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Vol. 3A 15-9
8086 EMULATION
15.2.1 Enabling Virtual-8086 Mode
The processor runs in virtual-8086 mode when the VM (virtual machine) flag in the EFLAGS
register is set. This flag can only be set when the processor switches to a new protected-mode
task or resumes virtual-8086 mode via an IRET instruction.
System software cannot change the state of the VM flag directly in the EFLAGS register (for
example, by using the POPFD instruction). Instead it changes the flag in the image of the
EFLAGS register stored in the TSS or on the stack following a call to an interrupt- or exception-
handler procedure. For example, software sets the VM flag in the EFLAGS image in the TSS
when first creating a virtual-8086 task.
The processor tests the VM flag under three general conditions:
• When loading segment registers, to determine whether to use 8086-style address
translation.
• When decoding instructions, to determine which instructions are not supported in virtual-
8086 mode and which instructions are sensitive to IOPL.
• When checking privileged instructions, on page accesses, or when performing other
permission checks. (Virtual-8086 mode always executes at CPL 3.)
15.2.2 Structure of a Virtual-8086 Task
A virtual-8086-mode task consists of the following items:
• A 32-bit TSS for the task.
• The 8086 program.
• A virtual-8086 monitor.
• 8086 operating-system services.
The TSS of the new task must be a 32-bit TSS, not a 16-bit TSS, because the 16-bit TSS does
not load the most-significant word of the EFLAGS register, which contains the VM flag. All
TSS’s, stacks, data, and code used to handle exceptions when in virtual-8086 mode must also be
32-bit segments.
The processor enters virtual-8086 mode to run the 8086 program and returns to protected mode
to run the virtual-8086 monitor.
The virtual-8086 monitor is a 32-bit protected-mode code module that runs at a CPL of 0. The
monitor consists of initialization, interrupt- and exception-handling, and I/O emulation proce-
dures that emulate a personal computer or other 8086-based platform. Typically, the monitor is
either part of or closely associated with the protected-mode general-protection (#GP) exception
handler, which also runs at a CPL of 0. As with any protected-mode code module, code-segment
descriptors for the virtual-8086 monitor must exist in the GDT or in the task’s LDT. The virtual-
8086 monitor also may need data-segment descriptors so it can examine the IDT or other parts
of the 8086 program in the first 1 MByte of the address space. The linear addresses above
10FFEFH are available for the monitor, the operating system, and other system software.
- IA-32 Intel 1
- Architecture 1
