÄÙ


10.2  Software Initialization for Real-Address Mode

In real-address mode a few structures must be initialized before a program
can take advantage of all the features available in this mode.


10.2.1  Stack

No instructions that use the stack can be used until the stack-segment
register (SS) has been loaded. SS must point to an area in RAM.


10.2.2  Interrupt Table

The initial state of the 80386 leaves interrupts disabled; however, the
processor will still attempt to access the interrupt table if an exception
or nonmaskable interrupt (NMI) occurs. Initialization software should take
one of the following actions:

  þ  Change the limit value in the IDTR to zero. This will cause a shutdown
     if an exception or nonmaskable interrupt occurs. (Refer to the 80386
     Hardware Reference Manual to see how shutdown is signalled externally.)

  þ  Put pointers to valid interrupt handlers in all positions of the
     interrupt table that might be used by exceptions or interrupts.

  þ  Change the IDTR to point to a valid interrupt table.


10.2.3  First Instructions

After RESET, address lines A{31-20} are automatically asserted for
instruction fetches. This fact, together with the initial values of CS:IP,
causes instruction execution to begin at physical address FFFFFFF0H. Near
(intrasegment) forms of control transfer instructions may be used to pass
control to other addresses in the upper 64K bytes of the address space. The
first far (intersegment) JMP or CALL instruction causes A{31-20} to drop
low, and the 80386 continues executing instructions in the lower one
megabyte of physical memory. This automatic assertion of address lines
A{31-20} allows systems designers to use a ROM at the high end of
the address space to initialize the system.


10.3  Switching to Protected Mode

Setting the PE bit of the MSW in CR0 causes the 80386 to begin executing in
protected mode. The current privilege level (CPL) starts at zero. The
segment registers continue to point to the same linear addresses as in real
address mode (in real address mode, linear addresses are the same physical
addresses).

Immediately after setting the PE flag, the initialization code must flush
the processor's instruction prefetch queue by executing a JMP instruction.
The 80386 fetches and decodes instructions and addresses before they are
used; however, after a change into protected mode, the prefetched
instruction information (which pertains to real-address mode) is no longer
valid. A JMP forces the processor to discard the invalid information.


10.4  Software Initialization for Protected Mode

Most of the initialization needed for protected mode can be done either
before or after switching to protected mode. If done in protected mode,
however, the initialization procedures must not use protected-mode features
that are not yet initialized.


10.4.1  Interrupt Descriptor Table

The IDTR may be loaded in either real-address or protected mode. However,
the format of the interrupt table for protected mode is different than that
for real-address mode. It is not possible to change to protected mode and
change interrupt table formats at the same time; therefore, it is inevitable
that, if IDTR selects an interrupt table, it will have the wrong format at
some time. An interrupt or exception that occurs at this time will have
unpredictable results. To avoid this unpredictability, interrupts should
remain disabled until interrupt handlers are in place and a valid IDT has
been created in protected mode.


10.4.2  Stack

The SS register may be loaded in either real-address mode or protected
mode. If loaded in real-address mode, SS continues to point to the same
linear base-address after the switch to protected mode.


10.4.3  Global Descriptor Table

Before any segment register is changed in protected mode, the GDT register
must point to a valid GDT. Initialization of the GDT and GDTR may be done in
real-address mode. The GDT (as well as LDTs) should reside in RAM, because
the processor modifies the accessed bit of descriptors.


10.4.4  Page Tables

Page tables and the PDBR in CR3 can be initialized in either real-address
mode or in protected mode; however, the paging enabled (PG) bit of CR0
cannot be set until the processor is in protected mode. PG may be set
simultaneously with PE, or later. When PG is set, the PDBR in CR3 should
already be initialized with a physical address that points to a valid page
directory. The initialization procedure should adopt one of the following
strategies to ensure consistent addressing before and after paging is
enabled:

  þ  The page that is currently being executed should map to the same
     physical addresses both before and after PG is set.

  þ  A JMP instruction should immediately follow the setting of PG.


10.4.5  First Task

The initialization procedure can run awhile in protected mode without
initializing the task register; however, before the first task switch, the
following conditions must prevail:

  þ  There must be a valid task state segment (TSS) for the new task. The
     stack pointers in the TSS for privilege levels numerically less than or
     equal to the initial CPL must point to valid stack segments.

  þ  The task register must point to an area in which to save the current
     task state. After the first task switch, the information dumped in this
     area is not needed, and the area can be used for other purposes.


10.5  Initialization Example

$TITLE ('Initial Task')

    NAME    INIT

init_stack  SEGMENT RW
            DW  20  DUP(?)
tos         LABEL   WORD
init_stack  ENDS

init_data   SEGMENT RW PUBLIC
            DW  20  DUP(?)
init_data   ENDS

init_code   SEGMENT ER PUBLIC

ASSUME      DS:init_data

    nop
    nop
    nop
init_start:
                                    ; set up stack
    mov ax, init_stack
    mov ss, ax
    mov esp, offset tos

    mov a1,1
blink:
    xor a1,1
    out 0e4h,a1
    mov cx,3FFFh
here:
    dec cx
    jnz here

    jmp SHORT blink

    hlt
init_code   ends

    END init_start, SS:init_stack, DS:init_data

$TITLE('Protected Mode Transition -- 386 initialization')
NAME  RESET

;*****************************************************************
; Upon reset the 386 starts executing at address 0FFFFFFF0H.  The
; upper 12 address bits remain high until a FAR call or jump is
; executed.
;
; Assume the following:
;
;
; -  a short jump at address 0FFFFFFF0H (placed there by the
;    system builder) causes execution to begin at START in segment
;    RESET_CODE.
;
;
; -  segment RESET_CODE is based at physical address 0FFFF0000H,
;    i.e.   at the start of the last  64K in the 4G address space.
;    Note that  this is the base of the CS register at reset.  If
;    you locate ROMcode above  this  address,  you  will  need  to
;    figure out an adjustment factor to address things within this
;    segment.
;
;*****************************************************************
$EJECT ;

; Define addresses to locate GDT and IDT in RAM.
; These addresses are also used in the BLD386 file that defines
; the GDT and IDT. If you change these addresses, make sure you
; change the base addresses specified in the build file.

