What determines dram speed?

mact

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Motherboard rating or cpu FSB?

i.e. (I'm inventing numbers here, so they may be nonsense) suppose the cpu is fsb 1066 (which I think means 553 ram--yes?) but the MoBo talks abut DDR2 400.

does this mean that should I buy 553 ram it will ratchet down to 400 because the mobo does not support it?

(no overclocking in mind...just trying to understand enough to spec out a new system I hope to assemble in the next 3-4 months.

Like pick a chip: E6600-E6800 (the candidates at this moment, depending on cost at the time I buy, althoug the last couple I bought were AMD they seem to be at the bottom of the cycle right now).

Then which mobo is most compatible/efficient with this cpu and which spec ram ought to be paired to get the best from the cpu/mobo combo.

et cetera et cetera.

(I'm astonished at how quickly things have progressed in 3 years. I used to build a new box every 2-3 years and it was pretty straight forward. CPM>NEC 20 (z80/8086 compatible) >286-10 >386-20 (boy, THAT cost a bunch>486-33>486-66>486-133>P-133>p-166/200>P3-500>P4-3.0>etcetera.

New generation at least retained the picture that THIS chip required THAT and stuff worked from there with no apparent options along the way.

more choices now. Good, I guess.

I've gone thru the FAQ and cannot see an answer to my question, but further study is warranted<G>

TIA for guidance.
 

mact

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Better example.

a gigabyte GA-965P-DQ6. with a E6600 chip

Kingston lists 533, 667, and 800 Mhz ram for this combo.

which ram is the optimum setup? and (if you can) why?

(showing my deficient understanding....the FSB on this chip is 1066 so the "real" ram access speed is 533 so 667 or 800 is a waste because they will ratchet down to 533 anyway. Am I wrong, why?)

no overclocking.
 

AlaskaFox

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Traditionally, the CPU FSB determines the memory speed, such as your 533 (x2, 1066). however if the memory needs to be slower or faster than this, it uses its own ratio, based on the FSB. on some mobo's, you can select it as 1:1, 2:3, and so on. other motherboards have you select it differently, by selecting DDR400, DDR533 etc. basically, this is the same thing just displayed differently.
example if the FSB of the motherboard is 200Mhz. quad pumped for the proc brings this to 800 for the processor. with a gate of 1:1 (or DDR400), the double pumped memory would be at 400Mhz. different ratios and different calculations will change this, but that is the way I have come to understand it. imho, there is too much of this double pump, quad pump and so on, but ah well. takes a little math and calculations, but it is possible to figure out. even w/o the math, most motherboards are smart enough where it will work with almost anything you throw at it with auto timings. good luck and hope this poor explanation helps, heh.
 

mact

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<<xample if the FSB of the motherboard is 200Mhz. quad pumped for the proc brings this to 800 for the processor. with a gate of 1:1 (or DDR400), the double pumped memory would be at 400Mhz. different ratios and different calculations will change this, but that is the way I have come to understand it. imho, there is too much of this double pump, quad pump and so on, but ah well. takes a little math and calculations, but it is possible to figure out. even w/o the math, most motherboards are smart enough where it will work with almost anything you throw at it with auto timings. good luck and hope this poor explanation helps, heh.>>

Ummm. Errr.

Huh?

<BG>

If 1066 is the FSB on the processor, than means 533 on the ram. Right? To profitably use >533 I need to twiddle with the BIOS and change the ram timing? IF i do not do that, then an 800 set will ratchet down to 533 and the extra cost has been wasted. Right?

Wrong?
 

SuperFly03

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Let's see if I can't break it down.

You are trying to understand RAM speed and how it is calculated.

It is determined by 2 things, the FSB and the Memory divider. The FSB on Conroe chips is 1066, but that is quad pumped (meaning 4 data paths working per clock cycle). The true FSB is 266, thus 266 x 4 = 1066 effective FSB. Next is the memory divider. It is set in the BIOS automatically by reading you memory's SPD (serial presence detect) module. It is in essence a separate memory module that contains data programmed by the manufacturer which specifies what speed and settings the RAM supports. The BIOS then defaults to the highest speed multiplier supported by both the motherboard and the memory.

Example:

Max motherboard support: 800
Max RAM supported: 667

266 FSB (explained above) x 5/4 Memory Divider = 332.5 x 2 = DDR2 667

Max Motherboard supported: 533
Max RAM supported: 800

266 FSB x 1/1 Memory divider = 266 x 2 = DDR2 533

Max Motherboard supported: 800
Max RAM supported: 800

266 FSB x 3/2 Memory Divider = 400 x 2 = DDR2 800

The divider, as I said before, is detected and set automatically by the BIOS (you can change it manually if you want). So, by changing either the FSB or the divider you change the memory speed.

NOTE: this does not work in the slightest for AMD based processors, they have a very different formula.
 

Mondoman

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If 1066 is the FSB on the processor, than means 533 on the ram.
For good performance, you want at least DDR2-533 RAM speed in that case.

To profitably use >533 I need to twiddle with the BIOS and change the ram timing?
It depends on your BIOS. Some BIOSs may default to a 1:1 ratio with the CPU (DDR2-533 in this case), while others may default to the fastest speed in the RAM's SPD. Since the price of RAM can rise steeply at the fastest speed grades, and increases in performance are not great when using RAM faster than a 1:1 ratio, buying faster RAM may not be the most cost-effective purchase if you will not be overclocking.
However, C2D CPUs seem very robust to mild/medium overclocking. If you OC your CPU to 1600MHz effective FSB (400MHz clock), then DDR2-800 RAM speed is a 1:1 ratio. Such OCing (or allowing such OCing in the future) is why many buy DDR2-800 RAM these days.