- Software Developer’s Manual 1
- CONTENTS FOR VOLUME 3A AND 3B 3
- Vol. 3A v 5
- Vol. 3A vii 7
- Vol. 3A ix 9
- Vol. 3A xi 11
- Vol. 3A xiii 13
- Vol. 3A xv 15
- Vol. 3A xvii 17
- Vol. 3A xix 19
- Vol. 3A xxi 21
- Vol. 3A xxiii 23
- Vol. 3A xxv 25
- Vol. 3A xxvii 27
- Vol. 3A xxix 29
- Vol. 3A xxxi 31
- Vol. 3A xxxiii 33
- About This Manual 35
- CHAPTER 1 37
- ABOUT THIS MANUAL 37
- 1.3 NOTATIONAL CONVENTIONS 40
- 1.3.1 Bit and Byte Order 41
- 1.3.3 Instruction Operands 42
- 1.3.5 Segmented Addressing 43
- 1.3.7 Exceptions 44
- 1.4 RELATED LITERATURE 45
- System Architecture 47
- Overview 47
- CHAPTER 2 49
- SYSTEM ARCHITECTURE OVERVIEW 49
- Linear Address 51
- Tab leDirectory 52
- 2.1.5 Memory Management 55
- 2.1.6 System Registers 56
- 2.1.7 Other System Resources 58
- 2.2 MODES OF OPERATION 58
- 2.4.4 Task Register (TR) 64
- 2.5 CONTROL REGISTERS 64
- Counters 76
- Protected-Mode 79
- CHAPTER 3 81
- Logical Address 82
- 3.2 USING SEGMENTS 83
- 3.2.1 Basic Flat Model 83
- 3.2.2 Protected Flat Model 83
- Figure 3-2. Flat Model 84
- 3.2.3 Multi-Segment Model 85
- 3.2.5 Paging and Segmentation 86
- 3.3 PHYSICAL ADDRESS SPACE 86
- Vol. 3A 3-7 87
- 3.4.2 Segment Selectors 88
- 3.4.3 Segment Registers 89
- Vol. 3A 3-11 91
- 3.4.5 Segment Descriptors 92
- Vol. 3A 3-13 93
- 3-14 Vol. 3A 94
- 3-16 Vol. 3A 96
- 3.5 SYSTEM DESCRIPTOR TYPES 97
- 3-20 Vol. 3A 100
- 3.6.1 Paging Options 101
- ADDRESSING 102
- Page Directory 103
- 3.7.4 Memory Aliasing 105
- 3-28 Vol. 3A 108
- Vol. 3A 3-29 109
- MECHANISM 110
- Vol. 3A 3-31 111
- Addressing Enabled 114
- Vol. 3A 3-35 115
- PAGING MECHANISM 117
- Vol. 3A 3-39 119
- Physical Addr 120
- 3-42 Vol. 3A 122
- Vol. 3A 3-43 123
- MOV CR3, EAX 126
- Vol. 3A 3-47 127
- 3-48 Vol. 3A 128
- Protection 129
- CHAPTER 4 131
- PROTECTION 131
- PAGE-LEVEL PROTECTION 132
- 4.3 LIMIT CHECKING 135
- • The type field 136
- 4.5 PRIVILEGE LEVELS 138
- Figure 4-3. Protection Rings 139
- SEGMENTS 141
- REGISTER 143
- From Various Privilege Levels 146
- • Call gates 147
- • Trap gates 147
- • Interrupt gates 147
- • Task gates 147
- 4.8.3 Call Gates 148
- Segment Selector 151
- 4.8.5 Stack Switching 153
- 4.9 PRIVILEGED INSTRUCTIONS 162
- 4.10 POINTER VALIDATION 162
- Instructions) 164
- Instruction) 164
- 4.10.5 Checking Alignment 167
- 4.11 PAGE-LEVEL PROTECTION 167
- 4.11.1 Page-Protection Flags 168
- 4.11.3 Page Type 168
- • IA-32e mode 170
- 4.13.3 Reserved Bit Checking 173
- • IA32_EFER.NXE = 1 174
- Interrupt and 175
- Exception Handling 175
- CHAPTER 5 177
- 5.3 SOURCES OF INTERRUPTS 178
- 5.3.1 External Interrupts 178
- 5-4 Vol. 3A 180
- • Machine-check exceptions 181
- 5.6 PROGRAM OR TASK RESTART 182
- Vol. 3A 5-7 183
- 5.7.1 Handling Multiple NMIs 184
- Vol. 3A 5-9 185
