RAM Speed codes, Speeds and RAM/FSB Dividers

[EtheAv8r]

Could some kind sole help an ‘old fool’ understand the RAM speed codes and the relationship between RAM speed and FSB and the use of ratio dividers in overclocking.

I have seen comments that for Core 2 Duo, anything over DDR2 PC6400 (800Mhz) is a waste of money, why and is this really true? Is 667 Mhz the fastest you need if you don’t want to OC, and 800 Mhz if you do? If so what is the point of the faster memory, is it just for extreme FSB OCs, or maybe to provide headroom so the RAM operates comfortably within it’s performance capability?

OK here is what I understand:

RAM codes and related speeds:
PC3200 = 400 Mhz
PC4000 = 500 Mhz
PC4200 = 533 Mhz (why not 525 Mhz)
PC4300 = 533 Mhz (why not 537.5 Mhz?)
PC4400 = 550 Mhz
PC5300 = 667 Mhz (why not 662.5 Mhz?)
PC5400 = 667 Mhz (why not 675 Mhz?)
PC6400 = 800 Mhz
PC8000 = 1000 Mhz
PC8500 = 1066Mhz (why not 1062.5 Mhz?)

So therefore the A-Data PC1000+ is PC8000?

My query is in relation to Core 2 Duo, I plan to build a system with an E6700 (maybe E6600) and to overclock it to a reliable, sustainable stable level for ongoing use, not to see how far I can take it. Reliability and Stability are paramount It will live in an Antec P180 and be air cooled. I had been planning an ATi X1950XTX GPU but now think I may splash out on an nForce 8800 GPU, but want a fast single graphics card, I am not interested in SLI or Crossfire.

I want to run Flight Sims (including new MS Flight Simulator X) as fast and at as high detail & quality level as possible. I currently have 1600*1200 monitor, but may upgrade to 1920*1200 one day. I was toying with a 3 monitor setup, but have given up that idea as a bit OTT - as I have a real plane I fly as well (OK so not in combat!).

Many thank to anyone who helps me gain a clear(er) understanding!

[Hottentot]

The current Core 2 Duo is designed to run with 533MHz DDR2 so if you dont overclock, thats the memory you should get. Any memory faster than 533MHz is only really needed for overclocking the FSB. Since DDR2 800MHz is 50% faster than DDR2 533MHz you should be able to get a 50% overclock with ease (CPU and mobo permitting). It looks like most Core 2 Duos will hit 2.8-3.0Ghz so E6400 or E6600 would be good buys.

[oralpain]

Generally the PCxxxx ratings (the theoretical maximum bandwidth) are rounded to the nearest 100. No system at stock FSB with stock dividers is going to run the memory at 525, or 675MHz DDR, for example.

Just know what the PCxxxx ratings typically corispond to in MHz. They are, in of themselves, irrelivant.

As for the rest of your system, I would recommend some DDR2 667 (most will oc to 800 no problem, if you need it to), an E6600, and an 8800. You don't really need fancy memory unless you are going for a very large OC, and you will see far more of a benifit from a faster video card regardless.

A mid-range, or even the proper budget P965 motherboard should be quite sufficent. I've been looking at the Gigabyte GA-965P-S3, or ASUS P5B-E, for my next build. Ideally, I would like to have an NVIDIA 680i board, but the least expensive one is more than double the price of some good P965 boards, and the performance difference would be negligable, unless I overclock to truly extreme levels, or need SLI.

[shaithis]

PC4200 = 533 Mhz (why not 525 Mhz)
PC4300 = 533 Mhz (why not 537.5 Mhz?)
PC5300 = 667 Mhz (why not 662.5 Mhz?)
PC5400 = 667 Mhz (why not 675 Mhz?)
PC8500 = 1066Mhz (why not 1062.5 Mhz?)

They round them to the nearest used FSB to avoid confusion

[kalniel]

The 'PC' speed for RAM is 16x the real speed in mhz. FSB is a bit of a misleading term these days, so I'll refer to it as CPU clock - you multiply this by the processors multiplier setting to get the speed in mhz that the CPU runs at.

Taking AMD64 as an example first off. The CPU clock is 200mhz. To run memory synchronously with the CPU clock you need 200mhz RAM. DDR ram actually performs two operations per clock cycle, hence the name Double Data Rate, which would be DDR400 or PC3200.

Core 2 Duo's have a CPU clock of 266mhz. So to run synchronously with RAM your RAM needs to be 266mhz, or DDR533, or PC4256. Obviously PC4256 looks really ugly, so they call it either PC4200 or PC4300 - they are both the same thing.