GDTbase         EQU    00001000H   ; physical address for GDT base
IDTbase         EQU    00000400H   ; physical address for IDT base

PUBLIC     GDT_EPROM
PUBLIC     IDT_EPROM
PUBLIC     START

DUMMY      segment rw      ; ONLY for ASM386 main module stack init
           DW 0
DUMMY   ends

;*****************************************************************
;
; Note: RESET CODE must be USEl6 because the 386 initally executes
;       in real mode.
;

RESET_CODE segment er PUBLIC    USE16

ASSUME DS:nothing, ES:nothing

;
; 386 Descriptor template

DESC       STRUC
    lim_0_15    DW  0              ; limit bits (0..15)
    bas_0_15    DW  0              ; base bits (0..15)
    bas_16_23   DB  0              ; base bits (16..23)
    access      DB  0              ; access byte
    gran        DB  0              ; granularity byte
    bas_24_31   DB  0              ; base bits (24..31)
DESC       ENDS

; The following is the layout of the real GDT created by BLD386.
; It is located in EPROM and will be copied to RAM.
;
; GDT[O]      ...  NULL
; GDT[1]      ...  Alias for RAM GDT
; GDT[2]      ...  Alias for RAM IDT
; GDT[2]      ...  initial task TSS
; GDT[3]      ...  initial task TSS alias
; GDT[4]      ...  initial task LDT
; GDT[5]      ...  initial task LDT alias

;
; define entries in GDT and IDT.

GDT_ENTRIES    EQU    8
IDT_ENTRIES    EQU    32

; define some constants to index into the real GDT

GDT_ALIAS      EQU    1*SIZE DESC
IDT_ALIAS      EQU    2*SIZE DESC
INIT_TSS       EQU    3*SIZE DESC
INIT_TSS_A     EQU    4*SIZE DESC
INIT_LDT       EQU    5*SIZE DESC
INIT_LDT_A     EQU    6*SIZE DESC

;
; location of alias in INIT_LDT

INIT_LDT_ALIAS    EQU    1*SIZE DESC

;
; access rights byte for DATA and TSS descriptors

DS_ACCESS   EQU   010010010B
TSS_ACCESS  EQU   010001001B


;
; This temporary GDT will be used to set up the real GDT in RAM.

Temp_GDT    LABEL   BYTE        ; tag for begin of scratch GDT

NULL_DES    DESC <>             ; NULL descriptor

                                ; 32-Gigabyte data segment based at 0
FLAT_DES    DESC <0FFFFH,0,0,92h,0CFh,0>

GDT_eprom     DP    ?           ; Builder places GDT address and limit
                                ; in this 6 byte area.

IDT_eprom     DP    ?           ; Builder places IDT address and limit
                                ; in this 6 byte area.

;
; Prepare operand for loadings GDTR and LDTR.


TGDT_pword     LABEL  PWORD                 ; for temp GDT
        DW     end_Temp_GDT_Temp_GDT -1
        DD     0

GDT_pword      LABEL  PWORD                 ; for GDT in RAM
        DW     GDT_ENTRIES * SIZE DESC -1
        DD     GDTbase

IDT_pword      LABEL   PWORD                ; for IDT in RAM
        DW     IDT_ENTRIES * SIZE DESC -1
        DD     IDTbase


end_Temp_GDT   LABEL   BYTE

;
; Define equates for addressing convenience.

GDT_DES_FLAT        EQU DS:GDT_ALIAS +GDTbase
IDT_DES_FLAT        EQU DS:IDT_ALIAS +GDTbase

INIT_TSS_A_OFFSET   EQU DS:INIT_TSS_A
INIT_TSS_OFFSET     EQU DS:INIT_TSS

INIT_LDT_A_OFFSET   EQU DS:INIT_LDT_A
INIT_LDT_OFFSET     EQU DS:INIT_LDT


; define pointer for first task switch

ENTRY POINTER LABEL DWORD
             DW 0, INIT_TSS

;******************************************************************
;
;   Jump from reset vector to here.

START:

    CLI                ;disable interrupts
    CLD                ;clear direction flag

    LIDT    NULL_des   ;force shutdown on errors

;
;   move scratch GDT to RAM at physical 0

    XOR DI,DI
    MOV ES,DI           ;point ES:DI to physical location 0


    MOV SI,OFFSET Temp_GDT
    MOV CX,end_Temp_GDT-Temp_GDT        ;set byte count
    INC CX
;
;   move table

    REP MOVS BYTE PTR ES:[DI],BYTE PTR CS:[SI]

    LGDT    tGDT_pword                ;load GDTR for Temp. GDT
                                      ;(located at 0)

;   switch to protected mode

    MOV EAX,CR0                       ;get current CRO
    MOV EAX,1                         ;set PE bit
    MOV CRO,EAX                       ;begin protected mode
;
;   clear prefetch queue

    JMP SHORT flush
flush:

; set DS,ES,SS to address flat linear space (0 ... 4GB)

    MOV BX,FLAT_DES-Temp_GDT
    MOV US,BX
    MOV ES,BX
    MOV SS,BX
;
; initialize stack pointer to some (arbitrary) RAM location

    MOV ESP, OFFSET end_Temp_GDT

;
; copy eprom GDT to RAM

    MOV ESI,DWORD PTR GDT_eprom +2 ; get base of eprom GDT
                                   ; (put here by builder).

    MOV EDI,GDTbase                ; point ES:EDI to GDT base in RAM.

    MOV CX,WORD PTR gdt_eprom +0   ; limit of eprom GDT
    INC CX
    SHR CX,1                       ; easier to move words
    CLD
    REP MOVS   WORD PTR ES:[EDI],WORD PTR DS:[ESI]

;
; copy eprom IDT to RAM
;
    MOV ESI,DWORD PTR IDT_eprom +2 ; get base of eprom IDT
                                   ; (put here by builder)

    MOV EDI,IDTbase                ; point ES:EDI to IDT base in RAM.

    MOV CX,WORD PTR idt_eprom +0   ; limit of eprom IDT
    INC CX
    SHR CX,1
    CLD
    REP MOVS   WORD PTR ES:[EDI],WORD PTR DS:[ESI]

; switch to RAM GDT and IDT
;
    LIDT IDT_pword
    LGDT GDT_pword

;
    MOV BX,GDT_ALIAS               ; point DS to GDT alias
    MOV DS,BX
;
; copy eprom TSS to RAM
;
    MOV BX,INIT_TSS_A              ; INIT TSS A descriptor base
                                   ; has RAM location of INIT TSS.

    MOV ES,BX                      ; ES points to TSS in RAM

    MOV BX,INIT_TSS                ; get inital task selector
    LAR DX,BX                      ; save access byte
    MOV [BX].access,DS_ACCESS      ; set access as data segment
    MOV FS,BX                      ; FS points to eprom TSS

    XOR si,si                      ; FS:si points to eprom TSS
    XOR di,di                      ; ES:di points to RAM TSS

    MOV CX,[BX].lim_0_15           ; get count to move
    INC CX

;
; move INIT_TSS to RAM.

    REP MOVS BYTE PTR ES:[di],BYTE PTR FS:[si]

    MOV [BX].access,DH             ; restore access byte

;
; change base of INIT TSS descriptor to point to RAM.

