20-16 Vol. 3B
8086 EMULATION
d. Clears the VM flag in the EFLAGS register.
e. Begins executing the selected protected-mode interrupt handler.
2. The recommended action of the protected-mode interrupt handler is to read the VM flag from the EFLAGS
image on the stack. If this flag is set, the handler makes a call to the virtual-8086 monitor.
3. The virtual-8086 monitor should read the VIF flag in the EFLAGS register.
— If the VIF flag is clear, the virtual-8086 monitor sets the VIP flag in the EFLAGS image on the stack to
indicate that there is a deferred interrupt pending and returns to the protected-mode handler.
— If the VIF flag is set, the virtual-8086 monitor can handle the interrupt if it “belongs” to the 8086 program
running in the interrupted virtual-8086 task; otherwise, it can call the protected-mode interrupt handler to
handle the interrupt.
4. The protected-mode handler executes a return to the program executing in virtual-8086 mode.
5. Upon returning to virtual-8086 mode, the processor continues execution of the 8086 program.
When the 8086 program is ready to receive maskable hardware interrupts, it executes the STI instruction to set the
VIF flag (enabling maskable hardware interrupts). Prior to setting the VIF flag, the processor automatically checks
the VIP flag and does one of the following, depending on the state of the flag:
•
If the VIP flag is clear (indicating no pending interrupts), the processor sets the VIF flag.
•
If the VIP flag is set (indicating a pending interrupt), the processor generates a general-protection exception
(#GP).
The recommended action of the protected-mode general-protection exception handler is to then call the virtual-
8086 monitor and let it handle the pending interrupt. After handling the pending interrupt, the typical action of the
virtual-8086 monitor is to clear the VIP flag and set the VIF flag in the EFLAGS image on the stack, and then
execute a return to the virtual-8086 mode. The next time the processor receives a maskable hardware interrupt, it
will then handle it as described in steps 1 through 5 earlier in this section.
If the processor finds that both the VIF and VIP flags are set at the beginning of an instruction, it generates a
general-protection exception. This action allows the virtual-8086 monitor to handle the pending interrupt for the
virtual-8086 mode task for which the VIF flag is enabled. Note that this situation can only occur immediately
following execution of a POPF or IRET instruction or upon entering a virtual-8086 mode task through a task switch.
Note that the states of the VIF and VIP flags are not modified in real-address mode or during transitions between
real-address and protected modes.
NOTE
The virtual interrupt mechanism described in this section is also available for use in protected
mode, see Section 20.4, “Protected-Mode Virtual Interrupts”.
20.3.3
Class 3—Software Interrupt Handling in Virtual-8086 Mode
When the processor receives a software interrupt (an interrupt generated with the INT n instruction) while in
virtual-8086 mode, it can use any of six different methods to handle the interrupt. The method selected depends
on the settings of the VME flag in control register CR4, the IOPL field in the EFLAGS register, and the software inter-
rupt redirection bit map in the TSS. Table 20-2 lists the six methods of handling software interrupts in virtual-8086
mode and the respective settings of the VME flag, IOPL field, and the bits in the interrupt redirection bit map for
each method. The table also summarizes the various actions the processor takes for each method.
The VME flag enables the virtual mode extensions for the Pentium and later IA-32 processors. When this flag is
clear, the processor responds to interrupts and exceptions in virtual-8086 mode in the same manner as an Intel386
or Intel486 processor does. When this flag is set, the virtual mode extension provides the following enhancements
to virtual-8086 mode:
•
Speeds up the handling of software-generated interrupts in virtual-8086 mode by allowing the processor to
bypass the virtual-8086 monitor and redirect software interrupts back to the interrupt handlers that are part of
the currently running 8086 program.
•
Supports virtual interrupts for software written to run on the 8086 processor.