- INTERRUPTS 186
- 5-12 Vol. 3A 188
- • Task-gate descriptor 189
- • Interrupt-gate descriptor 189
- • Trap-gate descriptor 189
- 5-16 Vol. 3A 192
- 5-18 Vol. 3A 194
- 5.12.2 Interrupt Tasks 195
- 5.13 ERROR CODE 197
- 5.14.1 64-Bit Mode IDT 198
- Vol. 3A 5-23 199
- 5.14.3 IRET in IA-32e Mode 200
- 5.14.5 Interrupt Stack Table 201
- 5-26 Vol. 3A 202
- Vol. 3A 5-27 203
- Interrupt 2—NMI Interrupt 205
- 5-30 Vol. 3A 206
- Vol. 3A 5-31 207
- 5-32 Vol. 3A 208
- Vol. 3A 5-33 209
- 5-34 Vol. 3A 210
- Vol. 3A 5-35 211
- 5-36 Vol. 3A 212
- Vol. 3A 5-39 215
- Exception Class Fault 216
- Description 216
- 5-42 Vol. 3A 218
- Vol. 3A 5-43 219
- 5-44 Vol. 3A 220
- Vol. 3A 5-45 221
- 5-46 Vol. 3A 222
- Vol. 3A 5-47 223
- 5-48 Vol. 3A 224
- Vol. 3A 5-49 225
- 5-50 Vol. 3A 226
- Vol. 3A 5-51 227
- MOV SS, AX 229
- MOV SP, StackTop 229
- 5-54 Vol. 3A 230
- Vol. 3A 5-55 231
- 5-56 Vol. 3A 232
- 5-58 Vol. 3A 234
- Vol. 3A 5-59 235
- 5-60 Vol. 3A 236
- Vol. 3A 5-61 237
- Vol. 3A 5-63 239
- 5-64 Vol. 3A 240
- Task Management 241
- CHAPTER 6 243
- TASK MANAGEMENT 243
- 6.1.2 Task State 244
- 6.1.3 Executing a Task 245
- • Task-state segment (TSS) 246
- • TSS descriptor 246
- • Task register 246
- 6.2.2 TSS Descriptor 249
- 6.2.4 Task Register 251
- Figure 6-5. Task Register 252
- 6.2.5 Task-Gate Descriptor 253
- 6.3 TASK SWITCHING 254
- 6.4 TASK LINKING 258
- Figure 6-8. Nested Tasks 259
- 6.4.2 Modifying Task Linkages 260
- 6.5 TASK ADDRESS SPACE 261
- Multiple-Processor 267
- Management 267
- CHAPTER 7 269
- MULTIPLE-PROCESSOR MANAGEMENT 269
- 7.1 LOCKED ATOMIC OPERATIONS 270
- 7.1.2 Bus Locking 271
- 7-6 Vol. 3A 274
- 7.2 MEMORY ORDERING 275
- 7.4 SERIALIZING INSTRUCTIONS 282
- 7.5.1 BSP and AP Processors 284
- Vol. 3A 7-19 287
- 7-20 Vol. 3A 288
- • Cores per Package 292
- 7.8 INTEL 294
- HYPER-THREADING TECHNOLOGY 294
- ARCHITECTURE 294
- 7.8.2 APIC Functionality 296
- 7.8.8 IA32_MISC_ENABLE MSR 298
- 7.8.9 Memory Ordering 298
- 7.8.12 Self Modifying Code 299
- 7.9 DUAL-CORE ARCHITECTURE 301
- 7.9.4 IA32_MISC_ENABLE MSR 302
- Platform 304
- Hyper-Threading Technology 305
- 7-38 Vol. 3A 306
- Vol. 3A 7-39 307
- 7-40 Vol. 3A 308
- Vol. 3A 7-41 309
- 7-42 Vol. 3A 310
- Vol. 3A 7-43 311
- 7-44 Vol. 3A 312
- 7.11.1 HLT Instruction 313
- 7.11.2 PAUSE Instruction 314
- Spin_Lock: 317
- JE Get_Lock 317
- 7-50 Vol. 3A 318
- Vol. 3A 7-51 319
- 7-52 Vol. 3A 320
- Advanced 323
- Programmable 323
- Interrupt Controller 323
- CHAPTER 8 325
- ADVANCED PROGRAMMABLE 325
- INTERRUPT CONTROLLER (APIC) 325
- 8-2 Vol. 3A 326
- 3-Wire APIC Bus 327
- MP systems 328
- 8.2 SYSTEM BUS VS. APIC BUS 329
- 8.3 THE INTEL 329
- 82489DX EXTERNAL APIC 329
- THE APIC, AND THE XAPIC 329
- 8.4 LOCAL APIC 329
- 8-6 Vol. 3A 330
