Help to understand speed.

[GarryL]

Is there a relationship required between the speeds of the motherboard (is this the FSB speed?), the CPU & RAM?

I thought that I read somewhere that there is a 2:1 (or something) ratio between the speed of one element (FSB?) & another (RAM?). Is this just a mis-reading on my part, & that I just need to ensure that the FSB & RAM can both handle the speed that I want?

And what about CPU speed...?

I am not interested in overclocking, just out of the box operation.

Thanks, Garry.

[bobafett_h]

Yes, there certainly is a relationship with the FSB, CPU and RAM in terms of their speeds.
More so with the current Intels than AMDs though do to the traditional FSB not being used in favor of HyperTransport and the integrated memory controller on AM2 and AM2+ processors. The next generation of Intels though (the Core i7 CPUs) will also feature an intergrated memory controller and will use a similar set up to HyperTransport...

The general rule though is that everything should be an even multiple of the core clock speed. The core clock speed of a Core 2 Duo E8400 for instance is 333Mhz. In order to get the total clock speed of 3GHz, that core clock is multiplied by 9. The FSB of that CPU is 1333 which is 333MHz multplied by 4 as the FSBs of the Core 2 CPUs are what is called "quad pumped". The ideal match in terms of DDR2 memory for this processor would be 667MHz RAM as DDR2 works by doubling the core clock speed. 1333MHz RAM would be ideal for DDR3 memory configurations as it matches the FSB exactly.

With AMD CPUs, the core clock speed is usually only 200MHz. However, because the memory controller is built into the CPU and HyperTransport adds additional multipliers to the total speed between the processor and memory, the transfer rate from the memory to the CPU is still faster than on Intels. Generally because of this, 800MHz DDR2 is the best option for RAM. AM2 CPUs will not support anything faster than that. AM2+ CPUs (Phenoms) support upto 1066MHz DDR2 but I have seen this to actually be slower when compared to 800MHz memory. DDR3 support will not be available for AMD processors until the AM3 motherboards and CPUs are released.

It is certainly possible to use higher or slower speeds of RAM (more so with Intels), but stability is not always guaranteed. Memory that is faster than the core clock speed should only be used when overclocking. For example, a friend of mine has his Core 2 Quad Q9450 overclocked to 400MHz for the core clock which pushes the total clock speed of the CPU up to 3.2GHz, the FSB upto 1600MHz and the ideal RAM speed to 800MHz.