what does the advertised RAM speed do in bit transfers?

Question

you know, usually, RAM is specified by xxxMHz, xxxMB, ECC or nonECC.

bits move from the FSB, through the RAM to the CPU, right?
now, what does the frequency of the RAM(the xxxMHz that they scream about in the ads) do during the transfer.
answer specifically for DDR2 SDRAMs.
case study:
if a PC is CPU 1.6GHz Duo Centrino tech, FSB 800MHz, RAM 1.0GB, 667MHz DDR2.
the ram is the slowest, right? now, what does this 667MHz do in relation to the transfer of bits btw the CPU and the motherboard. remember it is DDR2, so it works 2x on each FSB cycle
please explain simply...
thanks.

Answer 1

Well actually the bits move from RAM to the northbridge, then to the processor, through the FSB. The RAM speed is the speed at which they travel from RAM to the northbridge, the FSB speed is the speed at which they travel from the northbridge to the RAM. And vice versa, CPU to NB to RAM. DDR2 is double pumped, meaning its actual speed is 1 half its stated speed, so in your example 667Mhz DDR2 is actually running at 333Mhz, but it sends two pieces of data each clock cycle, so it has the same bandwidth as if ran at 667Mhz. The FSB is quad pumped, so an 800Mhz FSB runs at 200Mhz, but sends for pieces of data per clock cycle, giving it the effective bandwidth of 800Mhz. So with the FSB having the bandwidth at 800Mhz (I am not going to do the calcuations to put it in GBs/s, do so if you want) and the RAM having the bandwidth of 667Mhz, the RAM would appear to be the bottleneck. This is why dual chanel architecture was created. http://en.wikipedia.org/wiki/Dual_channel This effectively allows to channels of data to go to the RAM, effectively doubling its bandwidth. So you end up with the FSB effectively at 800Mhz bandwidth, but the RAM effectively at 1333Mhz bandwidth, leaving the FSB once again as the bottleneck. Sorry I know that was not simple, but it was as short as I could make it while explaining it effectively.