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From: jimc@ucla-cs.UUCP
Newsgroups: net.math,net.arch
Subject: Re: Integer division
Message-ID: <9091@ucla-cs.ARPA>
Date: Tue, 18-Feb-86 13:30:36 EST
Article-I.D.: ucla-cs.9091
Posted: Tue Feb 18 13:30:36 1986
Date-Received: Thu, 20-Feb-86 00:10:43 EST
References: <557@aesat.UUCP>
Reply-To: jimc@ucla-cs.UUCP (Jim Carter)
Organization: UCLA Computer Science Department
Lines: 27
Xref: linus net.math:2492 net.arch:2369

In article <557@aesat.UUCP> bmw@aesat.UUCP (Bruce Walker) writes:
>| Is there someone out there who *wants* a/b to round towards 0 (for reasons
>| ucbvax!brahms!weemba	Matthew P Wiener/UCB Math Dept/Berkeley CA 94720
>
>Yes, *I* want a/b to round toward 0, and for a good (although selfish)
>reason.  (I also want divide to produce *both* result *and* remainder
>which answers a question posed by someone else on just that point.)
>
>...When a logical block address is handed to me, I do an
>integer divide by the number of sectors per track ...

"Round" is a misnomer.  I think the problem is, if the logical block number
is negative, should the answer be made more or less positive?  In other words,
in (-16)/7 do you want the answer to be -2 or -3?  I don't think I have ever
had a situation where it mattered -- but remaindering is closely related, and
it *does* matter there.  Do you want (-16)%7 to be -2 or +5?  In your disc
example I think you would rather be on sector +5, track -3 rather than 
sector -2 track -2.  Similarly for circular caches, hash tables, etc.etc.
   One point, though: most divisions and remainderings are of known-to-be
positive quantities, and we want to avoid adding useless overhead to check
unsigned quantities for being negative.  It would be best if the hardware
would let the remainder have the sign of the divisor, whatever the dividend
is.  (Ask for the moon and they might give it to you.)

James F. Carter            (213) 206-1306
UCLA-SEASnet; 2567 Boelter Hall; 405 Hilgard Ave.; Los Angeles, CA 90024
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