DDR ram questions (long post)

[El_Brio]

Ok, so I am not exactly current on DDR ram and DDR mobos. I have a few questions:

As far as I can tell, ram speeds (ex 2100, 2700, 3200) correspond to 266mhz DDR, 333mhz DDR and 400 mhz ram clock speeds? So if I buy an a KT333 mobo and run the ram @ 333 mhz DDR the FSB will be 333/2 or 166mhz. So my proc speed will use a multiplier * 166 to get my cpu speed? So if you have a KT266 mobo, you buy 2100 ram which is 266 mhz DDR . So the FSB would be 266/2 or 133 so the proc speed is X*133. Is this correct? If so then what the heck is 3200 for? Is that 400 mhz DDR memory which uses a 200MHZ FSB? But as far as I can tell mobo’s only support memory speeds of 333 (FSB of 166). Maybe I am all confused. This is what I have managed to gather from various sources. I am probably completely wrong though. I built my last rig back in the good ol’ days of sdram. So if the various ram speeds (2100, 2700, 3200) correspond to different speeds (266 mhz DDR, 333 mhz DDR, 400 mhz DDR) why don’t they just call them 266,333,400 etc? One final question: the Athlon XP speeds ratings (2200 etc) I know they are approximate speed comparisons to P4 clock speeds. So the MHZ equivalent of these procs are actually quite different from the “named speed”. What I was wondering is what FSB settings are used to determine the clock speed? Do they test them at 200, 266 or 300 DDR (100, 133, 166 mhz multipliers?)

[BipolarBill]

The larger numbers represent bandwidth as opposed to frequency. Hence, 3200 = ~3.2GB/s. It's not exact - it just gives you an idea how each memory type scales.

CPU speed doesn't change with faster memory - memory speed does. CPU speed in MHz is computed by FSBxMultiplier alone. If you put DDR333 in a board and run it at FSB 266, the CPU speed is the same. If you could get it to run a 166Mhz, you would be overclocked by 25%. The Athlon won't tolerate that. There were plans for a 166MHz AMD CPU - we'll see.

[El_Brio]

so you put ddr333 in a board (pc 2700 correct?) and run the FSB at 133MHZ (266 ddr). So the athalon wont run at 166 FSB so the extra capability of the ram is wasted since you could get by with a 2100 stick (ddr266). (BTW i know that proc speed doesn't change becuase of ram, but with different FSB speeds you use different multipliers and get different speeds: ex 10*133 = 1330 mhz proc speed 13*100 = 1300) Memory speed is limited by the FSB setting correct? So if you have ddr333 in a board where the FSB is clocked to 133 or 266 ddr the stick will only run as fast as pc2100 (which is ddr266 mhz)?

[BipolarBill]

Yes to all. You do, however, benefit from the bandwidth increase. More is nearly always better. ;)

Remember - MHz is the speed of the memory where the 2100/2700/3400/4000 is the width of the pipeline.

[Rugor]

Actually the memory speed is not always dependent on the FSB. It depends on whether your chipset runs in synchronous or asynchronous mode.

I have a Gigabyte GA-7DX with the AMD 761 chipset which is synchronous in nature and only runs the Memory at the FSB. So in order to take advantage of PC2700 I would have to raise my FSB to 166MHz (333 DDR). Intel's 440BX chipset was also synchronous.

However the Via chipsets are all Asynchronous. They normally let you clock the Memory bus at either FSB+PCI or FSB-PCI. So the KT333 chipset lets you run a 133MHz FSB with a 166MHz Memory bus. It will give some boost in performance in most cases, not as much as running 166/166 but some.

Hope this helps.

[BipolarBill]

I must have gotten ripped off. The MSI KT3 doesn't let you turn up memory speed past 266(133Mhz). It does, however, allow pushing the FSB speed to 166Mhz.

[otheos]

Bill, the mem bus can and should run async to the FSB. 133/166 is what the KT333 offers, so that both CPU and mem run at full speed and within specs. I have no personal experience with that MSI, but every other KT333 and SiS745 chipset based motherboard I've seen has a setting in the bios to set the mem to 166Mhz asynchronously to the FSB.

[Turnip12]

The MSI KT3 lets you bump up your memory to 333. Check out your BIOS settings, you can choose FSB + 33 which would equal 133+33, x2 (DDR) bringing it up to 332, close enough to 333.

[BipolarBill]

Thanks guys - I'll check it out when my RMAed CPU arrives. :p