DDR Memory Confusion

[seismik]

Hi,

I've got a P4 2.66 processor with a 533 FSB. I was planning on buying a Soltek SL-86SPE-L (I865 S478 800MHZ DUAL DDR AGP8X 6PCI ATA100 SOUND LAN) motherboard and the some Corsair TWINX512 DDR PC3200 memory, and figured I had performance fairly well optimized for that CPU.

Then I came across some information stating that unless I'm running a P4 with a 800 FSB, running PC3200 RAM at DDR400 is useless and doesn't really give any performance gain over DDR333 with some PC2700 memory.

True? Is buying the faster RAM worth it until I upgrade to a CPU with a 800 FSB? I'm leaning towards buying the faster stuff anyway in the thought that I'll be moving to an 800FSB eventually and I'm better off to spend a little more now than the whole thing over again in the future.

Seis

 

[ScrapSilicon]

Hello & Welcome to AnandTech forums !

better off to spend a little more now than the whole thing over again in the future

you answered your own question..

 

[seismik]

you answered your own question..

But what about the perfomance question? It doesn't seem to make sense that faster RAM would give you the same performance as slower RAM, regardless of the speed of the FSB...

Seis

 

[Sharkmeat]

You be better off staying with what you got untill you get the ready for the upgrade.I suggest you buy all your upgrade parts from the same vendor.

 

[Idontcare]

Originally posted by: seismik

you answered your own question..

But what about the perfomance question? It doesn't seem to make sense that faster RAM would give you the same performance as slower RAM, regardless of the speed of the FSB...

Seis

Consider the "faster" ram as simply being rated to allow you go faster if dropped into a DDR400 mobo.

At 333MHz, both PC2700 and PC3200 rated ram will operate at PC2700 speed (give or take depending on the latency settings).

Invest in the faster ram that will last you through your next mobo/FSB upgrade if you upgrade in <1yr cycles.

 

[Pauli]

Originally posted by: seismik

you answered your own question..

But what about the perfomance question? It doesn't seem to make sense that faster RAM would give you the same performance as slower RAM, regardless of the speed of the FSB...

Seis

Look, this is a common misconception. As Idontcare already stated, the RAM itself isn't "fast". The RAM does not have a clock on it and, therefore, doesn't have a "speed". RAM is graded as being able to run at certain FSB speed. The FSB alone determines how fast the RAM runs. If the RAM is capable of running at that FSB speed, it will simply work. If it can't run at a certain FSB speed, it will cause the system to either not boot, or lock the system.

 

[Acanthus]

on my P42.4B i see only a 2% performance increase going from dual DDR333 cas2.5 to dual ddr400 cas2.5 on a SiS 655 chipset.

Im sure the 800fsb CPUs see a nice increase though.

 

[Merethrond]

Originally posted by: Pauli

Originally posted by: seismik

you answered your own question..

But what about the perfomance question? It doesn't seem to make sense that faster RAM would give you the same performance as slower RAM, regardless of the speed of the FSB...

Seis

Look, this is a common misconception. As Idontcare already stated, the RAM itself isn't "fast". The RAM does not have a clock on it and, therefore, doesn't have a "speed". RAM is graded as being able to run at certain FSB speed. The FSB alone determines how fast the RAM runs. If the RAM is capable of running at that FSB speed, it will simply work. If it can't run at a certain FSB speed, it will cause the system to either not boot, or lock the system.

The thing I do not understand is that he said his processor has a 533MHz FSB and his PC3200 RAM runs at 400MHz. So what is stopping the RAM reaching 400MHz, his FSB shouldn't, because it is running at 533MHz.

 

[peter7921]

Originally posted by: Merethrond

Originally posted by: Pauli

Originally posted by: seismik

you answered your own question..

But what about the perfomance question? It doesn't seem to make sense that faster RAM would give you the same performance as slower RAM, regardless of the speed of the FSB...

Seis

Look, this is a common misconception. As Idontcare already stated, the RAM itself isn't "fast". The RAM does not have a clock on it and, therefore, doesn't have a "speed". RAM is graded as being able to run at certain FSB speed. The FSB alone determines how fast the RAM runs. If the RAM is capable of running at that FSB speed, it will simply work. If it can't run at a certain FSB speed, it will cause the system to either not boot, or lock the system.

The thing I do not understand is that he said his processor has a 533MHz FSB and his PC3200 RAM runs at 400MHz. So what is stopping the RAM reaching 400MHz, his FSB shouldn't, because it is running at 533MHz.

You see P4 motherboards are quad-pumped which means that it send data four times per clock twice on the leading edge and twice on the trailing edge. Where DDR-SDRAM is double pumped which sends data 2 times per clock one on leading and one on trailing.
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DDR P4


The FSB is actually running at 133 * 4 which gives you the effective speed of 533MHz, where DDR-400 needs a FSB of 200 to run at 400 (200 * 2). But you can run RAM Asyncronously like with the SIS 655 chipset where you can run the FSB at 533MHz (133*4) and run the RAM at 400MHz (200*2).

 

[Merethrond]

Thankyou for the explanation peter7921.

 

[peter7921]

OMG i never checked my post after i entered it, i didn't realize my bad attempt at drawing clock pulses idn't work at all?
Weird it agot rid of all the spaces? Sorry if i caused any confusion.
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