Intel IA-32 - Manual (Page 559)

Vol. 3A 15-9
8086 EMULATION
15.2.1 Enabling Virtual-8086 Mode
The processor runs in virtual-8086 mode when the VM (virtual machine) flag in the EFLAGS
register is set. This flag can only be set when the processor switches to a new protected-mode
task or resumes virtual-8086 mode via an IRET instruction.
System software cannot change the state of the VM flag directly in the EFLAGS register (for
example, by using the POPFD instruction). Instead it changes the flag in the image of the
EFLAGS register stored in the TSS or on the stack following a call to an interrupt- or exception-
handler procedure. For example, software sets the VM flag in the EFLAGS image in the TSS
when first creating a virtual-8086 task.
The processor tests the VM flag under three general conditions:
•
When loading segment registers, to determine whether to use 8086-style address
translation.
•
When decoding instructions, to determine which instructions are not supported in virtual-
8086 mode and which instructions are sensitive to IOPL.
•
When checking privileged instructions, on page accesses, or when performing other
permission checks. (Virtual-8086 mode always executes at CPL 3.)
15.2.2 Structure of a Virtual-8086 Task
A virtual-8086-mode task consists of the following items:
•
A 32-bit TSS for the task.
•
The 8086 program.
•
A virtual-8086 monitor.
•
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 proce-
dures 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.