Introduction to 8086 emulation

Using ‘Virtual-8086’ mode to execute real-mode procedures in a protected-mode environment

Features of real-mode

• At power-up the Pentium begins executing in real-address mode (memory addressing does not require use of descriptor tables)

• CPU privilege-restrictions are not imposed

• Memory addresses are limited to 20-bits

• Interrupt-routing is handled using the IVT

• Multitasking and paging are unsupported

• Lots of ‘legacy’ software written for 8086

Rationale for 8086 emulation

• It is desirable to run multiple 8086 tasks in an environment that ‘protects’ each task from interference by other tasks, yet offers each task the illusion of being in control of the system (as in ‘real-mode’ environment)

• Duplicate the environment of an 8086 cpu

• Synchronize access to shared resources, (such as files and peripheral i/o devices)

Commercial imperatives

• 1981: IBM-PC (to compete with CP/M)

• 1984: Macintosh (introduces graphics)

• 1985: Windows 1.0 (to rival Macintosh)

• 1986: Windows 2.0 (80286 multitasking)

• Macintosh had 32-bit processor (M68000), but Windows 2.0 had 16-bit processor and was handicapped by slow mode-switching needed to execute its firmware routines

Intel’s 80386

• Windows 3.0 ran on new 32-bit processor:– Faster mode-switching (whenever needed)– Virtual memory support (for task isolation)– Hardware ‘breakpoint’ debugging support – Virtual-8086 (for firmware and legacy code)

The VM-bit in EFLAGS

• The CPU executes in ‘Virtual-8086’ mode when the VM-bit (bit #17) in EFLAGS is 1

• POPFL instruction cannot modify VM-bit• Two methods for entering VM86-mode:

1) use the IRET instruction (.code32)2) use a task-switch to a new 386 TSS

• The only way to leave VM86-mode is with an interrupt (either hardware or software) or by resetting the processor (i.e., reboot)

Entering a VM86-mode procedure

GS-image

FS-image

DS-image

ES-image

SS-image

SP-image

EFLAGS ( VM=1, NT=0 )

CS-image

IP-imageSS:ESP

Ring-0 Stack-Frame

Execute IRET instruction from 32-bit code-segment while in protected-mode at privilege-level 0

I/O-sensitive Instructions

• While in VM86-mode, certain instructions are ‘sensitive’ to the current value of the IOPL-field in EFLAGS:– The CLI and STI instructions– The PUSHF and POPF instructions– The PUSHFL and POPFL instructions– The IRET and IRETL instructions– The INT-nn instruction

• The above instructions will generate a General Protection Exception (INT-13) unless IOPL==3

The EFLAGS register

0 0 0 0 0 0 0 0 0 0ID

VIP

VIF

AC

VM

RF

0NT

IOPL

OF

DF

IF

TF

SF

ZF

0AF

0PF

1CF

31 17 13 12 0

Legend: VM = Virtual-8086 Mode (1=yes, 0=no)IOPL = I/O Privilege-Level (0,1,2,3)VIF = Virtual Interrupt-Flag (if CR4.0 = 1)VIP = Virtual Interrupt Pending (if CR4.0 = 1) ID = CPUID-supported (1=yes, 0=no)

CF = Carry-Flag TF = Trap-Flag PF = Parity-Flag IF = Interrupt-Flag AF = Auxilliary-Flag DF = Direction-Flag ZF = Zero-Flag RF = Resume-Flag SF = Sign-Flag NT = Nested Task OF = Overflow-Flag AC = Alignment Check

Emulating I/O-sensitive instructions

• Suppose a task executing in VM86-mode tries to disable device-interrupts, using a ‘cli’ instruction

• If IOPL ≠ 3, this instruction will cause a GP-fault (exception 0x0D) with an error-code equal to 0

• An exception-handler can examine the opcode (by using the saved CS:IP address on its stack)

• If that opcode equals 0xFA (i.e., ‘cli’), then the handler can clear bit #9 in the saved EFLAGS image (i.e., the IF-bit), increment the saved IP, then execute IRET to resume the VM86 task

When IOPL == 3

• A VM86-task executes at privilege-level 3

• If IOPL==3, then the VM86 task is allowed to execute all the IO-sensitive instructions (except INT-nn) without generating a fault

How to leave VM-8086 mode?

