8086 emulation

1. 8086 emulation Using Virtual-8086 mode to execute real-mode procedures in a protected-mode environment

2. 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 exists for 8086

3. 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)

4. The VM-bit in EFLAGS • The CPU executes in ‘Virtual-8086’ mode when the VM-bit (bit #17) in EFLAGS is 1 • POPFD instruction cannot modify VM-bit • Two methods for entering VM86-mode: 1) use the IRETD instruction 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)

5. Entering a VM86-mode procedure Ring-0 Stack-Frame GS-image FS-image DS-image Execute IRETD instruction while in protected-mode at privilege-level 0 ES-image SS-image SP-image EFLAGS ( VM=1, NT=0 ) CS-image IP-image SS:ESP

6. 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 PUSHFD and POPFD instructions • The IRET and IRETD instructions • The INT-nn instruction • The above instructions with generate a General Protection Exception (INT-13) unless IOPL==3

7. 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 • The exception-handler can examine the opcode (using the saved CS:EIP address on its stack) • If the 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 EIP, then execute IRETD to resume the VM86 task

8. 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 • If the VME-bit (bit #0) in Control Register 4 is set to 1, Virtual Mode Extensions will be enabled, and then INT-nn instructions can also be executed without triggering a fault

9. 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:EIP address on its ring0 stack) and, if it is 0xF4 (i.e., ‘hlt’), can terminate the VM86 task, if that is what is desired

10. 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

11. Layout of the Task-State Segment I/O Permission Bitmap IOMAP 0x66 TSS Base-Address

12. 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 al, #0x60’ is encountered

13. Demo-program: ‘tryvm86.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 a blue-colored background (‘turn_blue’) • It executes with device-interrupts disabled • It includes no ‘io-sensitive’ instructions

14. In-class exercise #1 • Try modifying the ‘tryvm86.s’ demo -- to do something that’s much more interesting • Replace the ‘turn_blue’ routine with code that would call ROM-BIOS services (via interrupt 0x10) to print a message at the current cursor location • You will need to add code to the GP-fault handler that ‘emulates’ an ‘int-nn’ opcode

15. Steps for ‘int-nn’ emulation • Determine the interrupt’s ID-number • Advance the saved IP-value by 2 bytes (to skip the emulated interrupt-instruction) • Simulate the ‘push’ of FLAGS, CS, and IP onto the VM86 task’s ring3 stack • Copy vector from IVT onto ring0 stack • Clear IF and TF bits in the saved EFLAGS • NOTE: You may need to block all device interrupts (by setting PIC mask registers)

16. Emulating ‘int-nn’ Ring-3 Stack Ring-0 Stack GS FS DS FLAGS ES CS SS IP SP EFLAGS Real-Mode IVT CS CS IP IP SS:ESP

17. In-class exercise #2 • Include a counter in your GP-fault handler • Display the counter-value when finishing • Then enable Virtual-Mode Extensions (by setting bit #0 in Control Register 4) • Re-execute your demo – notice the new value of the GP-fault handler’s counter!