From:	Jeff Weeks <pweeks AT execulink DOT com>
Newsgroups:	comp.lang.asm.x86,comp.os.msdos.djgpp
Subject:	Intel opcodes: ModR/M and SIB bytes
Date:	Thu, 24 Jul 1997 21:47:24 -0400
Organization:	Code X Software
Lines:	41
Message-ID:	<33D805AC.172F8780@execulink.com>
NNTP-Posting-Host:	ppp35.leo.execulink.com
Mime-Version:	1.0
CC:	dj AT delorie DOT com
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Alright.  I'm writting this message to two newsgroups and DJ Delorie, in
the hopes that one of you might know the answer to this question
regarding Intel opcode formatting.

Here goes:
I'm writting a little paper describing the intruction format of Intel
CPU's (which I'll later use to write my own assembler) and I'm confused
by the ModR/M and SIB bytes (the two optional bytes immediately
following the opcode).

I think I've got the SIB byte down:  The first two bits are the scale
(00 = 1, 01 = 2, 10 = 4, 11 = 8... is that correct?), the next three
bits are the index register, and the next three bits are the base
register.  Is that all correct?  Are there any catches there?

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?)?

Lastly, how do you know, for example, wether certain bits form a
register, or their other purpose?  Is that designated by each certain
opcode?

Thank you very much,
Jeff Weeks

PS to DJ: Sorry for writting to you directly.  I just figured you'd be
the best to ask regarding this.  It's my last question for quite a
while, I believe.

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