- 8-10 Vol. 3A 334
- 63 071011 8912 335
- 8.4.6 Local APIC ID 336
- 8.4.7 Local APIC State 336
- Vol. 3A 8-13 337
- 8-14 Vol. 3A 338
- 8.5 HANDLING LOCAL INTERRUPTS 339
- 8.5.1 Local Vector Table 339
- Vol. 3A 8-17 341
- 8.5.2 Valid Interrupt Vectors 342
- 8.5.3 Error Handling 343
- 8.5.4 APIC Timer 344
- 8-22 Vol. 3A 346
- 8-24 Vol. 3A 348
- Vol. 3A 8-25 349
- 8-28 Vol. 3A 352
- 8-30 Vol. 3A 354
- 8-32 Vol. 3A 356
- 8.8 HANDLING INTERRUPTS 357
- Processors 357
- Pentium Processors) 359
- vector / 16 360
- Figure 8-21. EOI Register 364
- 8.9 SPURIOUS INTERRUPT 365
- 8.10.1 Bus Message Formats 367
- 8-44 Vol. 3A 368
- 31 20 19 12 11 4 3 2 1 0 368
- Vol. 3A 8-45 369
- 8-46 Vol. 3A 370
- 31 16 15 14 13 11 10 8 7 0 370
- Vol. 3A 8-47 371
- 8-48 Vol. 3A 372
- Processor 373
- Management and 373
- Initialization 373
- CHAPTER 9 375
- PROCESSOR MANAGEMENT AND 375
- INITIALIZATION 375
- 9-2 Vol. 3A 376
- 9.2 X87 FPU INITIALIZATION 380
- 9.3 CACHE ENABLING 382
- Vol. 3A 9-9 383
- OPERATION 384
- 9.7.1 Real-Address Mode IDT 385
- 9.7.2 NMI Interrupt Handling 385
- 9-12 Vol. 3A 386
- 9.8.3 Initializing Paging 387
- 9-14 Vol. 3A 388
- Vol. 3A 9-15 389
- 9-16 Vol. 3A 390
- 9.9 MODE SWITCHING 391
- 9-18 Vol. 3A 392
- Vol. 3A 9-19 393
- 9-20 Vol. 3A 394
- FFFF FFFFH 395
- FFFF FFF0H 395
- FFFF 0000H 395
- 64K EPROM 395
- 9.10.1 Assembler Usage 396
- 9.10.2 STARTUP.ASM Listing 397
- 9-24 Vol. 3A 398
- Vol. 3A 9-25 399
- 9-26 Vol. 3A 400
- Vol. 3A 9-27 401
- 9-28 Vol. 3A 402
- Vol. 3A 9-29 403
- 162-172 of List File) 404
- RAM_START 405
- 9.10.3 MAIN.ASM Source Code 407
- 9.10.4 Supporting Files 407
- 9-34 Vol. 3A 408
- 9.11.1 Microcode Update 410
- Vol. 3A 9-41 415
- Table 9-10. Processor Flags 416
- Vol. 3A 9-43 417
- ; Offset of microcode update 418
- Vol. 3A 9-45 419
- 9-46 Vol. 3A 420
- Vol. 3A 9-47 421
- Access: Read/Write 421
- Vol. 3A 9-49 423
- 9-50 Vol. 3A 424
- Vol. 3A 9-51 425
- 9-52 Vol. 3A 426
- Vol. 3A 9-53 427
- 9-58 Vol. 3A 432
- Vol. 3A 9-59 433
- 9-60 Vol. 3A 434
- Table 9-16. Mnemonic Values 435
- Memory Cache 439
- CHAPTER 10 441
- MEMORY CACHE CONTROL 441
- 10.2 CACHING TERMINOLOGY 444
- 10.3.2 Choosing a Memory Type 449
- 10.4 CACHE CONTROL PROTOCOL 450
- 10.5 CACHE CONTROL 450
- and P6 family 452
- Pentium II Processors 456
- 10.5.3 Preventing Caching 458
- 10.6 SELF-MODIFYING CODE 461
- AND P6 FAMILY PROCESSORS) 462
- 10.8 EXPLICIT CACHING 462
- 10.10 STORE BUFFER 464
- Register Pair 471
- Vol. 3A 10-33 473
- 10-34 Vol. 3A 474
- 10.11.5 MTRR Initialization 475
- ≠ FirstType 476
- Vol. 3A 10-37 477
- 10-38 Vol. 3A 478
- 10.12.2 IA32_CR_PAT MSR 482
- 10.12.4 Programming the PAT 483
- Technology 487
- System Programming 487
- CHAPTER 11 489
- MMX™ TECHNOLOGY SYSTEM 489
- PROGRAMMING 489