    MOV AX,INIT_TSS_A_OFFSET.bas_0_15
    MOV INIT_TSS_OFFSET.bas_0_15,AX
    MOV AL,INIT_TSS_A_OFFSET.bas_16_23
    MOV INIT_TSS_OFFSET.bas_16_23,AL
    MOV AL,INIT_TSS_A_OFFSET.bas_24_31
    MOV INIT_TSS_OFFSET.bas_24_31,AL

;
; change INIT TSS A to form a save area for TSS on first task
; switch. Use RAM at location 0.

    MOV BX,INIT_TSS_A
    MOV WORD PTR [BX].bas_0_15,0
    MOV [BX].bas_16_23,0
    MOV [BX].bas_24_31,0
    MOV [BX].access,TSS_ACCESS
    MOV [BX].gran,O
    LTR BX                         ; defines save area for TSS

;
; copy eprom LDT to RAM

    MOV BX,INIT_LDT_A              ; INIT_LDT_A descriptor has
                                   ; base address in RAM for INIT_LDT.

    MOV ES,BX                      ; ES points LDT location in RAM.

    MOV AH,[BX].bas_24_31
    MOV AL,[BX].bas_16_23
    SHL EAX,16
    MOV AX,[BX].bas_0_15           ; save INIT_LDT base (ram) in EAX

    MOV BX,INIT_LDT                ; get inital LDT selector
    LAR DX,BX                      ; save access rights
    MOV [BX].access,DS_ACCESS      ; set access as data segment
    MOV FS,BX                      ; FS points to eprom LDT

    XOR si,si                      ; FS:SI points to eprom LDT
    XOR di,di                      ; ES:DI points to RAM LDT

    MOV CX,[BX].lim_0_15           ; get count to move
    INC CX
;
; move initial LDT to RAM

    REP MOVS BYTE PTR ES:[di],BYTE PTR FS:[si]

    MOV [BX].access,DH             ; restore access rights in
                                   ; INIT_LDT descriptor

;
; change base of alias (of INIT_LDT) to point to location in RAM.

    MOV ES:[INIT_LDT_ALIAS].bas_0_15,AX
    SHR EAX,16
    MOV ES:[INIT_LDT_ALIAS].bas_16_23,AL
    MOV ES:[INIT_LDT_ALIAS].bas_24_31,AH

;
; now set the base value in INIT_LDT descriptor

    MOV AX,INIT_LDT_A_OFFSET.bas_0_15
    MOV INIT_LDT_OFFSET.bas_0_15,AX
    MOV AL,INIT_LDT_A_OFFSET.bas_16_23
    MOV INIT_LDT_OFFSET.bas_16_23,AL
    MOV AL,INIT_LDT_A_OFFSET.bas_24_31
    MOV INIT_LDT_OFFSET.bas_24_31,AL

;
; Now GDT, IDT, initial TSS and initial LDT are all set up.
;
; Start the first task!
'
   JMP ENTRY_POINTER

RESET_CODE ends
   END START, SS:DUMMY,DS:DUMMY


10.6  TLB Testing

The 80386 provides a mechanism for testing the Translation Lookaside Buffer
(TLB), the cache used for translating linear addresses to physical
addresses. Although failure of the TLB hardware is extremely unlikely, users
may wish to include TLB confidence tests among other power-up confidence
tests for the 80386.

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
NOTE
  This TLB testing mechanism is unique to the 80386 and may not be
  continued in the same way in future processors. Sortware that uses
  this mechanism may be incompatible with future processors.
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

When testing the TLB it is recommended that paging be turned off (PG=0 in
CR0) to avoid interference with the test data being written to the TLB.


10.6.1  Structure of the TLB

The TLB is a four-way set-associative memory. Figure 10-3 illustrates the
structure of the TLB. There are four sets of eight entries each. Each entry
consists of a tag and data. Tags are 24-bits wide. They contain the
high-order 20 bits of the linear address, the valid bit, and three attribute
bits. The data portion of each entry contains the high-order 20 bits of the
physical address.


10.6.2  Test Registers

Two test registers, shown in Figure 10-4, are provided for the purpose of
testing. TR6 is the test command register, and TR7 is the test data
register. These registers are accessed by variants of the MOV
instruction. A test register may be either the source operand or destination
operand. The MOV instructions are defined in both real-address mode and
protected mode. The test registers are privileged resources; in protected
mode, the MOV instructions that access them can only be executed at
privilege level 0. An attempt to read or write the test registers when
executing at any other privilege level causes a general
protection exception.

The test command register (TR6) contains a command and an address tag to
use in performing the command:

C         This is the command bit. There are two TLB testing commands:
          write entries into the TLB, and perform TLB lookups. To cause an
          immediate write into the TLB entry, move a doubleword into TR6
          that contains a 0 in this bit. To cause an immediate TLB lookup,
          move a doubleword into TR6 that contains a 1 in this bit.

Linear    On a TLB write, a TLB entry is allocated to this linear address;
Address   the rest of that TLB entry is set per the value of TR7 and the
          value just written into TR6. On a TLB lookup, the TLB is
          interrogated per this value; if one and only one TLB entry
          matches, the rest of the fields of TR6 and TR7 are set from the
          matching TLB entry.

V         The valid bit for this TLB entry. The TLB uses the valid bit to
          identify entries that contain valid data. Entries of the TLB
          that have not been assigned values have zero in the valid bit.
          All valid bits can be cleared by writing to CR3.

D, D#     The dirty bit (and its complement) for/from the TLB entry.

U, U#     The U/S bit (and its complement) for/from the TLB entry.

W, W#     The R/W bit (and its complement) for/from the TLB entry.

          The meaning of these pairs of bits is given by Table 10-1,
          where X represents D, U, or W.

The test data register (TR7) holds data read from or data to be written to
the TLB.

Physical  This is the data field of the TLB. On a write to the TLB, the
Address   TLB entry allocated to the linear address in TR6 is set to this
          value. On a TLB lookup, if HT is set, the data field (physical
          address) from the TLB is read out to this field. If HT is not
          set, this field is undefined.

HT        For a TLB lookup, the HT bit indicates whether the lookup was a
          hit (HT  1) or a miss (HT  0). For a TLB write, HT must be set
          to 1.

REP       For a TLB write, selects which of four associative blocks of the
          TLB is to be written. For a TLB read, if HT is set, REP reports
          in which of the four associative blocks the tag was found; if HT
          is not set, REP is undefined.


