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From: davec@nucleus.amd.com (Dave Christie)
Newsgroups: comp.arch
Subject: Re: Workstation Data Integrity
Message-ID: <1990Sep6.141040.3244@mozart.amd.com>
Date: 6 Sep 90 14:10:40 GMT
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Reply-To: davec@nucleus.amd.com (Dave Christie)
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In article <2483@crdos1.crd.ge.COM> davidsen@crdos1.crd.ge.com (bill davidsen) writes:
>
>  I would expect EDAC on a 64 bit machine, however, since it is probably
>cheaper. Note that for byte parity it takes 8 bits of parity memory, but
>for EDAC you can use 1+log2(N) or 7 bits, and get 2 bit error detection,
>1 bit error correction, and use less memory.

Its been many years since I designed EDAC for a 64-bit machine, but what
I seem to remember is that using 7 bits would only allow you to correct
the 64-bit data portion, not the 7 check bits themselves.  To cover those
you need one more bit (and you really do want those covered as well). 

This extra bit is worthwhile for another reason - it gives you a lot more
freedom in arranging the matrix for generating (& regenerating) the
check bits, which translates into speed.  Eight bits would probably allow
faster encoding and decoding for a 64-bit word than seven.

So lower cost really isn't a factor.  For the 64-bit machines around today,
the money spent on EDAC is a drop in the bucket, and any performance penalty
is greatly reduced either by caches or vector operations.  Moreover, these
machines often run looooong jobs, and they are paid for by charging the 
users.  If a user's 3-day job bombs after 2 1/2 days because of a memory
error, you really can't charge him, and have lost significant revenue.
(Not to mention severely pissing off the user, who undoubtedly has things
timed to finish 1 hour before his paper on the results must be submitted :-).

Future 64-bit workstations will certainly have some different considerations
though.

---------------------------------
Dave Christie		My fuzzy memories only.