Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/5/84; site ism780c.UUCP
Path: utzoo!watmath!clyde!cbosgd!ukma!psuvm.bitnet!psuvax1!burdvax!sdcrdcf!ism780c!tim
From: tim@ism780c.UUCP (Tim Smith)
Newsgroups: net.math,net.arch
Subject: Re: Integer division
Message-ID: <628@ism780c.UUCP>
Date: Fri, 14-Feb-86 15:43:10 EST
Article-I.D.: ism780c.628
Posted: Fri Feb 14 15:43:10 1986
Date-Received: Tue, 18-Feb-86 06:49:04 EST
References: <557@aesat.UUCP>
Reply-To: tim@ism780c.UUCP (Tim Smith)
Organization: Interactive Systems Corp., Santa Monica, CA
Lines: 29
Xref: watmath net.math:2856 net.arch:2550

In article <557@aesat.UUCP> bmw@aesat.UUCP (Bruce Walker) writes:
>
>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 I write the firmware for the disk controller boards I design, I
>want that code to run as fast as possible (so do you, if your machine
>uses my board!).  When a logical block address is handed to me, I do an
>integer divide by the number of sectors per track which produces both a
>quotient and remainder.  The quotient will represent the track I should
>seek, and the remainder represents the sector I should read.  No extra
>work is done, and all the micros I would normally use for this kind of
>purpose do what I expect.
>

If your logical block numbers are >= 0, and your disks have a positive
number of sectors per track, then round toward zero and round toward
negative infinity are the same.

If you were given a relative block number to seek to, say N blocks from
the start of the current track, then N might be negative.  In this case
you would also want to round to negative infinity.  For example, if we have
16 sectors per track, and want the logical sector -13 from the start of
the current track, we want -13/16 = -1, and -13%16 = 3, so we go back
one track and read sector 3.  With round toward zero, we would get -13/16
= 0, -13%16 = -13, thus we try to read sector -13 of the current track.
--
Tim Smith       sdcrdcf!ism780c!tim || ima!ism780!tim || ihnp4!cithep!tim