Table 10-1. Meaning of D, U, and W Bit Pairs

X     X#      Effect during        Value of bit X
              TLB Lookup           after TLB Write

0     0       (undefined)          (undefined)
0     1       Match if X=0         Bit X becomes 0
1     0       Match if X=1         Bit X becomes 1
1     1       (undefined)          (undefined)


Figure 10-3.  TLB Structure

                                   ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
                                  7º       TAG       º      DATA      º
                                   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                                                                    
                 ÚÄÄÄÄÄÄÄ                                           
                 ³       SET 11                                     
                 ³    ÚÄÄ          ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                 ³    ³           1º       TAG       º      DATA      º
                 ³    ³            ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                 ³    ³           0º       TAG       º      DATA      º
                 ³    ³            ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                 ³    ³
                 ³    ³            ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
                 ³    ³           7º       TAG       º      DATA      º
                 ³    ³            ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                 ³    ³                                             
                 ³    ÀÄÄ                                           
                 ³       SET 10                                     
                 ³    ÚÄÄ          ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                 ³    ³           1º       TAG       º      DATA      º
      ³ D ³      ³    ³            ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
      ³ A ³      ³    ³           0º       TAG       º      DATA      º
      ³ T ÀÄÄÄÄÄÄÙ    ³            ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
      ³ A             ³
      ³   ÚÄÄÄÄÄÄ¿    ³            ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
      ³ B ³      ³    ³           7º       TAG       º      DATA      º
      ³ U ³      ³    ³            ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
      ³ S ³      ³    ³                                             
                 ³    ÀÄÄ                                           
                 ³       SET 01                                     
                 ³    ÚÄÄ          ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                 ³    ³           1º       TAG       º      DATA      º
                 ³    ³            ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                 ³    ³           0º       TAG       º      DATA      º
                 ³    ³            ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                 ³    ³
                 ³    ³            ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
                 ³    ³           7º       TAG       º      DATA      º
                 ³    ³            ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                 ³    ³                                             
                 ³    ÀÄÄ                                           
                 ³       SET 00                                     
                 ÀÄÄÄÄÄÄÄ          ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                                  1º       TAG       º      DATA      º
                                   ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÎÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
                                  0º       TAG       º      DATA      º
                                   ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ


Figure 10-4.  Test Registers

      31                23              15   11      7             0
     ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍØÍÍÍÍÍÍÍØÍÍÍÍÍÑÍÑÍÍÍÑÍÍÍ»
     º                                      ³             ³H³   ³   º
     º           PHYSICAL ADDRESS           ³0 0 0 0 0 0 0³ ³REP³0 0º TR7
     º                                      ³             ³T³   ³   º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÂÄÂÄÂÄÂÄÂÄÂÄÅÄÁÄÄÄÁÄÂĶ
     º                                      ³ ³ ³D³ ³U³ ³W³       ³ º
     º            LINEAR ADDRESS            ³V³D³ ³U³ ³ ³ ³0 0 0 0³Cº TR8
     º                                      ³ ³ ³#³ ³#³ ³#³       ³ º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍØÍÏÍÏÍÏÍØÍÏÍÏÍÏÍÍÍÍÍÍÍÏͼ

     NOTE: 0 INDICATES INTEL RESERVED. NO NOT DEFINE


10.6.3  Test Operations

To write a TLB entry:

  1.  Move a doubleword to TR7 that contains the desired physical address,
      HT, and REP values. HT must contain 1. REP must point to the
      associative block in which to place the entry.

  2.  Move a doubleword to TR6 that contains the appropriate linear
      address, and values for V, D, U, and W. Be sure C=0 for "write"
      command.

Be careful not to write duplicate tags; the results of doing so are
undefined.

To look up (read) a TLB entry:

  1.  Move a doubleword to TR6 that contains the appropriate linear address
      and attributes. Be sure C=1 for "lookup" command.

  2.  Store TR7. If the HT bit in TR7 indicates a hit, then the other
      values reveal the TLB contents. If HT indicates a miss, then the other
      values in TR7 are indeterminate.

For the purposes of testing, the V bit functions as another bit of
addresss.  The V bit for a lookup request should usually be set, so that
uninitialized tags do not match. Lookups with V=0 are unpredictable if any
tags are uninitialized.


Chapter 11  Coprocessing and Multiprocessing

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

The 80386 has two levels of support for multiple parallel processing units:

  þ  A highly specialized interface for very closely coupled processors of
     a type known as coprocessors.

  þ  A more general interface for more loosely coupled processors of
     unspecified type.


11.1  Coprocessing

The components of the coprocessor interface include:

  þ  ET bit of control register zero (CR0)
  þ  The EM, and MP bits of CR0
  þ  The ESC instructions
  þ  The WAIT instruction
  þ  The TS bit of CR0
  þ  Exceptions


11.1.1  Coprocessor Identification

The 80386 is designed to operate with either an 80287 or 80387 math
coprocessor. The ET bit of CR0 indicates which type of coprocessor is
present. ET is set automatically by the 80386 after RESET according to the
level detected on the ERROR# input. If desired, ET may also be set or reset
by loading CR0 with a MOV instruction. If ET is set, the 80386 uses the
32-bit protocol of the 80387; if reset, the 80386 uses the 16-bit protocol
of the 80287.


11.1.2  ESC and WAIT Instructions

The 80386 interprets the pattern 11011B in the first five bits of an
instruction as an opcode intended for a coprocessor. Instructions thus
marked are called ESCAPE or ESC instructions. The CPU performs the following
functions upon encountering an ESC instruction before sending the
instruction to the coprocessor:

  þ  Tests the emulation mode (EM) flag to determine whether coprocessor
     functions are being emulated by software.

  þ  Tests the TS flag to determine whether there has been a context change
     since the last ESC instruction.

  þ  For some ESC instructions, tests the ERROR# pin to determine whether
     the coprocessor detected an error in the previous ESC instruction.

The WAIT instruction is not an ESC instruction, but WAIT causes the CPU to
perform some of the same tests that it performs upon encountering an ESC
instruction. The processor performs the following actions for a WAIT
instruction:

  þ  Waits until the coprocessor no longer asserts the BUSY# pin.

  þ  Tests the ERROR# pin (after BUSY# goes inactive). If ERROR# is active,
     the 80386 signals exception 16, which indicates that the coprocessor
     encountered an error in the previous ESC instruction.

  þ  WAIT can therefore be used to cause exception 16 if an error is
     pending from a previous ESC instruction. Note that, if no coprocessor
     is present, the ERROR# and BUSY# pins should be tied inactive to
     prevent WAIT from waiting forever or causing spurious exceptions.


11.1.3  EM and MP Flags

The EM and MP flags of CR0 control how the processor reacts to coprocessor
instructions.

The EM bit indicates whether coprocessor functions are to be emulated. If
the processor finds EM set when executing an ESC instruction, it signals
exception 7, giving the exception handler an opportunity to emulate the ESC
instruction.

