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From: lamaster@pioneer.UUCP
Newsgroups: comp.arch
Subject: Re: Wirth's "challenge" (overflows)
Message-ID: <3457@ames.arpa>
Date: Thu, 19-Nov-87 21:13:53 EST
Article-I.D.: ames.3457
Posted: Thu Nov 19 21:13:53 1987
Date-Received: Sat, 21-Nov-87 17:15:01 EST
References: <1656@geac.UUCP> <863@winchester.UUCP>
Sender: usenet@ames.arpa
Reply-To: lamaster@ames.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 34
Keywords: integeroverflow

In article <6777@apple.UUCP> bcase@apple.UUCP (Brian Case) writes:

>
>As for the mode bit, we thought about it and discarded it as ugly.  The
>same effect can be had by substituting the desired version of the add/sub
>instructions for the ones that are actually in the code when the program

>modes make simulation of architectures much more difficult (although the
>big jump in difficulty comes as you go from zero modes to one).  We just
>didn't think there was really any justification for a mode bit.  However,
>we were wrong at least once, so we could have been wrong on this one.
>Comments?  Feelings?  Experiences?

I have the Feeling that a processor with floating point support is going to
need mode bits anyway.  But, I think it is a Good Idea to explicitly use
different instructions for those cases where integer arithmetic is intended
and those cases where modulo 32 bit arithmetic is intended.  Sometimes when
you are debugging something it sure is nice to be able to turn exceptions on
and off at will, though, for the "normal" integer, and also floating point,
arithmetic.  But not, strictly speaking, Necessary.

I must admit that while I worry about the cost of particular hardware
features, I don't worry about the cost of designing, simulating, and testing
them.  My job is usually to worry about the cost of debugging the software. 




  Hugh LaMaster, m/s 233-9,  UUCP {topaz,lll-crg,ucbvax}!
  NASA Ames Research Center                ames!pioneer!lamaster
  Moffett Field, CA 94035    ARPA lamaster@ames-pioneer.arpa
  Phone:  (415)694-6117      ARPA lamaster@pioneer.arc.nasa.gov

(Disclaimer: "All opinions solely the author's responsibility")