Intel IA-32 - Manual (Page 569)

Vol. 3A 15-19
8086 EMULATION
The virtual-8086 monitor runs at privilege level 0, like the protected-mode interrupt and excep-
tion handlers. It is commonly closely tied to the protected-mode general-protection exception
(#GP, vector 13) handler. If the protected-mode interrupt or exception handler calls the virtual-
8086 monitor to handle the interrupt or exception, the return from the virtual-8086 monitor to
the interrupted virtual-8086 mode program requires two return instructions: a RET instruction
to return to the protected-mode handler and an IRET instruction to return to the interrupted
program.
The virtual-8086 monitor has the option of directing the interrupt and exception back to an
interrupt or exception handler that is part of the interrupted 8086 program, as described in
Section 15.3.1.2, “Handling an Interrupt or Exception With an 8086 Program Interrupt or
Exception Handler”.
15.3.1.2 Handling an Interrupt or Exception With an
8086 Program Interrupt or Exception Handler
Because it was designed to run on an 8086 processor, an 8086 program running in a virtual-
8086-mode task contains an 8086-style interrupt vector table, which starts at linear address 0. If
the virtual-8086 monitor correctly directs an interrupt or exception vector back to the virtual-
8086-mode task it came from, the handlers in the 8086 program can handle the interrupt or
exception. The virtual-8086 monitor must carry out the following steps to send an interrupt or
exception back to the 8086 program:
1. Use the 8086 interrupt vector to locate the appropriate handler procedure in the 8086
program interrupt table.
2. Store the EFLAGS (low-order 16 bits only), CS and EIP values of the 8086 program on the
privilege-level 3 stack. This is the stack that the virtual-8086-mode task is using. (The
8086 handler may use or modify this information.)
3. Change the return link on the privilege-level 0 stack to point to the privilege-level 3
handler procedure.
4. Execute an IRET instruction to pass control to the 8086 program handler.
5. When the IRET instruction from the privilege-level 3 handler triggers a general-protection
exception (#GP) and thus effectively again calls the virtual-8086 monitor, restore the
return link on the privilege-level 0 stack to point to the original, interrupted, privilege-level
3 procedure.
6. Copy the low order 16 bits of the EFLAGS image from the privilege-level 3 stack to the
privilege-level 0 stack (because some 8086 handlers modify these flags to return
information to the code that caused the interrupt).
7. Execute an IRET instruction to pass control back to the interrupted 8086 program.
Note that if an operating system intends to support all 8086 MS-DOS-based programs, it is
necessary to use the actual 8086 interrupt and exception handlers supplied with the program.
The reason for this is that some programs modify their own interrupt vector table to substitute
(or hook in series) their own specialized interrupt and exception handlers.