Bill Cohane <[log in to unmask]> wrote:
> At 13:53 04-05-98 +0000, Javier wrote:
> >To single error correct 64 bits you need 7 bits, since the error
> >may be also in the parity bits. I agree this is covered with 72 bits, but
> >to be precise...
>

[stuff deleted]

>
> Does anyone know exactly what the difference is between plain parity RAM
> and ECC RAM? Is it just that they call modern parity RAM (for example,
> SDRAM DIMMs) by the name ECC because that is what chipsets like the LX
> and BX can do with "parity" RAM?
>
> I know that there are more exotic forms of ECC that have more extra bits
> than does parity RAM (for even more error protection). But I am asking
> about regular 72 bit wide DIMMs.


No, parity RAM has 1 extra bit per byte. It can detect single bit errors only.
Multiple bit errors may go undetected.

ECC RAM has enough extra bits to not only be able to detect single bit errors,
but also correct them. Double bit errors are detected (but not corrected).

SDRAM is a different type of interface for the clock and timing of the RAM. It
is unrelated to parity or correction. Conceivably, SDRAM can come in various
flavors of parity or ECC.

I have never seen ECC in 72 pin SIMM's, there aren't enough pins for the extra
signal. Instead, you see them more commonly in 168pin DIMM's, however to use
them, your system must be capable of using ECC memory.


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Russ Poffenberger               Engineering Specialist
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