The MP (monitor coprocessor) bit indicates whether a coprocessor is
actually attached. The MP flag controls the function of the WAIT
instruction. If, when executing a WAIT instruction, the CPU finds MP set,
then it tests the TS flag; it does not otherwise test TS during a WAIT
instruction. If it finds TS set under these conditions, the CPU signals
exception 7.

The EM and MP flags can be changed with the aid of a MOV instruction using
CR0 as the destination operand and read with the aid of a MOV instruction
with CR0 as the source operand. These forms of the MOV instruction can be
executed only at privilege level zero.


11.1.4  The Task-Switched Flag

The TS bit of CR0 helps to determine when the context of the coprocessor
does not match that of the task being executed by the 80386 CPU. The 80386
sets TS each time it performs a task switch (whether triggered by software
or by hardware interrupt). If, when interpreting one of the ESC
instructions, the CPU finds TS already set, it causes exception 7. The WAIT
instruction also causes exception 7 if both TS and MP are set. Operating
systems can use this exception to switch the context of the coprocessor to
correspond to the current task. Refer to the 80386 System Software Writer's
Guide for an example.

The CLTS instruction (legal only at privilege level zero) resets the TS
flag.


11.1.5  Coprocessor Exceptions

Three exceptions aid in interfacing to a coprocessor: interrupt 7
(coprocessor not available), interrupt 9 (coprocessor segment overrun), and
interrupt 16 (coprocessor error).


11.1.5.1  Interrupt 7 ÄÄ Coprocessor Not Available

This exception occurs in either of two conditions:

  1.  The CPU encounters an ESC instruction and EM is set. In this case,
      the exception handler should emulate the instruction that caused the
      exception. TS may also be set.

  2.  The CPU encounters either the WAIT instruction or an ESC instruction
      when both MP and TS are set. In this case, the exception handler
      should update the state of the coprocessor, if necessary.


11.1.5.2  Interrupt 9 ÄÄ Coprocessor Segment Overrun

This exception occurs in protected mode under the following conditions:

  þ  An operand of a coprocessor instruction wraps around an addressing
     limit (0FFFFH for small segments, 0FFFFFFFFH for big segments, zero for
     expand-down segments). An operand may wrap around an addressing limit
     when the segment limit is near an addressing limit and the operand is
     near the largest valid address in the segment. Because of the
     wrap-around, the beginning and ending addresses of such an operand
     will be near opposite ends of the segment.

  þ  Both the first byte and the last byte of the operand (considering
     wrap-around) are at addresses located in the segment and in present and
     accessible pages.

  þ  The operand spans inaccessible addresses. There are two ways that such
     an operand may also span inaccessible addresses:

     1.  The segment limit is not equal to the addressing limit (e.g.,
         addressing limit is FFFFH and segment limit is FFFDH); therefore,
         the operand will span addresses that are not within the segment
         (e.g., an 8-byte operand that starts at valid offset FFFC will span
         addresses FFFC-FFFF and 0000-0003; however, addresses FFFE and FFFF
         are not valid, because they exceed the limit);

     2.  The operand begins and ends in present and accessible pages but
         intermediate bytes of the operand fall either in a not-present page
         or in a page to which the current procedure does not have access
         rights.

The address of the failing numerics instruction and data operand may be
lost; an FSTENV does not return reliable addresses. As with the 80286/80287,
the segment overrun exception should be handled by executing an FNINIT
instruction (i.e., an FINIT without a preceding WAIT). The return address on
the stack does not necessarily point to the failing instruction nor to the
following instruction. The failing numerics instruction is not restartable.

Case 2 can be avoided by either aligning all segments on page boundaries or
by not starting them within 108 bytes of the start or end of a page. (The
maximum size of a coprocessor operand is 108 bytes.) Case 1 can be avoided
by making sure that the gap between the last valid offset and the first
valid offset of a segment is either no less than 108 bytes or is zero (i.e.,
the segment is of full size). If neither software system design constraint
is acceptable, the exception handler should execute FNINIT and should
probably terminate the task.


11.1.5.3  Interrupt 16 ÄÄ Coprocessor Error

The numerics coprocessors can detect six different exception conditions
during instruction execution. If the detected exception is not masked by a
bit in the control word, the coprocessor communicates the fact that an error
occurred to the CPU by a signal at the ERROR# pin. The CPU causes interrupt
16 the next time it checks the ERROR# pin, which is only at the beginning of
a subsequent WAIT or certain ESC instructions. If the exception is masked,
the numerics coprocessor handles the exception according to on-board logic;
it does not assert the ERROR# pin in this case.


11.2  General Multiprocessing

The components of the general multiprocessing interface include:

  þ  The LOCK# signal

  þ  The LOCK instruction prefix, which gives programmed control of the
     LOCK# signal.

  þ  Automatic assertion of the LOCK# signal with implicit memory updates
     by the processor


11.2.1  LOCK and the LOCK# Signal

The LOCK instruction prefix and its corresponding output signal LOCK# can
be used to prevent other bus masters from interrupting a data movement
operation. LOCK may only be used with the following 80386 instructions when
they modify memory. An undefined-opcode exception results from using LOCK
before any instruction other than:

  þ  Bit test and change: BTS, BTR, BTC.
  þ  Exchange: XCHG.
  þ  Two-operand arithmetic and logical: ADD, ADC, SUB, SBB, AND, OR, XOR.
  þ  One-operand arithmetic and logical: INC, DEC, NOT, and NEG.

A locked instruction is only guaranteed to lock the area of memory defined
by the destination operand, but it may lock a larger memory area. For
example, typical 8086 and 80286 configurations lock the entire physical
memory space. The area of memory defined by the destination operand is
guaranteed to be locked against access by a processor executing a locked
instruction on exactly the same memory area, i.e., an operand with
identical starting address and identical length.

The integrity of the lock is not affected by the alignment of the memory
field. The LOCK signal is asserted for as many bus cycles as necessary to
update the entire operand.


11.2.2  Automatic Locking

In several instances, the processor itself initiates activity on the data
bus. To help ensure that such activities function correctly in
multiprocessor configurations, the processor automatically asserts the LOCK#
signal. These instances include:

  þ  Acknowledging interrupts.

     After an interrupt request, the interrupt controller uses the data bus
     to send the interrupt ID of the interrupt source to the CPU. The CPU
     asserts LOCK# to ensure that no other data appears on the data bus
     during this time.

  þ  Setting busy bit of TSS descriptor.

     The processor tests and sets the busy-bit in the type field of the TSS
     descriptor when switching to a task. To ensure that two different
     processors cannot simultaneously switch to the same task, the processor
     asserts LOCK# while testing and setting this bit.

  þ  Loading of descriptors.

     While copying the contents of a descriptor from a descriptor table into
     a segment register, the processor asserts LOCK# so that the descriptor
     cannot be modified by another processor while it is being loaded. For
     this action to be effective, operating-system procedures that update
     descriptors should adhere to the following steps:

     ÄÄ  Use a locked update to the access-rights byte to mark the
         descriptor not-present.

