Intel IA-32 - Manual (Page 571)

Vol. 3A 15-21
8086 EMULATION
available or not enabled, maskable hardware interrupts are handled as class 1
interrupts. Here, if VIF and VIP flags are needed, the virtual-8086 monitor
can implement them in software.
Existing 8086 programs commonly set and clear the IF flag in the EFLAGS register to enable
and disable maskable hardware interrupts, respectively; for example, to disable interrupts while
handling another interrupt or an exception. This practice works well in single task environments,
but can cause problems in multitasking and multiple-processor environments, where it is often
desirable to prevent an application program from having direct control over the handling of
hardware interrupts. When using earlier IA-32 processors, this problem was often solved by
creating a virtual IF flag in software. The IA-32 processors (beginning with the Pentium
processor) provide hardware support for this virtual IF flag through the VIF and VIP flags.
The VIF flag is a virtualized version of the IF flag, which an application program running from
within a virtual-8086 task can used to control the handling of maskable hardware interrupts.
When the VIF flag is enabled, the CLI and STI instructions operate on the VIF flag instead of
the IF flag. When an 8086 program executes the CLI instruction, the processor clears the VIF
flag to request that the virtual-8086 monitor inhibit maskable hardware interrupts from inter-
rupting program execution; when it executes the STI instruction, the processor sets the VIF flag
requesting that the virtual-8086 monitor enable maskable hardware interrupts for the 8086
program. But actually the IF flag, managed by the operating system, always controls whether
maskable hardware interrupts are enabled. Also, if under these circumstances an 8086 program
tries to read or change the IF flag using the PUSHF or POPF instructions, the processor will
change the VIF flag instead, leaving IF unchanged.
The VIP flag provides software a means of recording the existence of a deferred (or pending)
maskable hardware interrupt. This flag is read by the processor but never explicitly written by
the processor; it can only be written by software.
If the IF flag is set and the VIF and VIP flags are enabled, and the processor receives a maskable
hardware interrupt (interrupt vector 0 through 255), the processor performs and the interrupt
handler software should perform the following operations:
1. The processor invokes the protected-mode interrupt handler for the interrupt received, as
described in the following steps. These steps are almost identical to those described for
method 1 interrupt and exception handling in Section 15.3.1.1, “Handling an Interrupt or
Exception Through a Protected-Mode Trap or Interrupt Gate”:
a. Switches to 32-bit protected mode and privilege level 0.
b. Saves the state of the processor on the privilege-level 0 stack. The states of the EIP,
CS, EFLAGS, ESP, SS, ES, DS, FS, and GS registers are saved (see Figure 15-4).
c. Clears the segment registers.
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.