fsb and memory speed

[Casper1676]

I am going to be building a PC for someone at work (Athlon 2400/266 FSB and Crucial 512MB PC2700) and I have a question about FSB speed vs. memory speed. Do they need to match? Is there a performance increase if they are equal? Anything else I should know?

Thanks

[kallikru]

FSB and memory speed do not need to match, but the memory speed should be AT LEAST as fast as the FSB. In your case PC2100 will work nicely with a 266 FSB processor, but PC2700 RAM will work just as well, maybe even better if it will run using better (more aggressive settings) parameters (CAS, RAS-TO-CAS ect.) than PC2100 RAM.

Avoid mixing different types of RAM though. It will usually work, but sometimes you may experience performance problems.

[Platypus]

Yes, agreed.

Furthermore, some motherboards have the capacity to run the memory bus faster, (called asynchronous buses), eg the PC2700, being DDR333, will function on a 166MHz bus. This increases the memory data throughput (bandwidth), but the memory cycles are no longer synchronised with the CPU cycles, so the overall performance increase will be nowhere near as much as the bus speed increase.

In this case, the 25% increase in memory bus speed may yield, say, a 10% improvement in data throughput, at the expense of there being waits when the buses get too far out of sync (this is called latency). It's often not easy to work out if your applications will benefit most from higher data throughput (faster memory bus) or lower latency (same bus speeds), so many people settle for synchronous operation (same bus speeds).