     ÄÄ  Update the fields of the descriptor.  (This may require several
         memory accesses; therefore, LOCK cannot be used.)

     ÄÄ  Use a locked update to the access-rights byte to mark the
         descriptor present again.

  þ  Updating page-table A and D bits.

     The processor exerts LOCK# while updating the A (accessed) and D 
     (dirty) bits of page-table entries.  Also the processor bypasses the
     page-table cache and directly updates these bits in memory.

  þ  Executing XCHG instruction.

     The 80386 always asserts LOCK during an XCHG instruction that
     references memory (even if the LOCK prefix is not used).


11.2.3  Cache Considerations

Systems programmers must take care when updating shared data that may also
be stored in on-chip registers and caches.  With the 80386, such  shared
data includes:

  þ  Descriptors, which may be held in segment registers.

     A change to a descriptor that is shared among processors should be
     broadcast to all processors.  Segment registers are effectively
     "descriptor caches".  A change to a descriptor will not be utilized by
     another processor if that processor already has a copy of the old
     version of the descriptor in a segment register.

  þ  Page tables, which may be held in the page-table cache.

     A change to a page table that is shared among processors should be
     broadcast to all processors, so that others can flush their page-table
     caches and reload them with up-to-date page tables from memory.

Systems designers can employ an interprocessor interrupt to handle the
above cases. When one processor changes data that may be cached by other
processors, it can send an interrupt signal to all other processors that may
be affected by the change. If the interrupt is serviced by an interrupt
task, the task switch automatically flushes the segment registers. The task
switch also flushes the page-table cache if the PDBR (the contents of CR3)
of the interrupt task is different from the PDBR of every other task.

In multiprocessor systems that need a cacheability signal from the CPU, it
is recommended that physical address pin A31 be used to indicate
cacheability. Such a system can then possess up to 2 Gbytes of physical
memory. The virtual address range available to the programmer is not
affected by this convention.


Chapter 12  Debugging

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

The 80386 brings to Intel's line of microprocessors significant advances in
debugging power. The single-step exception and breakpoint exception of
previous processors are still available in the 80386, but the principal
debugging support takes the form of debug registers. The debug registers
support both instruction breakpoints and data breakpoints. Data breakpoints
are an important innovation that can save hours of debugging time by
pinpointing, for example, exactly when a data structure is being
overwritten. The breakpoint registers also eliminate the complexities
associated with writing a breakpoint instruction into a code segment
(requires a data-segment alias for the code segment) or a code segment
shared by multiple tasks (the breakpoint exception can occur in the context
of any of the tasks). Breakpoints can even be set in code contained in ROM.


12.1  Debugging Features of the Architecture

The features of the 80386 architecture that support debugging include:

Reserved debug interrupt vector

Permits processor to automatically invoke a debugger task or procedure when
an event occurs that is of interest to the debugger.

Four debug address registers

Permit programmers to specify up to four addresses that the CPU will
automatically monitor.

Debug control register

Allows programmers to selectively enable various debug conditions
associated with the four debug addresses.

Debug status register

Helps debugger identify condition that caused debug exception.

Trap bit of TSS (T-bit)

Permits monitoring of task switches.

Resume flag (RF) of flags register

Allows an instruction to be restarted after a debug exception without
immediately causing another debug exception due to the same condition.

Single-step flag (TF)

Allows complete monitoring of program flow by specifying whether the CPU
should cause a debug exception with the execution of every instruction.

Breakpoint instruction

Permits debugger intervention at any point in program execution and aids
debugging of debugger programs.

Reserved interrupt vector for breakpoint exception

Permits processor to automatically invoke a handler task or procedure upon
encountering a breakpoint instruction.

These features make it possible to invoke a debugger that is either a
separate task or a procedure in the context of the current task. The
debugger can be invoked under any of the following kinds of conditions:

  þ  Task switch to a specific task.
  þ  Execution of the breakpoint instruction.
  þ  Execution of every instruction.
  þ  Execution of any instruction at a given address.
  þ  Read or write of a byte, word, or doubleword at any specified address.
  þ  Write to a byte, word, or doubleword at any specified address.
  þ  Attempt to change a debug register.


12.2  Debug Registers

Six 80386 registers are used to control debug features. These registers are
accessed by variants of the MOV instruction. A debug register may be either
the source operand or destination operand. The debug registers are
privileged resources; the MOV instructions that access them can only be
executed at privilege level zero. An attempt to read or write the debug
registers when executing at any other privilege level causes a general
protection exception. Figure 12-1 shows the format of the debug registers.


Figure 12-1.  Debug Registers

      31              23              15              7             0
     ÉÍÍÍÑÍÍÍÑÍÍÍÑÍÍÍØÍÍÍÑÍÍÍÑÍÍÍÑÍÍÍØÍÍÍÑÍÑÍÍÍÍÍÑÍÑÍØÍÑÍÑÍÑÍÑÍÑÍÑÍÑÍ»
     ºLEN³R/W³LEN³R/W³LEN³R/W³LEN³R/W³   ³ ³     ³G³L³G³L³G³L³G³L³G³Lº
     º   ³   ³   ³   ³   ³   ³   ³   ³0 0³0³0 0 0³ ³ ³ ³ ³ ³ ³ ³ ³ ³ º DR7
     º 3 ³ 3 ³ 2 ³ 2 ³ 1 ³ 1 ³ 0 ³ 0 ³   ³ ³     ³E³E³3³3³2³2³1³1³0³0º
     ÇÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÅÄÂÄÅÄÅÄÄÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÅÄÅÄÅÄÅĶ
     º                               ³B³B³B³                 ³B³B³B³Bº
     º0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0³ ³ ³ ³0 0 0 0 0 0 0 0 0³ ³ ³ ³ º DR6
     º                               ³T³S³D³                 ³3³2³1³0º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÁÄÁÄÁÄÁĶ
     º                                                               º
     º                            RESERVED                           º DR5
     º                                                               º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                                                               º
     º                            RESERVED                           º DR4
     º                                                               º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                                                               º
     º                 BREAKPOINT 3 LINEAR ADDRESS                   º DR3
     º                                                               º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                                                               º
     º                 BREAKPOINT 2 LINEAR ADDRESS                   º DR2
     º                                                               º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                                                               º
     º                 BREAKPOINT 1 LINEAR ADDRESS                   º DR1
     º                                                               º
     ÇÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄĶ
     º                                                               º
     º                 BREAKPOINT 0 LINEAR ADDRESS                   º DR0
     º                                                               º
     ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍͼ

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
NOTE
      0 MEANS INTEL RESERVED. DO NOT DEFINE.
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ


12.2.1  Debug Address Registers (DR0-DR3)

Each of these registers contains the linear address associated with one of
four breakpoint conditions. Each breakpoint condition is further defined by
bits in DR7.

