In addition, during the DDR heyday, Intel’s Pentium 4 DDR chipsets didn’t officially support asynchronous memory operation greater than PCICLK (33.3MHz) faster than the processor’s clock speed (note that I mean actual clock speeds, not effective bandwidth, in this comparison). This means that if your Pentium 4 runs on a “400MHz” front-side bus (which is an actual 100MHz quad-pumped), Intel did not officially support running the actual memory clock faster than 133MHz (this corresponds to PC2100, or DDR266). Some motherboards with Intel chipsets allowed you to overclock your memory above the officially supported speed differential (this means DDR333/PC2700 memory on an FSB400 CPU, or DDR400/PC3200 memory on an FSB533 CPU) – but then, once you exceeded the 33.3MHz differential (after all, DDR400 memory runs at an actual clock speed that’s 66.7MHz higher than the FSB533 processor’s actual FSB clock speed), the system would become increasingly unstable.
On the other hand, Intel’s modern DDR2 and DDR3 chipsets allow asynchronous memory operation (in terms of the memory’s effective speed rating) up to the effective speed rating of the processor’s FSB (this means that an FSB533 CPU can run DDR2 memory up to DDR2-533 speed, or an FSB800 CPU can run DDR2 memory up to DDR2-800, or an FSB1066 CPU can run DDR2 or DDR3 memory up to DDR2-1066 or DDR3-1066).
