From: email@example.com (John R. Mashey)
Subject: Re: Radiation and DRAM?
Date: 27 Nov 1995 18:50:59 GMT
Organization: Silicon Graphics, Inc.
In article <firstname.lastname@example.org>,
email@example.com (David Miller) writes:
|> He also insisted that the cache sram was as susceptible to parity errors
|> (I'll assume on a per-bit basis) and since it's not parity, having parity
|> on the dram was silly.
In many current CPUs:
1) On-chip caches usually have byte parity.
I don't know off-hand of anyone using ECC for the on-chip caches, since:
a) The caches are fairly small.
b) They usually need to support partial-word-writes, and would prefer
to avoid the read-modify-write usually implied by ECC.
c) This is especially clear if the on-chip cache is write-through
anyway, i.e., there is likely to be a good copy of the data in the
2) Off-chip caches mostly use ECC. (I think just about everybody does this
with exception of UltraSparc.)
a) The caches are much bigger.
b) These caches are usually write-back, and hence may have the only
copy of good data in the system.
c) In a big SMP, there may be a fairly substantial amount of memory:
for example, an SGI Challenge could have 36 R4400s, each with 4MB
of cache, or 144MB total of cache.
-john mashey DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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