As we all know, AMD Opteron and Athlon 64 processors have integrated memory controller which dramatically decreases memory latency (please refer toour AMD Athlon 64 preview for more information). A bit less known fact is that memory frequency in AMD 8^th generation processors depends on the core-clock, hence, even if you utilise PC2700 memory, there would be cases when such memory functions on lower than 166MHz frequencies.

We have managed to find a very interesting table on AMD.com web-site. The table shows us dependence between memory and on the core-clocks for different memory types. The table was found in the document related to AMD Opteron processors, but I believe that something similar will be found in AMD Athlon 64 CPU that comes in September.

As you may see from the table, the memory frequency for PC2100 memory is often (in half of the cases) lower than 133MHz. The situation is even worse for PC2700 DDR SDRAM - it works on 166MHz only when processor’s core-clock is 2GHz (1GHz processor will never be released). But why AMD have not implemented totally asynchronous memory controller, so memory can run on its own frequency? I think such step has been taken by AMD because performance penalty from asynchronous memory controller is larger than from slightly lower memory clock.

Note that PC1600 memory always works on its own frequency since 100MHz is actually the reference clock of the HyperTransport link, thus it doesn't need dividing. The same can be said about PC3200 (200MHz), which is supported by the upcoming AMD Athlon 64 processors.