Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!hao!gatech!bbn!rochester!crowl
From: crowl@cs.rochester.edu (Lawrence Crowl)
Newsgroups: comp.arch
Subject: Shift and Subtract is NOT Multiply
Message-ID: <7276@sol.ARPA>
Date: 1 Mar 88 16:57:35 GMT
References: <7649@pur-ee.UUCP>
Reply-To: crowl@cs.rochester.edu (Lawrence Crowl)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 12

In article <7649@pur-ee.UUCP> hankd@pur-ee.UUCP (Hank Dietz) writes:
>For example, a multiply by 7 (not a nice power of 2) is really shift to
>multiply by 8 and then subtract the original number.

If you use this method for multiplication by 7, and you do not provide for
a double width result, using subtraction will lose the overflow information.
For example, a*7 implemented as (a<<3)-a may overflow where (a<<2)+(a<<1)+a
will not.  If we detect an overflow with (a<<3)-a, then we cannot immediately
tell whether or not a*7 would overflow.
-- 
  Lawrence Crowl		716-275-9499	University of Rochester
		      crowl@cs.rochester.edu	Computer Science Department
...!{allegra,decvax,rutgers}!rochester!crowl	Rochester, New York,  14627