Intel IA-32 - Manual (Page 557)

Vol. 3A 15-7
8086 EMULATION
(For backward compatibility to Intel 8086 processors, the default base address and limit of the
interrupt vector table should not be changed.)
Table 15-1 shows the interrupt and exception vectors that can be generated in real-address mode
and virtual-8086 mode, and in the Intel 8086 processor. See Chapter 5, “Interrupt and Exception
Handling”, for a description of the exception conditions.
15.2 VIRTUAL-8086 MODE
Virtual-8086 mode is actually a special type of a task that runs in protected mode. When the
operating-system or executive switches to a virtual-8086-mode task, the processor emulates an
Intel 8086 processor. The execution environment of the processor while in the 8086-emulation
state is the same as is described in Section 15.1, “Real-Address Mode” for real-address mode,
including the extensions. The major difference between the two modes is that in virtual-8086
mode the 8086 emulator uses some protected-mode services (such as the protected-mode inter-
rupt and exception-handling and paging facilities).
As in real-address mode, any new or legacy program that has been assembled and/or compiled
to run on an Intel 8086 processor will run in a virtual-8086-mode task. And several 8086
programs can be run as virtual-8086-mode tasks concurrently with normal protected-mode
tasks, using the processor’s multitasking facilities.
Figure 15-2. Interrupt Vector Table in Real-Address Mode
0
2
4
8
12
0 15
Segment Selector
Offset
* Interrupt vector number 0 selects entry 0
Interrupt Vector 0*
Entry 1
Entry 2
Entry 3
Up to Entry 255
IDTR
(called “interrupt vector 0”) in the interrupt
vector table. Interrupt vector 0 in turn
points to the start of the interrupt handler
for interrupt 0.