If mobo only supports upto ddr2 667, how can fsb be 800?

[Techie333]

I've been looking at some Pentium D mobos, I plan on getting a Pentium D 805 w/ water cooler and OCing the crap out of it, BUT first I'm confused about this: some Pentium D mobos on newegg say they only support upto ddr2 667 (so no DDR2 800?) however, the fsb says it can support upto 800. Can someone fill me in? If I am going to be setting my fsb at around 190 or 200mhz, I would have to get ddr2 800mhz memory right?

 

[Heidfirst]

separate buses with different speeds ...

 

[o1die]

Not necessarily. You can overclock by setting the memory lower at 166 or less, while increasing the cpu fsb, if your board has built in ratios or memory settings of 133, 166, and 200.

 

[betasub]

Your memory speed and your CPU speed are both dervived from the same bus speed, but the memory speed is Double Data Rata (and can be modified by a divider, e.g. 1:1, 5:4 etc) whereas your CPU speed is a multiple of your Front Side Bus: for Pentium D processors (amongst others) this runs at Quadruple Data Rate.

So for a base bus speed of 200MHz, your CPU has a QDR FSB of 800MHz. Memory running at 1:1 will DDR2 400. DDR2 667 will allow to run your memory at 3:2, or keep 1:1 and overclock your CPU FSB up to 1333MHz. There is also an intermediate standardised memory speed of DDR2 533 suitable for running at 1:1 with the Conroe QDR FSB of 1066MHz.