theoretically pc XXx should be fully compatible with all systems using sd ram, the only two exceptions i have ever ran into is the occasional mac, and almost all Intel boards of the time, including the rotting p4 board i have in the drawer, that will refuse to post with pc 66 and pc 133 ram

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Well, systems should work with their rated speed or faster. I can guarantee your P4 board will not work with PC-66, no matter what. And some PC-133 RAM is non-compliant with PC-100; most notably 'single-sided' 512 MB modules. (The chips are too high density for PC-100 or PC-66 chipsets to recognize.) A 'double-sided' 512 MB PC-133 module should work just fine in any PC-100 or PC-66 system, though. (And there are some oddball single-sided 512 MB modules that work just fine in older systems.)