i386/i386.reference/s15_04.htm

   1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
   2 <HTML>
   3 <HEAD>
   4 <TITLE>80386 Programmer's Reference Manual -- Section 15.4</TITLE>
   5 </HEAD>
   6 <BODY>
   7 <B>up:</B> <A HREF="c15.htm">
   8 Chapter 15 -- Virtual 8086 Mode</A><BR>
   9 <B>prev:</B> <A HREF="s15_03.htm">15.3  Entering and Leaving V86 Mode</A><BR>
  10 <B>next:</B> <A HREF="s15_05.htm">15.5  Virtual I/O</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>15.4  Additional Sensitive Instructions</H1>
  15 When the 80386 is executing in V86 mode, the instructions
  16 <A HREF="PUSHF.htm">PUSHF</A>,
  17 <A HREF="POPF.htm">POPF</A>,
  18 <A HREF="INT.htm">INT n</A>, and
  19 <A HREF="IRET.htm">IRET</A> are sensitive to IOPL. The instructions
  20 <A HREF="IN.htm">IN</A>,
  21 <A HREF="INS.htm">INS</A>,
  22 <A HREF="OUT.htm">OUT</A>, and
  23 <A HREF="OUTS.htm">OUTS</A>,
  24 which are ordinarily sensitive in protected mode, are not sensitive
  25 in V86 mode. Following is a complete list of instructions that are sensitive
  26 in V86 mode:
  27 <UL>
  28 <LI> <A HREF="CLI.htm">CLI</A> -- Clear Interrupt-Enable Flag
  29 <LI> <A HREF="STI.htm">STI</A> -- Set Interrupt-Enable Flag
  30 <LI> <A HREF="LOCK.htm">LOCK</A> -- Assert Bus-Lock Signal
  31 <LI> <A HREF="PUSHF.htm">PUSHF</A> -- Push Flags
  32 <LI> <A HREF="POPF.htm">POPF</A> -- Pop Flags
  33 <LI> <A HREF="INT.htm">INT n</A> -- Software Interrupt
  34 <LI> <A HREF="RET.htm">RET</A> -- Interrupt Return
  35 </UL>
  36 CPL is always three in V86 mode; therefore, if IOPL < 3, these instructions
  37 will trigger a general-protection exceptions. These instructions are made
  38 sensitive so that their functions can be simulated by the V86 monitor.
  39
  40 <H2>15.4.1  Emulating 8086 Operating System Calls</H2>
  41 <A HREF="INT.htm">INT n</A>
  42 is sensitive so that the V86 monitor can intercept calls to the
  43 8086 OS. Many 8086 operating systems are called by pushing parameters onto
  44 the stack, then executing an
  45 <A HREF="INT.htm">INT n</A> instruction. If IOPL < 3,
  46 <A HREF="INT.htm">INT n</A>
  47 instructions will be intercepted by the V86 monitor. The V86 monitor can
  48 then emulate the function of the 8086 operating system or reflect the
  49 interrupt back to the 8086 operating system in V86 mode.
  50
  51 <H2>15.4.2  Virtualizing the Interrupt-Enable Flag</H2>
  52 When the processor is executing 8086 code in a V86 task, the instructions
  53 <A HREF="PUSHF.htm">PUSHF</A>,
  54 <A HREF="POPF.htm">POPF</A>, and
  55 <A HREF="IRET.htm">IRET</A> are sensitive to IOPL so that the V86 monitor can
  56 control changes to the interrupt-enable flag (IF). Other instructions that
  57 affect IF
  58 (<A HREF="STI.htm">STI</A> and
  59 <A HREF="CLI.htm">CLI</A>) are IOPL sensitive both in 8086 code and in
  60 80386/80386 code.
  61 <P>
  62 Many 8086 programs that were designed to execute on single-task systems set
  63 and clear IF to control interrupts. However, when these same programs are
  64 executed in a multitasking environment, such control of IF can be
  65 disruptive. If IOPL is less than three, all instructions that change or
  66 interrogate IF will trap to the V86 monitor. The V86 monitor can then
  67 control IF in a manner that both suits the needs of the larger environment
  68 and is transparent to the 8086 program.
  69 <P>
  70 <HR>
  71 <P>
  72 <B>up:</B> <A HREF="c15.htm">
  73 Chapter 15 -- Virtual 8086 Mode</A><BR>
  74 <B>prev:</B> <A HREF="s15_03.htm">15.3  Entering and Leaving V86 Mode</A><BR>
  75 <B>next:</B> <A HREF="s15_05.htm">15.5  Virtual I/O</A>
  76 </BODY>