DDR2 confuses me

Vertigo · 4 Jul 2007 at 22:28

Upgrade time soon which will finally involve the switch from DDR (which my brain can cope with) to DDR2.

Now DDR2 confuses the hell out of me. There are so many speed ratings and latency timings available that I haven't a clue what's best any more.

Will most likely be getting an E6850 and clocking it a bit so we're looking at 3Ghz+ on a 1333FSB. What's are the best options memory wise?

If anyone can point me in the direction of some tech articles which explain all the DDR2 specs that'd be excellent.

helmutcheese · 4 Jul 2007 at 22:35

http://forums.overclockers.co.uk/showthread.php?t=17750168

DDR1 was for example 200mhz real clock x2 = 400mhz (as it worked on rise and fall of cycle where as old SDRAM worked only on rise.

DDR2 is for example 200mhz real clock x2 = 400mhz then again x2 = 800mhz (200x2x2)

Vertigo · 4 Jul 2007 at 23:05

DDR2 is still double data rate though, not quad data rate.

What I can't wrap my brain around is how these speeds relate to the FSB on Intel systems, which are quad-pumped.

On a 1066Mhz FSB C2D for example, the actual FSB speed is 266Mhz, it's just the quad-pumped nature that produces the 1066Mhz effective at the processor.

As the memory is only double data rate, does mean that you need memory which can run at twice the real FSB speed (266Mhz in this case) to get the 1:1 ratio and thus the best performance?

I've read people discussing various FSB/memory ratios but I don't fully understand the relationship when one is double and the other quad data rate plus I don't know what the performance implications are of running at 1:1 compared to 3:2 or whatever other permutations are allowed.

helmutcheese · 4 Jul 2007 at 23:13

Aint 100% on Intel but you dont need memory to be that fast to be in sync as of quad pumped method you mention

Vertigo · 5 Jul 2007 at 07:51

Well done some reading and much more confident now.

From what I can determine, running memory faster than the FSB speed is fairly pointless as the FSB becomes the bottleneck, so for a 266(1066)MHz processor, the best memory is DDR2-533 whereas for the new 333(1333)MHz ones, DDR2-667 would be better utilised.

Of course overclocking comes into it so, if I was planning on pushing my FSB up towards 400Mhz then DDR2-800 would be beneficial. As the price jump for stuff faster than this is fairly large I think I'll go with DDR2-800, probably paying the premium for CL4 over CL5.

Does the above sound correct and make sense?

lozza23_uk · 5 Jul 2007 at 08:58

if this is the case, then I can just get cheapo RAM to go with a 6420 then

saves a few quid.

by DDR2 533 you are relatting to the MHZ of the RAM, as opposed to the model descriptions (DDR2 5300, which actually runs at 667 mhz)?

edit, barely any cheaper...and after a bit more reading, are we saying that the newer chipsets can allow ram to run upto 4x faster than the FSB? if so then faster RAM would be beneficial in any system?

oh dear. wish my xp2500+ was good enough. goto go in the bin soon though

KangooVanMan · 5 Jul 2007 at 09:30

I could be way off mark, but I think for overclocking c2d with 800fsb PC6400 is good, if you have 1066 or 1333fsb PC8500 or higher is better. Then depending on how much you want to spend, get the lowest latency of that particular speed of ram.

Vertigo · 5 Jul 2007 at 10:33

KangooVanMan said:
I could be way off mark, but I think for overclocking c2d with 800fsb PC6400 is good, if you have 1066 or 1333fsb PC8500 or higher is better. Then depending on how much you want to spend, get the lowest latency of that particular speed of ram.

From what I've discovered, it appears that the FSB is the bottleneck when it comes to increasing memory performance on C2D systems, as the memory controller is still in the Northbridge (unlike AMD systems) and thus all main memory access must traverse the main bus.

So on a current 266MHz (1066 effective) C2D system, anything above DDR2-533 is a waste. You will get slightly improved speeds but only marginally and nowhere near enough to justify the extra expense.

The forthcoming 333Mhz (1333 effective) processors will likewise benefit from DDR2-667 but not much from anything faster.

Overclocking the FSB, however, allows the system to make much better use of faster memory, so a 400Mhz FSB will utilise all of the bandwidth of DDR2-800 modules. As memory faster than this starts to cost a packet, it's only worth it if you're planning to overclock your FSB up to the 500Mhz region.

Of course the faster rated memory will often run at tighter timings at lower clock speeds so you could buy this and run it at DDR2-667 or DDR2-800 speeds and get better timings than if you'd gone for memory nominally rated at this speed, IYSWIM. Personally I think DDR2-800 C4 is the sweet spot right now on price/performance.