Quick Q about Ram..

[ghosty]

I didn't know where else to put this question but anyways... does ran only run at the speed that your FSB is set at.. if its at 100 MHz does the ram run at 100MHz.. if so.. how the hell do you run DDR400MHz ram?

Thanks

 

[pspada]

DDR = Double Data Rate. So, the actual FSB speed is 200Mhz, but since you double it, it effectively operates at 400Mhz. Basically they figured out a way to use both the rising edge of the memory clock as well as the falling edge, doubling the data rate.

 

[ghosty]

so if your using a board that has a max FSB of 133.. (setting the jumper on the board to that) you can't run that 400MHz (actual 200MHz) at its best performance?

is this true or false..

 

[SpideyCU]

If your board cannot be set to run at speeds above 133 MHz (266 DDR), then yes, you can't reach 400 MHz. However, most boards will let you run the memory bus asyncronously from the FSB, but this is a chipset-by-chipset situation (older ones won't), and in some cases (nForce2), there's no advantage to doing so.

 

[Jeff7]

This all depends on what kind of motherboard you've got, or are looking at. Some things to know though:
There was SDR sDRAM, usually just called sDRAM (Single Data Rate Synchronous Dynamic Random Access Memory); it came in PC66, PC100, and PC133 - the numbers there indicate the MHz rating. There's no double rate here; the MHz rating there is what the RAM runs at.
Then came DDR - Double Data Rate. It is still sDRAM, but as pspada pointed out, it transfers data twice per clock cycle. It has a different number of pins, and uses a different socket than SDR RAM did. It comes in many varieties nowadays, the norms being PC1600, PC2100, PC2700, and PC3200. Those numbers come from the theoretical effective speeds of the RAM - PC1600 was supposed to have 1.6GB/sec of bandwidth, PC2100 2.1GB/sec, etc. PC1600 ran at 100MHz, but had an effective speed of 200, since it transferred twice per cycle. PC2100 - 133MHz, 266 effective. PC2700 - 166MHz, 333 effective. PC3200 - 200MHz, 400 effective.

So, when people talk about FSB400, that's actually 200MHz, but since it's DDR, it's effectively 400.
Hopefully that made some sense, and even cleared up things for you.

 

[ghosty]

But my question is you have to have the equal FSB to run it at that MHz...?

so your 400MHz you have to have the 200 MHz FSB correct?

 

[lbmcleod]

With new boards you can run a different CPU FSB to the memory (older ones have to be the same). Just check the manual to see. And of course the speed you want to run the memory at and the CPU FSB both have to be supported by the board in the first place.

 

[Jeff7]

Originally posted by: ghosty
But my question is you have to have the equal FSB to run it at that MHz...?

so your 400MHz you have to have the 200 MHz FSB correct?

Well I don't exactly understand the question - MHz is being thrown around wildly these days. For instance, PC2100 runs at 133MHz, but its effective speed is 266. That's how I put it - I either say 266 effective or else I just don't use the MHz at all unless I really mean Megahertz.