Q about FSB, Ram, & dual-channel mode

PC2 6400 DDR2 800 ram runs at 800mhz, right? So as it is at stock speeds, you've got an equal amount of bandwidth going back and forth to a 800mhz FSB, right? Well, I've got it set up in dual-channel mode, so does that literally mean I have 1600mhz of bandwidth from the ram to the fsb? Or have I misunderstood dual-channel mode?

But if I've got it right then wouldn't single-channel mode be more efficient since dual-channel mode would seem to create a bottleneck? And if you did do dual-channel wouldn't you want a ratio of 1:2 (even though that ratio doesn't exist as one you can choose, at least not on my system) so you don't create a bottleneck?

As I understand it you can think of dual channel as RAID-0 for RAM. Basically the requests go back and forth from one module to the next instead of hammering one stick repeatedly. This creates a theoretical doubling in bandwidth, but real world is more like 20% methinks.

--Illah

You're a little off. DDR ram is double pumped, so essentially 800mhz is really just 400mhz x 2. This would mean that your FSB on your cpu would have to be 400 to be 1:1.

AMD CPU's are 2x pumped (the old ones, not sure on new but I think it's the same) so you'd pick 200 for the FSB setting, some of the mobo's show the FSB after the 2x multi though, so in that case you'd pick 400.

Intel's latest and greatest (not sure how far this goes back, someone please clarify, I know the whole 'core' series works liek this though) is quad-pumped. So to have 1:1, your FSB is 1600 and your ram is 800 (or 400 if it shows it before it's double-pumped, usually the boards will have both of them one way or the other). Some mobos may have you choose 400 as the FSB, or it may have you choose 1600 as the FSB to be 1:1, depends on how they display it, both mean the same thing.

As far as dual-channel, I think (I know very little on this) it's just set up so there are 2 independent ram channels on the mobo that are then sent to the northbridge and then to the CPU and back again. This is why on your mobo, to get dual channel, you put the sticks of ram into opposite color ram slots. Each of the colors is a different channel, thus to get dual channel you need some in each bank, If you were to put 2 sticks in 1 bank, or 4 sticks in all banks, then the sticks would share the busses (for the 4, 2 would share 1, the other 2 would share the other).

This make sense? People please feel free to clarify as my knowledge is limited.

OP is right, he just forgot one factor and thats the bus width. RAM is only 64bit wide for each stick, while the FSB is 128bits wide, so dual channel is required to fill up the theoretical available bandwidth of the FSB.

1:1 is the optimal ratio when talking effective frequencies. For real frequencies, like in Pearljammzz post, the optimal ratio is 1:2, again because FSB is twice as effective when at the same frequency.

PearlJammzz said:

You're a little off. DDR ram is double pumped, so essentially 800mhz is really just 400mhz x 2. This would mean that your FSB on your cpu would have to be 400 to be 1:1...

Click to expand...

To quote wikipedia.org:

"Like DDR before it, DDR2 cells transfer data both on the rising and falling edge of the clock (a technique called double pumping). The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the speed of the memory cells, so four words of data can be transferred per memory cell cycle. Thus, without speeding up the memory cells themselves, DDR2 can effectively operate at twice the bus speed of DDR."

DDR2-800 runs at a 200 MHz Memory Clock; With a 400 MHz I/O bus; With a single channel bandwidth of 6.400 GB/s.

I am still on a quest to fully understand Double-Data-Rate-Dynamic-Random Access-Memory on all its levels. Remember that there are fundamental differences between the DDRs. Such as sacrificing Latency for bandwidth and more operations per cycle at lower voltages as you go from DDR to DDR2 to DDR3.

Wikipedia.org is a great read on this BTW.