i386/i386.reference/c15.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Chapter 15</TITLE>
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  10 <A HREF="s14_08.htm">14.8  Differences From 80286 Real-Address Mode</A><BR>
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  14 <P>
  15 <H1>Chapter 15  Virtual 8086 Mode</H1>
  16 <P>
  17 The 80386 supports execution of one or more 8086, 8088, 80186, or 80188
  18 programs in an 80386 protected-mode environment. An 8086 program runs in
  19 this environment as part of a V86 (virtual 8086) task. V86 tasks take
  20 advantage of the hardware support of multitasking offered by the protected
  21 mode. Not only can there be multiple V86 tasks, each one executing an 8086
  22 program, but V86 tasks can be multiprogrammed with other 80386 tasks.
  23 <P>
  24 The purpose of a V86 task is to form a "virtual machine" with which to
  25 execute an 8086 program. A complete virtual machine consists not only of
  26 80386 hardware but also of systems software. Thus, the emulation of an 8086
  27 is the result of cooperation between hardware and software:
  28 <UL>
  29 <LI>The hardware provides a virtual set of registers (via the TSS), a
  30      virtual memory space (the first megabyte of the linear address space of
  31      the task), and directly executes all instructions that deal with these
  32      registers and with this address space.
  33
  34 <LI>The software controls the external interfaces of the virtual machine
  35      (I/O, interrupts, and exceptions) in a manner consistent with the
  36      larger environment in which it executes. In the case of I/O, software
  37      can choose either to emulate I/O instructions or to let the hardware
  38      execute them directly without software intervention.
  39 </UL>
  40 Software that helps implement virtual 8086 machines is called a V86
  41 monitor.
  42 <P>
  43 <A HREF="s15_01.htm">15.1  Executing 8086 Code</A><BR>
  44 <A HREF="s15_02.htm">15.2  Structure of a V86 Task</A><BR>
  45 <A HREF="s15_03.htm">15.3  Entering and Leaving V86 Mode</A><BR>
  46 <A HREF="s15_04.htm">15.4  Additional Sensitive Instructions</A><BR>
  47 <A HREF="s15_05.htm">15.5  Virtual I/O</A><BR>
  48 <A HREF="s15_06.htm">15.6  Differences From 8086</A><BR>
  49 <A HREF="s15_07.htm">15.7  Differences From 80286 Real-Address Mode</A>
  50 <P>
  51 <HR>
  52 <P>
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  56 <A HREF="s14_08.htm">14.8  Differences From 80286 Real-Address Mode</A><BR>
  57 <B>next:</B> <A HREF="s15_01.htm">15.1  Executing 8086 Code</A>
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