Can't get my head around DDR2 + Intel. Can people help explain?

Firegod · 12 Dec 2007 at 20:38

Hello all.

I've been doing some reading and searching on the forum but seem to be getting in a muddle about how DDR2 speeds/ratings and Intel CPU's fit together. (Damn my A64+DDR is so easier to understand!).

What I can work out is that basically:-

PC2-5300 = 667Mhz
PC2-6400 = 800Mhz
PC2-8500 = 1066Mhz

Now, Intel CPU's. I keep hearing that they quad-pump the FSB to give them their 1066Mhz FSB. So a bit of maths means that actually, the FSB of the Intel CPU is 266Mhz? Is that correct?

However, as it stands, to run the CPU and Memory in a 1:1 ratio you need PC2-8500 memory. As the CPU FSB = 1066Mhz and the Memory = 1066Mhz?

So my question being, why is PC2-6400 (800Mhz) seen to be enough. Surely it's running slower? (Ignoring overclocking at the moment to keep things very simple for me. lol).

Thanks for any help in clearing this up for me. Diagrams and simple colouring in's most welcome.

Voodle · 12 Dec 2007 at 21:06

The easiest way to think of it is like this:

You're right that the actual MHz speed of a 1066MHz Intel CPU is 266MHz but memory running at 1:1 with that will also be running at 266MHz - because the memory is double data rate that becomes 533MHz (effective) which is about the slowest DDR2 you can get.

So to take advantage of the full speed of PC2-6400 (800MHz DDR, 400MHz actual) you would also have to be running a 1600MHz FSB on the CPU - which is 400 actual MHz again without the quad-pumping. I guess you could also use a divider of something which would leave the CPU at 266/1066MHz and the memory at 400/800MHz too

The interest in high speed memory is there (I think?) because a lot of motherboards don't like running the memory asynchronously or can only run the memory faster than the FSB and it may be necessary to up the FSB on a CPU to get the best speed out of it, and running 1:1 with the memory would require more expensive memory once you're past 400MHz (800MHz DDR2, 1600MHz quad pumped FSB) - last year it was a limitation at least, not so much now that DDR2 is so cheap.

helmutcheese · 12 Dec 2007 at 21:17

DDR be it 1, 2 or 3 all is Double Date Rate, just way it works is slightly different (dont need to know).

When the 1st DDR came out there was a legal matter to the name they wanted to call it as another Vendor used that name, PC266, so they called it after the amount of Bandwidth it had, so DDR266 is called PC2100 (2100mhz or 2.1ghz of Bandwidth).

And on to Intel, all their CPU's are artificially Quad Pumped, the old 400mhz FSB CPU's were really 100mhz FSB while AMD were 133mhz then 266mhz DDR.

If you buy a new Intel CPU and its got a 1333mhz FSB its really running a 333mhz FSB but Quad Pumped so x4.

The Memory has to be running at 667mhz (PC5300) to be in Sync with it ( 667x2=1333).

When new 1600FSB CPU arrive the Memory will have to run at 800mhz so you buy PC6400

WJA96 · 13 Dec 2007 at 05:59

It's never going to be a bad thing to buy faster RAM. PC2-6400 is probably fine, but I've actually got a situation with my Q6600 where I've had to swap my Corsair PC2-6400 XMS2 for Crucial Ballistix PC8500 because it was limiting my overclock to "only" 3.8GHz. With Ballistix I can run over 4GHz (452MHz) stable but the PC2-6400 (400MHz) stuff won't run over 445MHz so it's limiting my overclock.

Work out what you reckon your maximum overclock will be and buy RAM fast enough - I hate overclocking RAM as I do think it shortens it's life - especially with Crucial chips.

Firegod · 13 Dec 2007 at 17:53

Thanks guys, I think I start to get it!

So the 1066Mhz CPU's are actually just 266Mhz. Obviously, double for DDR which makes 533Mhz. In other words, you need PC2-4200 Memory.

Therefore, when overclocking, PC2-6400 is more than enough (I guess you could argue that PC2-5400 would be ok for a small overclock?)

However, the newer CPU's for example with 1600Mhz would have an FSB of 400Mhz and therefore, DDR means it is 800Mhz. Therefore needing PC2-6400 Memory, but to overclock you should get PC2-8500.

Does that seem to make sense?

I think I can see why PC2-6400 is enough!

Cob · 13 Dec 2007 at 17:59

That's spot-on

Firegod · 13 Dec 2007 at 18:17

Cob said:
That's spot-on

Yey. Lol.

pegasus1 · 14 Dec 2007 at 15:04

If your chip has a high multi then pc5400 might be enough, a E2180 has a 10 multi so 5400 memory will run at 1:1 if the CPU is clocked to 3 GHZ (333 fsb).