Memor speed:CPU FSB - need clarification

Hi all,

I am new to Tom's Hardware and a bit of a n00b to hardware specs and how they all fit together and provide "true collective" vs. "stock individual component" performance. Sorry for my made up terms if they are just plain daft and better bona fide terminology exists out there for these conecpts.

I have been reading quite a bit and so have tried to "self help", but there is one thing I cannot seem to find a definitive answer to. Let's say you get the Intel E8500 which has a speed of 3.16GHZ and an FSB of 1333MHZ. Being Intel, this is quad pumped and so we have a system clock of 333MHZ making the internal multiplier 9.5 to give you the stock speed of the processor. All this I understand.

So we now want to get memory that can make the most use of our dandy new CPU. We are going with DDR, obviously. DDR is dual rate, so 2 x system clock which, as we calculated earlier, is set to 333MHZ = 667MHZ. Now we can see that there is DDR-3 1333MHZ available nowadays that matches the 1333MHZ of the CPU's FSB. Ahhhh, but hang on. Even though the memory can run at this speed it is still only DDR meaning 2 x clock speed. Correct? This IS my question. At the moment I understand the answer to this question to be "yes", which means the memory will run at 667MHZ. So, latency issues aside, why not just go for DDR2-667 or DDR2-800? Of course, I realise the 800 will run slower, but I read that even though it won't be 20% faster it will be marginally better than the step below. Is this correct? If so, then I may as well do it as the price gap is negligible in my estimation.

But back to the original question. Apparently, the ideal is to do a 1:1 so you are getting the most out of the "4x" of your processor's and "2x" of your memory's capabilities. So, if you have 1066MHZ processor FSB, get 533MHZ RAM ("4x" / "2x" ). Faster memory will provide a "diminishing returns" gain in performance, so don't waste the money if you don't have it. However, what about when the ratio flips back to even, i.e. not a 1:5 or 1:3 or 5:4 ratio but a 2:1 (or 1:2 however you are supposed to look at it). Have you overcome the latency caused by things not being in sync due to the non-even ratio, but still have memory running at half the speed of the processor? In other words, what you have gained is the latency improvements delivered by the faster memory but not the clock speed?

I know this question is all jumbled up, but I am assuming you guys are so aware of how this all get's put together that you probably got what I was getting at without all my blabbing.

TIA,

TIUK-3000

You are fine, you just have a lot you are talking about up there haha. I'm not 100% sure, but from what I understand having the RAM faster than the FSB (above 1:1 ratio), the gain is practically nothing.

So if you say your FSB is 333, then 667 for the RAM is good. Lower the RAM and the RAM is the bottleneck. Raise the RAM and the FSB is the bottleneck.


If you meant something else, please clarify.

You cannot lower the RAM to less the 1:1 the machine will not POST... c2d arch requires the cpu:ram ratio be at least 1:1 or greater.

If you are running the RAM at 1066mhz as manufacturer specs state, yes you do no matter what FSB the cpu is. However, if you are running 1:1 with the cpu then your RAM will not be running at 1066mhz therefore you should be able to get away with something lower.