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From: boykin@custom.UUCP (Joseph Boykin)
Newsgroups: comp.sys.ibm.pc
Subject: Interesting 80X86 instruction
Message-ID: <767@custom.UUCP>
Date: Wed, 5-Aug-87 15:31:59 EDT
Article-I.D.: custom.767
Posted: Wed Aug  5 15:31:59 1987
Date-Received: Sat, 8-Aug-87 04:29:23 EDT
Organization: Custom Software Systems; Natick, MA
Lines: 30

I was looking through my handy-dandy 80X86 programmers reference
and noticed something that I thought was a little strange.
The AAD instruction (ASCII Adjust before division),
does the following:
	AL = AL + (10 * AH).
	AH = 0

This instruction has a 'two' byte op-code, which really didn't
seem necessary; i.e. why couldn't the instruction be encoded
in one byte?  OK, one-byte opcodes are valuable and you don't
want to waste it on a semi-useless instruction.  The other
point was that the second byte was 0A.  Humm, an instruction
which multiplies the least significant nibble by 10 has a 10
as its second byte!  Taking out my handy debugger I found that
AAD *is* a one-byte instruction with the second byte acting
as a parameter.  Try coding the following:

	db	0d5h, 10h

and you'll create an instruction which does:
	AL = AL + (16 * AH)
	AH = 0.

I'm not sure what value such an instruction might have. Looking
at the instruction timings a MUL is faster, but I'm sure someone
will find a case where this is useful.  If you do, let me know!

Joe Boykin
Custom Software Systems
...necntc!custom!boykin