Re: Huge uptimes & cosmic rays

H. Peter Anvin (hpa@transmeta.com)
30 Jul 1997 02:15:11 GMT

Followup to: <199707141659.JAA12609@cray.varesearch.com>
By author: "Larry M. Augustin" <lma@varesearch.com>
In newsgroup: linux.dev.kernel
>
> Be careful asking for "ECC". I know of at least two major memory
> suppliers (Centon and Transcend) that have a ECC SIMM that gives you 5
> check bits per 32-bit word. Asking them for ECC SIMMs creates further
> confusion.
>
> That's probably not what people on this mailing list want. You want
> "true quad-cast parity", which is one extra bit per byte. 4-bits per
> 32-bit word isn't enough to do ECC.
>

Or just "true 36-bit" should suffice. Any vendor which ships you
anything but 36-bit DRAM on that would be fraudulent and should be
reported.

>
> PC motherboards that do ECC combine two 36-bit wide SIMMs to get a
> 72-bit word + 8 check bits and use the 8 check bits to do ECC across
> the SIMM pair. That's not exactly the same as an ECC SIMM, which
> would give you ECC checking per SIMM not per SIMM pair.
>

Well, it is still ECC, just on a 64-bit basis. Since Pentium and
higher x86 CPU's have a 64-bit memory bus, this works. The
probability for uncatched error is larger than for 32-bit-word ECC,
but not enough to be significant.

By-the-word ECC improves bit error rate by many orders of magnitude;
so much that it is effectively zero.

-hpa

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