Intel IA-32 Manuale Utente

15-20 Vol. 3A

When an interrupt or exception vector points to a task gate in the IDT, the processor performs a
task switch to the selected interrupt- or exception-handling task. The following actions are
carried out as part of this task switch:

1.

The EFLAGS register with the VM flag set is saved in the current TSS.

2.

The link field in the TSS of the called task is loaded with the segment selector of the TSS
for the interrupted virtual-8086-mode task.

3.

The EFLAGS register is loaded from the image in the new TSS, which clears the VM flag
and causes the processor to switch to protected mode.

4.

The NT flag in the EFLAGS register is set.

5.

The processor begins executing the selected interrupt- or exception-handler task.

When an IRET instruction is executed in the handler task and the NT flag in the EFLAGS
register is set, the processors switches from a protected-mode interrupt- or exception-handler
task back to a virtual-8086-mode task. Here, the EFLAGS and segment registers are loaded from
images saved in the TSS for the virtual-8086-mode task. If the VM flag is set in the EFLAGS
image, the processor switches back to virtual-8086 mode on the task switch. 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. 

Maskable hardware interrupts are those interrupts that are delivered through the INTR# pin or
through an interrupt request to the local APIC (see Section 5.3.2, “Maskable Hardware Inter-
rupts”). These interrupts can be inhibited (masked) from interrupting an executing program or
task by clearing the IF flag in the EFLAGS register.

When the VME flag in control register CR4 is set and the IOPL field in the EFLAGS register is
less than 3, two additional flags are activated in the EFLAGS register:

•

VIF (virtual interrupt) flag, bit 19 of the EFLAGS register.

•

VIP (virtual interrupt pending) flag, bit 20 of the EFLAGS register.

These flags provide the virtual-8086 monitor with more efficient control over handling
maskable hardware interrupts that occur during virtual-8086 mode tasks. They also reduce inter-
rupt-handling overhead, by eliminating the need for all IF related operations (such as PUSHF,
POPF, CLI, and STI instructions) to trap to the virtual-8086 monitor. The purpose and use of
these flags are as follows.

The VIF and VIP flags are only available in IA-32 processors that support the
virtual mode extensions. These extensions were introduced in the IA-32
architecture with the Pentium processor. When this mechanism is either not