From: rhyde AT cs DOT ucr DOT edu (Randy Hyde)
Newsgroups: comp.lang.asm.x86,comp.os.msdos.djgpp
Subject: Re: Intel opcodes: ModR/M and SIB bytes
Date: 25 Jul 1997 17:09:42 GMT
Organization: UC Riverside, Dept. of Computer Science
Lines: 16
Message-ID: <5ramkm$c3i$1@skylark.ucr.edu>
References: <33D805AC DOT 172F8780 AT execulink DOT com>
NNTP-Posting-Host: cs.ucr.edu
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

. Okay, now what about the ModR/M byte?  That one really confuses me.  I
. really don't know much about it at all.  I don't understand the function
. of the first two bits.  The next three bits, to my understanding hold
. either a register (the first operand?), or another three bits of the
. opcode, is that right?  And the last three bits, to my understanding,
. hold either another register (the second operand?) or something else
. (what?)?



Chapter Four of "The Art of Assembly Language" describes the
MOD-REG-R/M byte.  Check it out at http://webster.ucr.edu.
Appendix D also contains an expanded instruction/opcode list
that you might find useful.  Best to grab the PDF version...
Randy Hyde