Vol. 3B 20-7

8086 EMULATION

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When checking privileged instructions, on page accesses, or when performing other permission checks. 
(Virtual-8086 mode always executes at CPL 3.)

20.2.2 

Structure of a Virtual-8086 Task

A virtual-8086-mode task consists of the following items:

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A 32-bit TSS for the task.

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The 8086 program.

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A virtual-8086 monitor.

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8086 operating-system services.

The TSS of the new task must be a 32-bit TSS, not a 16-bit TSS, because the 16-bit TSS does not load the most-
significant word of the EFLAGS register, which contains the VM flag. All TSS’s, stacks, data, and code used to handle 
exceptions when in virtual-8086 mode must also be 32-bit segments.
The processor enters virtual-8086 mode to run the 8086 program and returns to protected mode to run the virtual-
8086 monitor.
The virtual-8086 monitor is a 32-bit protected-mode code module that runs at a CPL of 0. The monitor consists of 
initialization, interrupt- and exception-handling, and I/O emulation procedures that emulate a personal computer 
or other 8086-based platform. Typically, the monitor is either part of or closely associated with the protected-mode 
general-protection (#GP) exception handler, which also runs at a CPL of 0. As with any protected-mode code 
module, code-segment descriptors for the virtual-8086 monitor must exist in the GDT or in the task’s LDT. The 
virtual-8086 monitor also may need data-segment descriptors so it can examine the IDT or other parts of the 8086 
program in the first 1 MByte of the address space. The linear addresses above 10FFEFH are available for the 
monitor, the operating system, and other system software.
The 8086 operating-system services consists of a kernel and/or operating-system procedures that the 8086 
program makes calls to. These services can be implemented in either of the following two ways:

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They can be included in the 8086 program. This approach is desirable for either of the following reasons:
— The 8086 program code modifies the 8086 operating-system services.
— There is not sufficient development time to merge the 8086 operating-system services into main operating 

system or executive.

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They can be implemented or emulated in the virtual-8086 monitor. This approach is desirable for any of the 
following reasons:
— The 8086 operating-system procedures can be more easily coordinated among several virtual-8086 tasks.
— Memory can be saved by not duplicating 8086 operating-system procedure code for several virtual-8086 

tasks.

— The 8086 operating-system procedures can be easily emulated by calls to the main operating system or 

executive.

The approach chosen for implementing the 8086 operating-system services may result in different virtual-8086-
mode tasks using different 8086 operating-system services.

20.2.3 

Paging of Virtual-8086 Tasks

Even though a program running in virtual-8086 mode can use only 20-bit linear addresses, the processor converts 
these addresses into 32-bit linear addresses before mapping them to the physical address space. If paging is being 
used, the 8086 address space for a program running in virtual-8086 mode can be paged and located in a set of 
pages in physical address space. If paging is used, it is transparent to the program running in virtual-8086 mode 
just as it is for any task running on the processor.
Paging is not necessary for a single virtual-8086-mode task, but paging is useful or necessary in the following situ-
ations: