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<html><head> <title>Operating Systems FAQ :: v8086</title> <link rel=stylesheet type="text/css" href="default.css"></head><body><TABLE border="0" width="100%"> <TR> <TD><H2><A name="whats_v8086">What is Virtual 8086, why is it causing problems for me?</A></H2> </TD> </TR> <TR> <TD>Virtual 8086 mode is a sub-mode of protected mode. In short, virtual 8086 mode is whereby the cpu (in protected mode) is running a "Emulated" 16bit 'segmented' model (real mode) machine.<P>V8086 mode can cause all different kinds of problems for OS programmers.</P><P>The most common problem with v86 mode is that, you can't "enter" protected mode inside a v86 task.</P><P>That meaning, if you are running Windows or have emm386 in memory, you can't do a "raw" switch into protected mode (it causes an exception if I remember correctly!:)</P><P>There are a few other more "technical" problems people have when using v86 mode, mostly because v86 has some instructions "emulated" by what's known as a v86-monitor program, as the cpu is in protected mode, some instructions are high up on the security/protection level and running those directly would cause no-end of trouble for the OS.</P><P>Such technicalities are beyond the scope of a simple FAQ</P> </TD> </TR></TABLE><P> </P><TABLE border="0" width="100%"> <TR> <TD><H2><A name="detect_v86">How do I detect if my machine is in Virtual 8086 mode?</A></H2> </TD> </TR> <TR> <TD>EFLAGS.VM is NEVER pushed onto the stack if the V86 task uses PUSHFD. You should check ifCR0.PE=1 and then assume it's V86 if that bit is set. <PRE>detect_v86: smsw ax and eax,1 ;CR0.PE bit ret <!-- /* how to detect if a machine is in v86 mode by Dark Fiber - 22nov98 * * in: nothing * out: eax; 0=task not in v86 mode * 1=task is in v86 mode * destroys: eax */ detect_v86: pushfd pop eax shr eax, 17 and eax, 1 ret --> </PRE> </TD> </TR></TABLE></body></html>⌨️ 快捷键说明
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