Ya know what they say: nothing is 100%. That goes for ram as well.

The (relatively) short answer to your question is that downward compatibility of PC100 with PC66 memory does not relate to either being buffered or unbuffered (both may or may not be, and it is best not to mix buffered and unbuffered regardless). It has much more to do with the timing specifications of the (in this case) PC100 memory, specifically the CAS timings, and what is programmed in to the SPD (serial presence detect) eprom on the ram module. There are also isues in terms of the DRAM timing configuration in the bios. The end result must be that a match between the timings (not just the bus speed) for all installed modules must be obtainable.

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