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Path: utzoo!watmath!kadorken
From: kadorken@watmath.UUCP
Newsgroups: comp.sys.intel
Subject: 386 instruction encoding query
Message-ID: <4979@watmath.UUCP>
Date: Tue, 10-Feb-87 16:34:23 EST
Article-I.D.: watmath.4979
Posted: Tue Feb 10 16:34:23 1987
Date-Received: Wed, 11-Feb-87 07:07:59 EST
Distribution: na
Organization: U of Waterloo, Ontario
Lines: 16


	The 386 programmers reference manual has for the full encoding of
the "jo" instruction the following bit pattern: 
0x0F 0x80 <full displacement>.  The bit encoding for pushing
a segment register (long form; 3-bit segment register name) is:
0x0F  10 seg3 000   where seg3 is one of {es, cs, ss, ds, fs or gs}.


	Thus the two byte opcode 0x0F 0x80 could be interpreted as
being either a "jo" instruction or the long form instruction "push es".
(the 3-bit encoding for es is 000).  Executing this instruction results
in a jump, not a push.  However, other encodings (using the other 3 bit
segment register names) do result in a value being pushed on the stack.

	Is the above behaviour just a fluke on our processor or will all
future '386 processors behave this way?