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From: rh@craycos.com (Robert Herndon)
Newsgroups: comp.arch
Subject: ECC (was: Re: 1st 64 Megabit DRAM)
Message-ID: <1990Jun14.220128.7904@craycos.com>
Date: 14 Jun 90 22:01:28 GMT
Organization: Cray Computer Corporation
Lines: 25


  Saying that 'ECC' can't correct multiple bit errors is too
strong.  Traditional SECDED won't help you, but there are
S2ECD2ED codes that will correct any errors in a single two
bit block of a machine word, S4ECD4ED codes that will correct
any errors in a single four bit block, etc., while recognizing
double block errors.
  ECC is generally only for people who worry about large words
(or perhaps cache lines), say, 64 bits or larger.  There is
some additional cost over simple SECDED, as SECDED for a 64
bit word requires 8 check bits, S2ECD2ED for a 64 bit word
requires 10 or 12 check bits, and S4ECD4ED for a 64 bit word
requires 16 check bits.
   Block-error correcting codes are probably not that well known/
popular; this may have some effect.  IBM has published (and
patented) some stuff on this, but I don't have the references
handy.  If you're interested, e-mail me and I can probably dig
them up.
  Even so, what percentage of machines "out there" really use
SECDED codes?  What percentage of memory chip production do
these products consume?

Robert Herndon				Cray Computer Corp.
rh@craycos.com				1110 Bayfield Dr.
719/540-4240				Colorado Springs, CO