Intel architecture ia-32 Manuale Utente

15-18 Vol. 3A

Interrupt and exception handlers can examine the VM flag on the stack to determine if the inter-
rupted procedure was running in virtual-8086 mode. If so, the interrupt or exception can be
handled in one of three ways:

•

The protected-mode interrupt or exception handler that was called can handle the interrupt
or exception.

•

The protected-mode interrupt or exception handler can call the virtual-8086 monitor to
handle the interrupt or exception.

•

The virtual-8086 monitor (if called) can in turn pass control back to the 8086 program’s
interrupt and exception handler.

If the interrupt or exception is handled with a protected-mode handler, the handler can return to
the interrupted program in virtual-8086 mode by executing an IRET instruction. This instruction
loads the EFLAGS and segment registers from the images saved in the privilege level 0 stack
(see Figure 15-4). A set VM flag in the EFLAGS image causes the processor to switch back to
virtual-8086 mode. The CPL at the time the IRET instruction is executed must be 0, otherwise
the processor does not change the state of the VM flag.

Unused

Old GS

Old ESP

ESP from

Old FS

Old DS

Old ES

Old SS

Old EFLAGS

Old CS

Old EIP

Error Code

New ESP

TSS

Unused

Old GS

Old ESP

ESP from

Old FS

Old DS

Old ES

Old SS

Old EFLAGS

Old CS

Old EIP

New ESP

TSS