Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site turtlevax.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!genrad!decvax!decwrl!turtlevax!ken
From: ken@turtlevax.UUCP (Ken Turkowski)
Newsgroups: net.arch,net.micro.68k
Subject: Re: Integer Multiplier Question
Message-ID: <975@turtlevax.UUCP>
Date: Sun, 1-Dec-85 23:51:08 EST
Article-I.D.: turtleva.975
Posted: Sun Dec  1 23:51:08 1985
Date-Received: Thu, 5-Dec-85 04:43:51 EST
References: <5456@allegra.UUCP> <291@l5.uucp>
Organization: CIMLINC, Inc. @ Menlo Park, CA
Lines: 21
Xref: watmath net.arch:2209 net.micro.68k:1377
Summary: I agree: don't just return the least significant part of the product. The most significant part is required to implement digital filters.

In article <291@l5.uucp>, gnu@l5.uucp (John Gilmore) writes:
> In article <5456@allegra.UUCP>, geno@allegra.UUCP (E.Rice) writes:
> > In many languages, the result of a integer multiply has the same length
> > as the operands.  Therefore it makes sense when building a parallel
> > multiplier to only generate the low order half of the product.
> 
> I proposed that the 68020 do this, and Motorola implemented it, but
> in the process several people pointed out that if the arguments are 
> considered as fractions (eg as in rational arithmetic, or in computing
> floating point) then the relevant bits are the *high order* bits of the
> double-length product.

As long as you just want to compute addresses for arrays, the least
significant part of the product is all that is needed.  However, for
any kind of digital signal processing, the most significant part is
needed for digital filters, if you don't want to have to deal with
overflow.
-- 
Ken Turkowski @ CIMLINC (formerly CADLINC), Menlo Park, CA
UUCP: {amd,decwrl,hplabs,seismo,spar}!turtlevax!ken
ARPA: turtlevax!ken@DECWRL.DEC.COM