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From: ark@alice.UucP (Andrew Koenig)
Newsgroups: net.arch
Subject: Re: Right shift vs. divide
Message-ID: <4772@alice.UUCP>
Date: Fri, 3-Jan-86 23:17:41 EST
Article-I.D.: alice.4772
Posted: Fri Jan  3 23:17:41 1986
Date-Received: Sat, 4-Jan-86 06:03:46 EST
References: <124000005@ima.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 12

> Now that we know that right shifting is not the same as division, has 
> anybody ever come up with a plausible reason for computers to have 
> arithmetic shift instructions on twos complement machines (other than that 
> they're easy to implement and poorly educated engineers might have thought 
> they were the same as division)?  For positive numbers, they give the same 
> result as unsigned shifts, and for negative numbers they give the wrong 
> answer.  Followups to net.arch, please.  

I can think of two reasons:  (1) sometimes you want to divide with
truncation toward -infinity and an arithmetic right shift gives you
that;  (2) on some machines, an arithmetic left shift lets you multiply
by a power of 2 with overflow checking.