Keeping things synchronous for the time being, in the absence of way of changing the multiplier of a CPU (as most are locked), the only way of overclocking a CPU is to change what I'm calling the CPU clock. So you could run an AMD system at CPU clock of 250mhz, or Core 2 Duo at 333mhz. To do this your RAM has to be able to run at these speeds as well, so DDR500 (PC4000) or DDR667 (PC5328 - ugly, so both 5300 and 5400 used again) should be used respectively if you want it to be guarenteed at those speeds.

However, it was quickly realised that if you uncouple the RAM speed from the CPU clock, you gain a whole bunch of flexibility. One use would be to overclock the CPU but keep the RAM running at it's original speed - this is often done with AMD systems, so you could run say a 4:5 RAM:CPU clock ratio, letting you run your RAM at 200mhz (DDR400) and your CPU at 250mhz.

On Intel chips people are tending to do the opposite - run the RAM at higher speeds than the CPU, for example 5:4 RAM:CPU clock ratio runs the CPU at 266 and the ram at 333 (DDR667).

The different approaches are due to both how different systems access their memory, and the memory technology. AMD64 (prior to AM2 sockets) used DDR (one) memory - together with the onchip memory controller it was found that increasing the speed of memory at the expense of the memory latency (timings) didn't really improve performance at all. However improving the latency did have a slight benefit - so the AMD strategy is to run the memory slower but with tight timings.

Intel chips do not have an on chip memory controller, this is handled by the motherboard instead, and the core 2 duo uses DDR2 memory. DDR2 memory is readily available in speeds well above that needed for the CPU clock. At the same time having poorer latency does not affect the performance as much as the gain that you find in this case from the increased speeds. You can't increase RAM speeds forever though - eventually you are just held back by the speed at which the CPU is talking to the RAM.

So what does this mean for you?

If you're tight for cash, all you need is DDR(2)533 ram to allow the core 2 duo to run at full speeds. However, if you have the spare cash, going up to DDR667 or 800 gives you several options - you can run on a divider to run your RAM faster than your CPU clock, or you can overclock your CPU clock knowing that you have the headroom to keep your RAM running synchronously. Or you can do both

[EtheAv8r]

Thank you everybody, this is a great response, and an extra thank you kalneil for taking the time and effort to explain so thoroughly.

As it happens, I am now at a time in my life when cash is not so tight, and I can afford to indulge somewhat. I have actually ordered an E6700 – mainly because I am visiting the USA next week, and it is 528 US$ - which with shipping at today’s exchange rate is a under £290. As an E6600 from Scan is £210, and an E6700 is £350 I figure WTH go for the E6700 which with a 50% target OC will be a little faster than an E6600 with a 50%OC.

[aur0ra]

...the E6700 which with a 50% target OC will be a little faster than an E6600 with a 50%OC.

Not at all correct!The two processors are differentiated (performance wise) only by their clock frequencies because the cores of the CPUs are identical.The plus-side of going for the E6700 is that it has a higher multiplier ,in it's case a x10 compared to the E6600's x9 and therefore you can achieve the same results easier with the E6700 because you don't have to set such a high FSB unlike the E6600 and it's lower multiplier

[merdat]

A great post,

As regards the E6700 having a *10 multiplier as opposed to the E6600 having a * 9 multiplier with the cores being identical . Then at the end of the day the E6700 is going to be the faster chip and will achieve a shorter time in a 'super pi' test. Similar scenario to a E4300 against a E 6300 due to the fact that the E4300 has a higher multiplier.

[Clunk]

Not at all correct!The two processors are differentiated (performance wise) only by their clock frequencies because the cores of the CPUs are identical.The plus-side of going for the E6700 is that it has a higher multiplier ,in it's case a x10 compared to the E6600's x9 and therefore you can achieve the same results easier with the E6700 because you don't have to set such a high FSB unlike the E6600 and it's lower multiplier

Thats assuming that both chips are identical in every way apart from the multi. You may find that you get a 6700 that doesnt overclock well and might need a lot of voltage to hit 3ghz, but then again, you might get a 6600 that will do 4ghz on a little over stock volts. Swings and roundabouts.

[aur0ra]

Thats assuming that both chips are identical in every way apart from the multi. You may find that you get a 6700 that doesnt overclock well and might need a lot of voltage to hit 3ghz, but then again, you might get a 6600 that will do 4ghz on a little over stock volts. Swings and roundabouts.

Yep Those pesky companies and their cheap ,faulty ,china-maid ,you-better-be-lucky-and-hope-your-chip-is-good manufacturing ways (oopsy ,got carried away)

P.S. thanks @ merdat for the recognition