• In VM86-mode, certain instructions trigger a General Protection Fault regardless of the current value in EFLAGS’ IOPL-field

• One of these is the halt-instruction (‘hlt’)• The GP fault-handler can examine the

opcode that triggered the fault (using the saved CS:IP address on its ring0 stack) and, if it is 0xF4 (i.e., ‘hlt’), can terminate the VM86 task, if that is what is desired

Example: ‘vm86demo.s’

• This demo illustrates entering and leaving a Virtual-8086 procedure within a 386 task that is executing in protected-mode

• The procedure draws directly to video ram, changing all the characters’ attribute-bytes to white on a blue-colored background

• It executes with device-interrupts disabled• It includes no ‘io-sensitive’ instructions • It uses ‘hlt’ to exit from Virtual-8086 mode

In-class exercise #1

• We want to modify ‘vm86demo.s’ -- to do something that’s much more interesting!

• Let’s add a ‘software interrupt’ instruction, to try executing some ROM-BIOS code

• Easiest to try is ‘int $0x1C’ -- because it normally does nothing but return (‘iret’)

• We will need to add code to our GP-fault handler that ‘emulates’ an ‘int-nn’ opcode

Steps for ‘int-nn’ emulation

• Increment the saved IP-image by 2 bytes (to simulate fetching the instruction)

• Simulate the ‘push’ of FLAGS, CS, and IP onto the VM86 task’s ring3 stack

• Identify the interrupt’s ID-number, and copy its vector from IVT onto ring0 stack

• Clear IF and TF bits in the saved EFLAGS

• Use ‘iret’ to resume ‘virtual-8086’ mode

Emulating ‘int-nn’

GS

FS

DS

ES

SS

SP

EFLAGS

CS

IP

FLAGS

CS

IP

Ring-0 StackRing-3 Stack

SS:ESP

CS

IP

Real-Mode IVT

int nn

Ring-3 code-segment

Emulating ‘iret’

GS

FS

DS

ES

SS

SP

EFLAGS

CS

IP

FLAGS

CS

IP

Ring-0 StackRing-3 Stack

SS:ESP

Other emulations if IOPL < 3

• If you try executing code in Virtual-8086 mode without IOPL==3, then you’re likely to need to emulate the other io-sensitive instructions (iret, cli, sti, pushf, popf)

• The CLI and STI instructions are easy

• The PUSHF/POPF are a little harder

• The IRET is the most complex of these

Emulating ‘cli’ or ‘sti’

GS

FS

DS

ES

SS

SP

EFLAGS

CS

IP

Ring-0 Stack

SS:ESP

Simply adjust bit number 9 in the saved image of the EFLAGS register on the ring0 stack

Emulating ‘popf’

GS

FS

DS

ES

SS

SP

EFLAGS

CS

IP

FLAGS

Ring-0 StackRing-3 Stack

SS:ESP

Copy the topmost word from the ring3 stack to the low-half of the saved EFLAGS-image on the ring0 stack; Add 2 to the saved SP-value; Add 1 to the saved IP-value; then execute IRET to resume

Emulating ‘pushf’

GS

FS

DS

ES

SS

SP

EFLAGS

CS

IP

FLAGS

Ring-0 StackRing-3 Stack

SS:ESP

Subtract 2 from the saved SP-image; Copy low-half of the saved EFLAGS-image from ring0 stack to top word of ring3 stack; Add 1 to the saved IP-value; then execute IRET to resume

IO-permission Bitmap

• For tasks that execute in VM86-mode, the ability to execute IN/OUT instructions can be controlled on a port-by-port basis, using a bitmap data-structure within the TSS

• The bitmap can be up to 8192 bytes long (one bit for each of the 65536 i/o ports)

• The CPU finds this bitmap by using the value at offset 0x66 within the TSS, which holds the bitmap’s starting TSS offset

Layout of the Task-State Segment

I/O PermissionBitmap

IOMAP

TSS Base-Address

0x66

Trapping I/O

• If you do not want a VM86 task to directly perform I/O operations on a specific port, you can set that port’s bit within the bitmap

• For example, to prevent a VM86 task from reading mouse-data (io-port 0x60), just set bit $0x60 within that task’s io-permission bitmap: this will causes a GP-fault if the instruction ‘in $0x60, %al’ is encountered

Pentium improves VM86-mode

• The Pentium processor introduced some speed-improvements to Virtual-8086 mode (largely for the benefit of Windows 3.0)

• So-called ‘Virtual Mode Extensions’ (VME) can be enabled (by setting bit #0 in a new Control Register named CR4)

• Then even software-interrupt instructions don’t require use of ‘emulation’ code