800 FSB and 667 MHZ SDRAM

[kenwood850]

As a newbie I am having a problem understanding the relationship between FSB speed and the speed of the memory. Dell is offering an Optiplex 320 with a FSB speed of 800 MHz. and the only option for memory is DDR2 Non-ECC SDRAM, 667 MHz. DIMMs. According the MEMORY FAQ at the beginning of this forum, SDRAM is “Synchronous DRAM” which means, “The clock speed of an SDRAM module is synchronous with the CPU bus.” How can the RAM be synchronous with the bus if it is running 667 MHz and the bus is running at 800 MHz.? What am I missing here?

 

[tlmck]

The CPU FSB is what they call "quad pumped". The FSB is actually 200mhz quad pumped to 800mhz.

Ram is "double pumped", meaning the actual FSB match for an 800mhz CPU is 400mhz ram. If you could still find it, this was called DDR2 400(PC 3200).

667 ram is actually 333 mhz double pumped so it is actually more than is needed for the 800mhz cpu. Its exact match would actually be a CPU with a 1333 FSB.

DDR2 533 would still be more than needed as well as it is 266 double pumped. Its FSB CPU match would be 1066.

The reason that most manufacturers are using 667 is that it is the most plentiful and cheap. They also only have to stock one speed to cover all current mainstream CPU's.

 

[kenwood850]

That was an excellent explanation. Thanks for taking the time to reply.