15 May
2006
15 May
'06
8:54 a.m.
Doesn't that mean that there's roughly a 10%
chance that a memory
error is going to be in the ECC bits rather than the main operating
memory - and that a problem there could result in the system
correcting an imaginary fault?
Only if the ECC is incompetently done; a single-bit error, in the ECC
world, can be in any of the bits. ECC is not simply adding checking
bits to a normal word; if you use 71 bits of memory to store 64 bits of
data ECC-protected, there normally will not be any 64 of those 71 bits
where you can look to always find the upper-layer data. Instead, the
coding scheme just ensures there are 2^64 distinct 71-bit words with a
Hamming distance of at least 3 (for SEC) or 4 (for SECDED) between any
two of them, with structure that makes it relatively easy to extract
the 64 bits of data from a 71-bit codeword.
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