The debug address registers are effective whether or not paging is enabled.
The addresses in these registers are linear addresses. If paging is enabled,
the linear addresses are translated into physical addresses by the
processor's paging mechanism (as explained in Chapter 5). If paging is not
enabled, these linear addresses are the same as physical addresses.

Note that when paging is enabled, different tasks may have different
linear-to-physical address mappings. When this is the case, an address in a
debug address register may be relevant to one task but not to another. For
this reason the 80386  has both global and local enable bits in DR7. These
bits indicate whether a given debug address has a global (all tasks) or
local (current task only) relevance.


12.2.2  Debug Control Register (DR7)

The debug control register shown in Figure 12-1 both helps to define the
debug conditions and selectively enables and disables those conditions.

For each address in registers DR0-DR3, the corresponding fields R/W0
through R/W3 specify the type of action that should cause a breakpoint. The
processor interprets these bits as follows:

   00 ÄÄ Break on instruction execution only
   01 ÄÄ Break on data writes only
   10 ÄÄ undefined
   11 ÄÄ Break on data reads or writes but not instruction fetches

Fields LEN0 through LEN3 specify the length of data item to be monitored. A
length of 1, 2, or 4 bytes may be specified. The values of the length fields
are interpreted as follows:

   00 ÄÄ one-byte length
   01 ÄÄ two-byte length
   10 ÄÄ undefined
   11 ÄÄ four-byte length

If RWn is 00 (instruction execution), then LENn should also be 00. Any other
length is undefined.

The low-order eight bits of DR7 (L0 through L3 and G0 through G3)
selectively enable the four address breakpoint conditions. There are two
levels of enabling: the local (L0 through L3) and global (G0 through G3)
levels. The local enable bits are automatically reset by the processor at
every task switch to avoid unwanted breakpoint conditions in the new task.
The global enable bits are not reset by a task switch; therefore, they can
be used for conditions that are global to all tasks.

The LE and GE bits control the "exact data breakpoint match" feature of the
processor. If either LE or GE is set, the processor slows execution so that
data breakpoints are reported on the instruction that causes them. It is
recommended that one of these bits be set whenever data breakpoints are
armed. The processor clears LE at a task switch but does not clear GE.


12.2.3  Debug Status Register (DR6)

The debug status register shown in Figure 12-1 permits the debugger to
determine which debug conditions have occurred.

When the processor detects an enabled debug exception, it sets the
low-order bits of this register (B0 thru B3) before entering the debug
exception handler. Bn is set if the condition described by DRn, LENn, and
R/Wn occurs. (Note that the processor sets Bn regardless of whether Gn or
Ln is set. If more than one breakpoint condition occurs at one time and if
the breakpoint trap occurs due to an enabled condition other than n, Bn may
be set, even though neither Gn nor Ln is set.)

The BT bit is associated with the T-bit (debug trap bit) of the TSS (refer
to 7 for the location of the T-bit). The processor sets the BT bit before
entering the debug handler if a task switch has occurred and the T-bit of
the new TSS is set. There is no corresponding bit in DR7 that enables and
disables this trap; the T-bit of the TSS is the sole enabling bit.

The BS bit is associated with the TF (trap flag) bit of the EFLAGS
register. The BS bit is set if the debug handler is entered due to the
occurrence of a single-step exception. The single-step trap is the
highest-priority debug exception; therefore, when BS is set, any of the
other debug status bits may also be set.

The BD bit is set if the next instruction will read or write one of the
eight debug registers and ICE-386 is also using the debug registers at the
same time.

Note that the bits of DR6 are never cleared by the processor. To avoid any
confusion in identifying the next debug exception, the debug handler should
move zeros to DR6 immediately before returning.


12.2.4  Breakpoint Field Recognition

The linear address and LEN field for each of the four breakpoint conditions
define a range of sequential byte addresses for a data breakpoint. The LEN
field permits specification of a one-, two-, or four-byte field. Two-byte
fields must be aligned on word boundaries (addresses that are multiples of
two) and four-byte fields must be aligned on doubleword boundaries
(addresses that are multiples of four). These requirements are enforced by
the processor; it uses the LEN bits to mask the low-order bits of the
addresses in the debug address registers. Improperly aligned code or data
breakpoint addresses will not yield the expected results.

A data read or write breakpoint is triggered if any of the bytes
participating in a memory access is within the field defined by a breakpoint
address register and the corresponding LEN field. Table 12-1 gives some
examples of breakpoint fields with memory references that both do and do not
cause traps.

To set a data breakpoint for a misaligned field longer than one byte, it
may be desirable to put two sets of entries in the breakpoint register such
that each entry is properly aligned and the two entries together span the
length of the field.

Instruction breakpoint addresses must have a length specification of one
byte (LEN = 00); other values are undefined. The processor recognizes an
instruction breakpoint address only when it points to the first byte of an
instruction. If the instruction has any prefixes, the breakpoint address
must point to the first prefix.


Table 12-1. Breakpoint Field Recognition Examples

                                    Address (hex)          Length

                       DR0             0A0001          1 (LEN0 = 00)
Register Contents      DR1             0A0002          1 (LEN1 = 00)
                       DR2             0B0002          2 (LEN2 = 01)
                       DR3             0C0000          4 (LEN3 = 11)

Some Examples of Memory                0A0001          1
References That Cause Traps            0A0002          1
                                       0A0001          2
                                       0A0002          2
                                       0B0002          2
                                       0B0001          4
                                       0C0000          4
                                       0C0001          2
                                       0C0003          1

Some Examples of Memory                0A0000          1
References That Don't Cause Traps      0A0003          4
                                       0B0000          2
                                       0C0004          4


12.3  Debug Exceptions

Two of the interrupt vectors of the 80386 are reserved for exceptions that
relate to debugging. Interrupt 1 is the primary means of invoking debuggers
designed expressly for the 80386; interrupt 3 is intended for debugging
debuggers and for compatibility with prior processors in Intel's 8086
processor family.


12.3.1  Interrupt 1 ÄÄ Debug Exceptions

The handler for this exception is usually a debugger or part of a debugging
system. The processor causes interrupt 1 for any of several conditions. The
debugger can check flags in DR6 and DR7 to determine what condition caused
the exception and what other conditions might be in effect at the same time.
Table 12-2 associates with each breakpoint condition the combination of
bits that indicate when that condition has caused the debug exception.

Instruction address breakpoint conditions are faults, while other debug
conditions are traps. The debug exception may report either or both at one
time. The following paragraphs present details for each class of debug
exception.


