ddr2 overclocking?

bigKr33

Senior member
Oct 6, 2005
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Okay i did make a parts list of some of the parts i want to buy. Its most likely going to get revamped pretty quick here anyway. Most ddr2 compatible motherboards are compatible with ddr667 and ddr800, i did the math and if the fsb stock is 266; then the ram can either be double or triple pumped corect?

I posted a link to some ddr1200 ram and someone said i would need a 600mhz fsb. That would make sense if you double pump, but couldn't you triple pump with a 400mhz fsb to get to that speed just as an example.

I bring this up because the only thing thats holding me back from making a decision in computer hardware is the ram, i just simple don't how it works (too used to ddr) I'm stuck between buying a dual core or quad core. I intend to overclock of course.
 

DrMrLordX

Lifer
Apr 27, 2000
20,670
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What you need to know is this:

With DDR2, the chips are essentually "quad pumped". The DIMMS themselves run at a speed 1/4 their DDR2 rating, while the memory BUS that connects the DIMMS to the memory controller runs at a speed 1/2 the DDR2 rating.

Example: DDR2-800

DIMMS: 200 mhz
Memory BUS: 400 mhz

For the most part, all you need to know is the speed of the memory BUS, and you only really need to know that for Core 2 systems.

On a Core 2 system, most people wisely use a 1:1 ratio, which means they will run their memory BUS at the same speed as their Front Side Bus (FSB). The trick then is to figure out what speed RAM you need based on the FSB you plan to run.

For example, if you want a 3.6 ghz E6600 (9x CPU multi), you need 400 FSB. That means you will want your memory BUS running at 400 mhz as well, which means you need DDR2-800. Beyond that, the trick is to tighten up your timings to reduce latency a bit if possible.

That guy who said you "needed 600 mhz FSB" was basically assuming that you were going to stick to a 1:1 ratio while trying to get the most performance out of those DDR2-1200 DIMMs. Realistically speaking, getting 600 mhz FSB on a Core 2 motherboard is damn near impossible.

There are also other memory dividers (or ratios) you can experiment with that will run your memory BUS (and RAM) at a speed faster than your FSB, which might also reduce your memory latency a bit. However, tighter timings (and 1T command rate) seem to matter more on Core 2 systems. The additional memory bandwidth that should be available while running a ratio like 1:2 never materializes on Core 2 systems since your actual memory bandwidth is limited by your FSB speed.

On an AM2 system, just about any memory speed will do bandwidth-wise. What you need is the lowest possible latency. Due to the memory controller on AM2 chips, mostly this involves running your memory as fast as you can, timings be damned. My basic strategy was to set my timings at 5-5-5-15 2T and start using ratios like 1:2 or 2:3 to achieve high memory speeds. Tight timings didn't help much, but memory speed did.
 

DrMrLordX

Lifer
Apr 27, 2000
20,670
9,778
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Both are probably D9s, I guess I'd go with the DDR2-1066 stuff. Better rebate on them, and a better rated speed at the same voltage with the same timings. Should be fine for most any Core 2 system.
 

cmdrdredd

Lifer
Dec 12, 2001
27,039
344
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Originally posted by: DrMrLordX
Both are probably D9s, I guess I'd go with the DDR2-1066 stuff. Better rebate on them, and a better rated speed at the same voltage with the same timings. Should be fine for most any Core 2 system.

Crucial stopped using the original D9s and swapped to a much lower quality IC...gotta check xtremesystems to find out for sure. I'll get back

edit: after checking around I didn't see anything to backup my info... still says D9GMH everywhere. They're good.
 

AlabamaCajun

Member
Mar 11, 2005
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Unless you have to money for DDR2-1000-1200 just get DDR800.
GSkill, OCZ or Winteck. They will do 900 at CL5-5-5-14-25 at 2.0V. Bandwidth is around 9100Mbs which is fast.

DDR2-800 is Ram running at 200mhz with double data rate reading 2 bits per clock to achive 400 then adding dual channel gets you 800.
Best thing to work with in calculating ram speed on AM2 is to forget 1:1, 2:1 that's confusing and irrelevant since all ram speeds are based on numerated dividers off the CPU speed. What you have to do is find the right divider based on BIOS computed RAM timing.

The formula is CPU stock speed/ 200 rounded up giving you the divider for DDR2-800. Then divide this number into the overclocked CPU speed to get OCed ram speed.

Stock 1900/200 = 9.5 or 10; 1900 /10 yields 190 x 4 or DDR2-760. Some feel ripped off, just oc a little to reach DDR2-800
Oced 2000/200 = 9.5 or 10; 1900 /10 yields 200 x 4 or DDR2-800. Most systems will do this with a settable HTT.
Oced 1900/166 = 11.4 or 12; 2400 / 12 yields 200 x 4 = DDR2-800. You really don't loose any ram performance
Oced ---------------------------; 2700 / 12 yields 225 x 4 = DDR2-900 Even cheap DDR2-800 should do this on 1.9-2.0V and CL5 getting about 9000 MBS
 

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