virtual 86 mode

1 definition found

virtual 86 mode - Free On-line Dictionary of Computing (26 May 2007) :

  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.