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Vol. 3 17-13
8086 EMULATION
Figure 17-3. Entering and Leaving Virtual-8086 Mode
Monitor
Virtual-8086
Real Mode
Code
Protected-
Mode Tasks
Virtual-8086
Mode Tasks
(8086
Programs)
Protected-
Mode Interrupt
and Exception
Handlers
Task Switch
1
VM = 1
Protected
Mode
Virtual-8086
Mode
Real-Address
Mode
RESET
PE=1
PE=0 or
RESET
#GP Exception
3
CALL
RET
Task Switch
VM=0
Redirect Interrupt to 8086 Program
Interrupt or Exception Handler
6
IRET
4
Interrupt or
Exception
2
VM = 0
NOTES:
- CALL or JMP where the VM flag in the EFLAGS image is 1.
- IRET where VM is 1 and NT is 1.
4. Normal return from protected-mode interrupt or exception handler.
3. General-protection exception caused by software interrupt (INT n), IRET,
POPF, PUSHF, IN, or OUT when IOPL is less than 3.
2. Hardware interrupt or exception; software interrupt (INT n) when IOPL is 3.
5. A return from the 8086 monitor to redirect an interrupt or exception back
to an interrupt or exception handler in the 8086 program running in virtual-
6. Internal redirection of a software interrupt (INT n) when VME is 1,
IOPL is <3, and the redirection bit is 1.
IRET
5
8086 mode.
1. Task switch carried out in either of two ways: