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Date: | Mon, 17 Jan 2000 23:09:02 -0800 |
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On 17 Jan 00, at 9:54, Carlos Cordeiro wrote:
> Can anyone explain me what is ecc RAM. Is it a parity bit ?
Yes, and no.
Yes, it's an extra bit per byte, used to detect memory errors.
No, it doesn't follow the "parity" rules to do the detection.
Parity is a very simple system that (a) can be cheaply done in
hardware without slowing down anything else, and (b) can detect
single-bit errors, but not double-bit errors, and cannot correct
errors.
ECC uses a much more complicated system, that has only become
practical now that RAM is typically accessed at least 64 bits at a
time. The ECC bits are claculated according to "Hamming code"; it
can not only detect double-bit errors, but actually *correct* single-
bit errors without stopping execution.
ECC stands for "Error-Correcting Code" (except on some
certification exams where it apparently stands for "Error Checking
and Correcting"). Because ECC may carry a slight performance
penalty, it's usually enabled only on servers.
David G
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