virtual 86 mode

processor, programming (Or "virtual mode" or "virtual 8086

mode") An operating mode provided by the Intel 80386 and

later processors to allow real mode programs to run under

operating systems which use protected mode. In this

sub-mode of protected mode, an operating environment is

created which mimics the address calculation in real mode.

In virtual 86 mode the segment MMU is practically turned off

and the segment registers exhibit the same behaviour as in

real mode. The paged MMU, however, still operates. This

means that the one megabyte address space of real mode can

be remapped in four kilobyte pages to anywhere in the 32 bit

physical address space. Each page can be protected

separately from read or write accesses.

Virtual mode is handled on a per-task-basis, so each

exception (from protection violations or interrupts)

switches the processor back into protected mode. It is

therefore possible to have multiple tasks in virtual mode

which run concurrently under the control of an operating

system which runs in protected mode.

Most operating system services in MS-DOS systems are called

by software interrupts, which are a kind of exception. If

an MS-DOS application runs in virtual mode under the control

of a protected mode operating system, each call to MS-DOS

causes a switch to protected mode. The operating system

emulates the MS-DOS service and switches back to the

application in virtual mode. From the viewpoint of the

application nothing differs from real mode.

Microsoft Windows, Windows NT, and OS/2 use this feature

to implement "DOS-boxes" in which both MS-DOS and real mode

application programs can run.