Path: utzoo!attcan!uunet!husc6!bloom-beacon!apple!baum
From: baum@Apple.COM (Allen J. Baum)
Newsgroups: comp.arch
Subject: Re: parity/ecc (was Re: Cray Quotations)
Message-ID: <22732@apple.Apple.COM>
Date: 23 Dec 88 00:22:22 GMT
References: <2496@ficc.uu.net> <8841@sequent.UUCP> <2523@ficc.uu.net> <582@m3.mfci.UUCP>
Reply-To: baum@apple.UUCP (Allen Baum)
Organization: Apple Computer, Inc.
Lines: 17

[]
>In article <582@m3.mfci.UUCP> colwell@mfci.UUCP (Robert Colwell) writes:
>
>Almost all the commercial machines I know about use SEC/DED ECC on
>their memories but parity checking on the buses.  Are there really
>machines that use ECC on their buses?  Why?

There are machines that have an ECC code on the command lines- like a two
out of 5 code. I think the VAXBI might do that. In general, I think that
most designers believe that parity is adequate on anything that can be
retried, like a bus transfer, but ECC is necessary for things that can't,
like storage. Even then, static storage inside a CPU (registers, etc.)
is usually only parity corrected, while dynamic storage (DRAMS),
which are much more susceptible to failures, are ECC protected.
--
		  baum@apple.com		(408)974-3385
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