Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!cornell!uw-beaver!microsoft!danno
From: danno@microsoft.UUCP (Daniel A. Norton)
Newsgroups: comp.sys.ibm.pc
Subject: Re: How to tell the difference between 80386 virtual 8086 mode and 286 real mode?
Summary: Use SMSW
Message-ID: <1657@microsoft.UUCP>
Date: 29 Jul 88 16:54:19 GMT
References: <1873@looking.UUCP>
Organization: Microsoft Corp.
Lines: 30

In article <1873@looking.UUCP>, brad@looking.UUCP (Brad Templeton) writes:
> Does anybody have any info on this?  Please respond by mail.
> I know you can spot a 386 in real mode by trying to set the IOPL bits in
> the flag word, but this doesn't work in virtual 8086 mode.

I sent you mail on this, but I've since been told of a better method.
The best way is to use the SMSW instruction.  It is non-privileged (as
opposed to LMSW) so it works fine in Virtual 8086 mode.  This instruction
stores some flags into a 16-bit operand.  The low bit indicates whether
or not the system is in protect mode.  A 286 in real-mode has a zero in
this bit.  An 80386 in V86 mode (presumably also in protect mode) has
a one in this bit.  I'm not sure that it makes sense to have the V86
mode set while in real-mode, so this should be safe.

Example:
	SMSW	AX
	RCR	AX,1
	JC	V86Mode
R286Mode:
	.
	.
	.
V86Mode:

This presumes, of course, that you know that you're not executing on
an 8088, because SMSW is illegal on that chip.
-- 
Any opinions expressed are mine, not my employer's.
nortond@microsof.beaver.washington.EDU nortond%microsof@uw-beaver.ARPA
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