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From: chase@Ozona.orc.olivetti.com (David Chase)
Newsgroups: comp.arch
Subject: Re: Parity (was: Time between memory failure)
Message-ID: <48950@ricerca.UUCP>
Date: 9 Feb 90 00:37:45 GMT
References: <1911@sunquest.UUCP> <38420@apple.Apple.COM> <2127@bnr-rsc.UUCP>
Sender: news@orc.Olivetti.Com
Reply-To: David Chase <chase@orc.olivetti.com>
Organization: Olivetti Research California, Menlo Park, CA
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In article <2127@bnr-rsc.UUCP> bcarh185!schow@bnr-rsc.UUCP (Stanley T.H. Chow) writes:
>But surely the point is Total-undetected-error-count. Yes, it is annoying
>to have errors introduced by the checking circuitary but that is secondary
>to catching all real errors.

If you really mean what you say, then it is ok to have a parity
checker that (incorrectly, of course) always indicates that an error
has occurred.  I can guarantee you that it will catch every single
error that occurs, which is better than *all* error-detecting schemes
used in computers today (which will only guarantee that the
probability of a missed error is very low, which is not the same as
catching all errors).

Of course, my error-detector won't let you get much done with your
computer, but we've already agreed that that is a "secondary" concern.

Seriously, there are tradeoffs.  What we're probably talking about
here is "given (a) parity checking, what trade-off would you make
between (b) a slower clock and (c) frequent crashes due to
misidentified errors?"  Probably (a) and (c) is unacceptable -- people
would decide your machine was flaky, so the choice is (a) and (b),
which means that your machine is slower.  (How much slower I don't
know -- one could also spend more money on the checking circuits, but
then the computer gets more expensive).

Either way, a computer without parity looks better most of the time
(until that error comes along which silently trashes bezillions of
dollars worth of data), which means that the computer without parity
is the one that sells.  Tandem, of course, makes money selling
reliable computers, but they aren't selling PCs.

David