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From: lamaster@pioneer.arpa (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: Wirth's "challenge" (overflows)
Message-ID: <3440@ames.arpa>
Date: Wed, 18-Nov-87 20:13:49 EST
Article-I.D.: ames.3440
Posted: Wed Nov 18 20:13:49 1987
Date-Received: Sat, 21-Nov-87 11:17:27 EST
References: <1656@geac.UUCP> <863@winchester.UUCP> <197@m2.mfci.UUCP> <902@mips.UUCP> <8933@utzoo.UUCP> <6743@apple.UUCP>
Sender: usenet@ames.arpa
Reply-To: lamaster@ames.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 33
Keywords: integeroverflow

In article <6743@apple.UUCP> bcase@apple.UUCP (Brian Case) writes:

>Very early in the design of the Am29000, it was decided that instructions
>for integer arithmetic would be provided in both trap-on-overflow and
>no-trap-on-overflow versions.  Without question, the thrust behind this

>The Am29000 has probably got way too many add and subtract instructions;
>while I was at AMD and making presentations to various groups, we got
>plenty of strange looks and laughingly-said remarks like "What the hell
>are these for?"  In at least one case, the most caustic remarks came from
>some influential computer scientists.  Whatever you think of these
>instrucdions, there is no incremental cost to trap 32-bit, two's complement,
>integer arithmeitc overflow.

Do the "trap" type instructions interact with a mask in the user context which
sets which conditions will trap (one machine I know of allows users to specify
their own routines for error handling of specific trapped errors); then, the
user can turn error checking on for a specific process.  I understand that
some "non-trap" type instructions are there to support certain kinds of
unsigned arithmetic, but there is virtually no cost even on a completely
hardwired machine for some kind of system and user programmable control
registers to give the user options on a per-process basis.






  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")