Path: utzoo!attcan!uunet!wuarchive!texbell!cs.utexas.edu!usc!apple!mips!winchester!mash
From: mash@mips.COM (John Mashey)
Newsgroups: comp.arch
Subject: Re: MIPS 64x32 Divide?
Message-ID: <39213@mips.mips.COM>
Date: 1 Jun 90 20:13:00 GMT
References: <9462@pt.cs.cmu.edu>
Sender: news@mips.COM
Reply-To: mash@mips.COM (John Mashey)
Organization: MIPS Computer Systems, Inc.
Lines: 24

In article <9462@pt.cs.cmu.edu> chiles@cs.cmu.edu writes:
>This is a general query of how to do 64-bit by 32-bit division when a
>machine only gives you a 32-bit by 32-bit instruction.  At first I was
>shocked that MIPS didn't offer this since a 32-bit by 32-bit multiply
>potentially results in a 64-bit result, and I expected to be able to
>reverse the operation.  I suspect all of their intensive profiling has
>revealed that C programs only ever divide a long integer by a long integer
>at best.  I wonder if they profiled any languages that support infinite
>precision integer arithmetic?  Maybe they did, and what they found is that
>conventional computing almost never requires a 64x32 divide.
Whether they require it or not, in general, it's something hard to
specify from most languages, and so of coruse, it never shows up in
thr profiles.  Although I think it was discussed, it dropped out
as a possibility very early [like, 1Q85].
However, in any case, it turns out that 64/32->32 requires a 64-bit
wide datapath, which would have caused serious chip layout problems
in the middle of something where every other integer thing was 32-wide.
(32/32->32 is OK, and so is 32*32->64, i.e., / is a worse problem
than *).
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
UUCP: 	 mash@mips.com OR {ames,decwrl,prls,pyramid}!mips!mash 
DDD:  	408-524-7015, 524-8253 or (main number) 408-720-1700
USPS: 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086