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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