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virtual 86 mode
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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.