Table 12-2. Debug Exception Conditions

Flags to Test              Condition

BS=1                       Single-step trap
B0=1 AND (GE0=1 OR LE0=1)  Breakpoint DR0, LEN0, R/W0
B1=1 AND (GE1=1 OR LE1=1)  Breakpoint DR1, LEN1, R/W1
B2=1 AND (GE2=1 OR LE2=1)  Breakpoint DR2, LEN2, R/W2
B3=1 AND (GE3=1 OR LE3=1)  Breakpoint DR3, LEN3, R/W3
BD=1                       Debug registers not available; in use by ICE-386.
BT=1                       Task switch


12.3.1.1  Instruction Addrees Breakpoint

The processor reports an instruction-address breakpoint before it executes
the instruction that begins at the given address; i.e., an instruction-
address breakpoint exception is a fault.

The RF (restart flag) permits the debug handler to retry instructions that
cause other kinds of faults in addition to debug faults. When it detects a
fault, the processor automatically sets RF in the flags image that it pushes
onto the stack. (It does not, however, set RF for traps and aborts.)

When RF is set, it causes any debug fault to be ignored during the next
instruction. (Note, however, that RF does not cause breakpoint traps to be
ignored, nor other kinds of faults.)

The processor automatically clears RF at the successful completion of every
instruction except after the IRET instruction, after the POPF instruction,
and after a JMP, CALL, or INT instruction that causes a task switch. These
instructions set RF to the value specified by the memory image of the EFLAGS
register.

The processor automatically sets RF in the EFLAGS image on the stack before
entry into any fault handler. Upon entry into the fault handler for
instruction address breakpoints, for example, RF is set in the EFLAGS image
on the stack; therefore, the IRET instruction at the end of the handler will
set RF in the EFLAGS register, and execution will resume at the breakpoint
address without generating another breakpoint fault at the same address.

If, after a debug fault, RF is set and the debug handler retries the
faulting instruction, it is possible that retrying the instruction will
raise other faults. The retry of the instruction after these faults will
also be done with RF=1, with the result that debug faults continue to be
ignored. The processor clears RF only after successful completion of the
instruction.

Real-mode debuggers can control the RF flag by using a 32-bit IRET. A
16-bit IRET instruction does not affect the RF bit (which is in the
high-order 16 bits of EFLAGS). To use a 32-bit IRET, the debugger must
rearrange the stack so that it holds appropriate values for the 32-bit EIP,
CS, and EFLAGS (with RF set in the EFLAGS image). Then executing an IRET
with an operand size prefix causes a 32-bit return, popping the RF flag
into EFLAGS.


12.3.1.2  Data Address Breakpoint

A data-address breakpoint exception is a trap; i.e., the processor reports
a data-address breakpoint after executing the instruction that accesses the
given memory item.

When using data breakpoints it is recommended that either the LE or GE bit
of DR7 be set also. If either LE or GE is set, any data breakpoint trap is
reported exactly after completion of the instruction that accessed the
specified memory item. This exact reporting is accomplished by forcing the
80386 execution unit to wait for completion of data operand transfers before
beginning execution of the next instruction. If neither GE nor LE is set,
data breakpoints may not be reported until one instruction after the data is
accessed or may not be reported at all. This is due to the fact that,
normally, instruction execution is overlapped with memory transfers to such
a degree that execution of the next instruction may begin before memory
transfers for the prior instruction are completed.

If a debugger needs to preserve the contents of a write breakpoint
location, it should save the original contents before setting a write
breakpoint. Because data breakpoints are traps, a write into a breakpoint
location will complete before the trap condition is reported. The handler
can report the saved value after the breakpoint is triggered. The data in
the debug registers can be used to address the new value stored by the
instruction that triggered the breakpoint.


12.3.1.3  General Detect Fault

This exception occurs when an attempt is made to use the debug registers at
the same time that ICE-386 is using them. This additional protection feature
is provided to guarantee that ICE-386 can have full control over the
debug-register resources when required. ICE-386 uses the debug-registers;
therefore, a software debugger that also uses these registers cannot run
while ICE-386 is in use. The exception handler can detect this condition by
examining the BD bit of DR6.


12.3.1.4  Single-Step Trap

This debug condition occurs at the end of an instruction if the trap flag
(TF) of the flags register held the value one at the beginning of that
instruction.  Note that the exception does not occur at the end of an
instruction that sets TF. For example, if POPF is used to set TF, a
single-step trap does not occur until after the instruction that follows
POPF.

The processor clears the TF bit before invoking the handler.  If TF=1 in
the flags image of a TSS at the time of a task switch, the exception occurs
after the first instruction is executed in the new task.

The single-step flag is normally not cleared by privilege changes inside a
task.  INT instructions, however, do clear TF.  Therefore, software
debuggers that single-step code must recognize and emulate INT n or INTO
rather than executing them directly.

To maintain protection, system software should check the current execution
privilege level after any single step interrupt to see whether single
stepping should continue at the current privilege level.

The interrupt priorities in hardware guarantee that if an external
interrupt occurs, single stepping stops. When both an external interrupt and
a single step interrupt occur together, the single step interrupt is
processed first. This clears the TF bit. After saving the return address or
switching tasks, the external interrupt input is examined before the first
instruction of the single step handler executes.  If the external interrupt
is still pending, it is then serviced. The external interrupt handler is not
single-stepped. To single step an interrupt handler, just single step an INT
n instruction that refers to the interrupt handler.


12.3.1.5  Task Switch Breakpoint

The debug exception also occurs after a switch to an 80386 task if the
T-bit of the new TSS is set.  The exception occurs after control has passed
to the new task, but before the first instruction of that task is executed.
The exception handler can detect this condition by examining the BT bit of
the debug status register DR6.

Note that if the debug exception handler is a task, the T-bit of its TSS
should not be set. Failure to observe this rule will cause the processor to
enter an infinite loop.


12.3.2  Interrupt 3 ÄÄ Breakpoint Exception

This exception is caused by execution of the breakpoint instruction INT 3.
Typically, a debugger prepares a breakpoint by substituting the opcode of
the one-byte breakpoint instruction in place of the first opcode byte of the
instruction to be trapped. When execution of the INT 3 instruction causes
the exception handler to be invoked, the saved value of ES:EIP points to the
byte following the INT 3 instruction.

With prior generations of processors, this feature is used extensively for
trapping execution of specific instructions. With the 80386, the needs
formerly filled by this feature are more conveniently solved via the debug
registers and interrupt 1.  However, the breakpoint exception is still
useful for debugging debuggers, because the breakpoint exception can vector
to a different exception handler than that used by the debugger. The
breakpoint exception can also be useful when it is necessary to set a
greater number of breakpoints than permitted by the debug registers.