- REGISTERS 492
- • System exceptions: 493
- Exceptions 494
- 11.6 DEBUGGING MMX CODE 494
- 11-8 Vol. 3A 496
- SSE, SSE2 and SSE3 497
- CHAPTER 12 499
- • CPUID.1:EDX.SSE[bit 25] = 1 500
- • CPUID.1:ECX.SSE3[bit 0] = 1 500
- SSE3, EM, MP, and TS 501
- • Memory Access Exceptions: 502
- Vol. 3A 12-5 503
- 12-6 Vol. 3A 504
- OR CONTEXT SWITCHES 505
- CONTEXT SWITCHES 505
- 12-8 Vol. 3A 506
- • Clears the TS flag 508
- Power and Thermal 509
- CHAPTER 13 511
- POWER AND THERMAL MANAGEMENT 511
- IA32_MPERF (Addr: E7H) 512
- IA32_APERF (Addr: E8H) 512
- Vol. 3A 13-3 513
- MANAGEMENT 514
- 13.4.2 Thermal Monitor 516
- Thermal Status Log 518
- Clock Modulation Facilities 521
- 16222327 522
- 16222324 523
- Machine Check 525
- Architecture 525
- CHAPTER 14 527
- MACHINE-CHECK ARCHITECTURE 527
- 14.3 MACHINE-CHECK MSRS 528
- . . . . 531
- 14.3.3 Mapping of the Pentium 537
- 14-12 Vol. 3A 538
- 14.6.1 Simple Error Codes 539
- 14.6.2 Compound Error Codes 540
- Vol. 3A 14-19 545
- 14.7.3 Pentium 546
- Vol. 3A 14-21 547
- 8086 Emulation 549
- CHAPTER 15 551
- 8086 EMULATION 551
- 15.2 VIRTUAL-8086 MODE 557
- 15.2.7 Sensitive Instructions 564
- 15.2.8 Virtual-8086 Mode I/O 564
- IN VIRTUAL-8086 MODE 565
- Mixing 16-Bit 579
- CHAPTER 16 581
- MIXING 16-BIT AND 32-BIT CODE 581
- 16.4.4 Parameter Translation 588
- IA-32 Architecture 591
- Compatibility 591
- CHAPTER 17 593
- 17.2. RESERVED BITS 594
- THROUGH SOFTWARE 594
- 17.5. INTEL MMX TECHNOLOGY 595
- 17.10. DUAL-CORE TECHNOLOGY 596
- PROCESSORS 596
- 17.13. OBSOLETE INSTRUCTIONS 598
- 17.14. UNDEFINED OPCODES 598
- 17.16. STACK OPERATIONS 599
- 17.16.1 PUSH SP 599
- 17.17. X87 FPU 600
- 17.17.2 x87 FPU Status Word 601
- 17.17.3 x87 FPU Control Word 602
- 17.17.4 x87 FPU Tag Word 602
- 17.17.5 Data Types 602
- Vol. 3A 17-11 603
- 17-12 Vol. 3A 604
- Vol. 3A 17-13 605
- 17-14 Vol. 3A 606
- Vol. 3A 17-15 607
- 17-16 Vol. 3A 608
- Vol. 3A 17-17 609
- 17.17.9 Obsolete Instructions 610
- Vol. 3A 17-19 611
- 17.20. CONTROL REGISTERS 613
- 17-22 Vol. 3A 614
- Vol. 3A 17-23 615
- 17.22. DEBUG FACILITIES 616
- Vol. 3A 17-25 617
- 17-26 Vol. 3A 618
- 17.25. INTERRUPTS 619
- 17.25.2 NMI Interrupts 619
- 17.25.3 IDT Limit 620
- Family and Pentium Processors 621
- 17.27. TASK SWITCHING AND TSS 621
- 17.27.2 TSS Selector Writes 622
- Offset FH from beginning of 623
- 17-32 Vol. 3A 624
- 17.29. PAGING 625
- 17.29.1 Large Pages 625
- 17.29.2 PCD and PWT Flags 625
- 17.30. STACK OPERATIONS 626
- 17.30.2 Error Code Pushes 627
- 17.32.1 Segment Wraparound 628
- Vol. 3A 17-37 629
- 17.34. BUS LOCKING 630
- 17.35. BUS HOLD 631
- 17-40 Vol. 3A 632
- Vol. 3A 17-41 633
- 17-42 Vol. 3A 634
- INTEL SALES OFFICES 635
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