Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!stanford.edu!neon.Stanford.EDU!kaufman
From: kaufman@neon.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac.programmer
Subject: Re: 68000 Assembly question -- overflow
Message-ID: <1991May30.020707.11193@neon.Stanford.EDU>
Date: 30 May 91 02:07:07 GMT
References: <12423.284125F3@stjhmc.fidonet.org> <1991May29.210531.7174@midway.uchicago.edu>
Organization: Computer Science Department, Stanford University, Ca , USA
Lines: 15

In article <1991May29.210531.7174@midway.uchicago.edu> tisu@quads.uchicago.edu (Seth Tisue) writes:

>I wasn't advocating one's complement arithmetic -- I'm not that naive.  What
>I was wondering was, why not have $80000000 (-2^31) be an invalid number
>rather than -2^31, thus removing the asymmetry?  I'm assuming there's some
>good reason for this...  What uses does one "often have" for the extra
>number?

And pray tell, what would you do, special, about this number.  How would you
set the Flag bits, and how would you modify the CPU arithmetic to deal with
it?  If the number offends thee, pluck it out.  Program tests in your code
to deal with it.  It's convenient for most of the rest of us that the +1
operation works in a consistent manner for ALL bit combinations.

Marc Kaufman (kaufman@Neon.stanford.edu)