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From: tisu@quads.uchicago.edu (Seth Tisue)
Newsgroups: comp.sys.mac.programmer
Subject: Re: 68000 Assembly question -- overflow
Message-ID: <1991May25.205252.6668@midway.uchicago.edu>
Date: 25 May 91 20:52:52 GMT
References: <1991May24.213815.16227@midway.uchicago.edu> <1991May25.021530.17126@neon.Stanford.EDU>
Sender: news@midway.uchicago.edu (NewsMistress)
Organization: Plaster Cramp Press
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In article <1991May25.021530.17126@neon.Stanford.EDU> kaufman@neon.Stanford.EDU (Marc T. Kaufman) writes:

>Sigh! What kind of class is this where they don't explain the difference
>between 1's complement, 2's complement, and sign-magnitude representations?
>The native mode of a 68000 for instance is 2's complement.  In that mode,
>$80000001 = -2147483647, and that minus 1 gives $80000000 - -2147483648.
>Yes, it is the case that there is no representation in 2's comp form of the
>positive number +2147483648 (at least in 32 bits).
>
>In 1's complement and sign-magnitude notation there are, indeed, 2
>representations of zero (+0 and -0), and the positive and negative ranges
>are symmetric.  But you don't have that, in terms of the 68000 Flag bits
>at least.

Thanks to everyone who straightened me out on this.  It still seems like a
poor situation to have a negative number, -214783648, which has no valid
corresponding positive representation; I'll have my program check for it.
(I don't think the user should have to worry about taking the negative of
something yield an overflow.)

>[a believer in the principle that there should be no such thing as an
>UNDERGRADUATE CS major]

You'd be happy here at the University of Chicago; there's no CS major
here.  You have to major in "Applied Math" (grr) and then take
computer courses.


-- 
---- Seth Tisue                     USMail: c/o Plaster Cramp Press  
---- (tisu@midway.uchicago.edu)                       P.O. Box 5975
"Please to be restful.  It is only a few           Chicago IL 60680
crazies who have from the crazy place outbroken."    --------------