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From: ken@turtlevax.UUCP (Ken Turkowski)
Newsgroups: net.arch
Subject: Re: Right shift vs. divide
Message-ID: <1016@turtlevax.UUCP>
Date: Mon, 6-Jan-86 03:39:43 EST
Article-I.D.: turtleva.1016
Posted: Mon Jan  6 03:39:43 1986
Date-Received: Tue, 7-Jan-86 06:50:42 EST
References: <124000005@ima.UUCP> <4772@alice.UUCP>
Reply-To: ken@turtlevax.UUCP (Ken Turkowski)
Organization: CIMLINC, Inc. @ Menlo Park, CA
Lines: 28

In article <4772@alice.UUCP> ark@alice.UucP (Andrew Koenig) writes:
>> 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.

Several more reasons are: (3) arithmetic right shift is an
approximation to division by a power of two that is only one LSB off.
Sometimes you may want to divide a signed number by a power of two, but
can tolerate a small error, but not excessive computation time; thus
the popularity of the arithmetic right shift.  (4) Arithmetic right
shift is necessary to implement multiplication without a multiplier
array.  (5) Arithmetic left shift is necessary to implement division
and square root.


-- 
Ken Turkowski @ CIMLINC, Menlo Park, CA
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