Confused about FSB/memory speed in relation to multiple cores

[archcommus]

I haven't entirely kept up on recent FSB speeds and their relation to DDR2 speeds since I haven't been interested in purchasing. But I don't know like not knowing so I'm asking.

I understand that, say, with my current system, Athlon 64 3200+, and DDR-400 memory, that the HT speed is 200 MHz, and so that the DDR speed is twice that = 400 MHz.

Now let's say you have a single core Intel processor with a 533 MHz FSB. Does this mean you want 533x2 = DDR2-1066 memory? If not, why is it different from my Athlon situation?

Then, let's say you have a dual core processor with a 667 FSB. Does this mean you want 667x2 = 1333 speed memory? I'm guessing not, since DDR2-1333 doesn't even exist. Or, should you take 667, divide it by 2 to get the FSB per core (333), and then take THAT times 2 to get your DDR2 speed? So you'd want DDR2-667 for that 667 FSB dual core CPU. What's weird about that is, why would you want slower memory for a dual core CPU than the single core 533 FSB CPU, where I calculated you'd want 533x2 = 1066 memory? Or if that's wrong, then once again, I don't understand why it's different from my Athlon.

Any info that would clear me up here would be appreciated, thanks.

 

[F1shF4t]

With Athlons the memory controller runs at cpu frequency and the memory speed is just that frequency divided by a memory divider. (In your case 2000mhz, mem divider 10 resulting in 200mhz memory speed, or 400mhz (DDR400) effective). A divider of 12 gives DDR333 mem speed etc. That is why for Athlons you just use the fastest mem clock with lowest latency.

For Intel the memory is accessed through FSB and best performance is when FSB clock = mem clock (effective). The FSB is for all cores, so they have to share the bus. So say for 1066 MHz FSB using 533MHz memory is ideal (assuming dual channel resulting in 1066 effective memory clock)
Also say for slower FSB cpu models having memory in a 1:2 ratio would also work well. (For example my laptop has 667FSB and dual channel 667Mhz memory)

 

[archcommus]

Thanks! That really helps. Still a little confused though.

Say with the Athlon XP 2100+. It has a 133 FSB, so you bought 133 MHz memory, which is labeled as twice that, DDR-266. With the Intel FSB of 1066, are you saying the reason you buy 533 MHz memory instead of 1066 MHz memory is because of the dual channel configuration doubling your effective rate? Right now I have an Epox board with 4 memory slots. My memory is currently in the first and second slots, so I'm guessing that means I'm NOT using dual channel, as that I'm guessing would require using the first and third slots. If I set it up that way could I be getting 800 MHz with my DDR-400 memory and then drop my memory divider to accomodate that?

Even with my memory in the first and second slots, when I go to CPU-Z it says "Channel #: Dual." Hmm.

 

[F1shF4t]

Originally posted by: archcommus
Thanks! That really helps. Still a little confused though.

Say with the Athlon XP 2100+. It has a 133 FSB, so you bought 133 MHz memory, which is labeled as twice that, DDR-266. With the Intel FSB of 1066, are you saying the reason you buy 533 MHz memory instead of 1066 MHz memory is because of the dual channel configuration doubling your effective rate? Right now I have an Epox board with 4 memory slots. My memory is currently in the first and second slots, so I'm guessing that means I'm NOT using dual channel, as that I'm guessing would require using the first and third slots. If I set it up that way could I be getting 800 MHz with my DDR-400 memory and then drop my memory divider to accomodate that?

Athlon XP 2100+ has a 133 MHz FSB, but its 266 effective cause it transfers data twice per clock. (Similar to Intel 266MHz FSB but transfer 4 times per clock which is 1066 effective)
Because the memory data goes through FSB there really isn't any reason to have the memory clocked higher than it as you will be limited by the FSB transfer rate. Reason people buy higher clocked memory is it allows them to run 1:1 for a much higher bus speed.

With AMD Athlon 64 there is no FSB, the memory controller is on the processor. So the faster you run your memory the better it is. In your case configure the memory to run in dual channel do not drop the memory divider and run the sticks in DDR400 speed which should give you 800 MHz effective.

 

[F1shF4t]

Epox EP-9NPA+ Ultra, to run memory in dual channel use slot 1 and 2.
Majority of mainboards are set up to use slot 1 and 3, this helps in cooling when only 2 memory sticks are used.

 

[archcommus]

I see, so the Athlon XP was 133, but 266 effective. The Intel is 266, but 1066 effective. In each case you want the FSB effective 1:1 with the memory effective. For the Athlon, the memory is 133, so 266 effective, thus labeled DDR-266. For the Intel, you need 533 effective in dual channel mode, or just spend more and get 1066 effective memory.

So does that mean the "actual" speed of the DDR2-533 memory is 533/2 = 266 MHz, but it's doubled to get the 533 effective rate that it's labeled with like the DDR-400?

Thanks for all this help.

Regarding my Epox, thank you, that's good to know, so CPU-Z is right and I've been in dual channel mode all this time. I never set the divider manually, however - everything is set to stock.

 

[F1shF4t]

Originally posted by: archcommus

So does that mean the "actual" speed of the DDR2-533 memory is 533/2 = 266 MHz, but it's doubled to get the 533 effective rate that it's labeled with like the DDR-400?

Its spot on for DDR, but DDR2 is different. DDR2 I/O clock rate is double the clock rate of the memory cells. So essentially DDR2-533 has a I/O bus clocked at 266Mhz and memory cells are clocked at 133Mhz.