Memory vs Motherboard speeds 1

[excalibr]

Hi all,

Given that ... PC2100 = 133 Mhz DDR-SDRam (DDR 266) and PC2700 = 166 Mhz DDR-SDRam (DDR 333)

I realise memory speeds are backwards compatible, for instance PC2700 will automatically run as PC2100 when placed in a motherboard that only supports PC2100. Meaning it drops the speed down from 166 to 133Mhz to be compatible with the mobo. What about the converse? If you have a mobo that supports PC2700 but you put PC2100 memory in it, does the mobo speed drops to accomodate memory?

There are these other technologies floating around, eg. double/quad-pumped FSB, how does this affect selection of memory with respect to optimal speeds? eg. do you want to match PC3200 = 200 Mhz DDR-SDRam (DDR 400) to a mobo which runs at 200 Mhz (or 800FSB)? Or we can basically ignore the double/quad-pumped factors and consider "native FSB"? I'm assuming FSB = bus that connects CPU and memory...

Thanks, Excalibur

 

[paparazi]

Of course there are ideal senario's where everything is maxed out and running at it's design limits ie:
A 400fsb XP3.2+ CPU running on a 400fsb motherboard with PC3200 (400fsb memory) so basically all the top components for this setup.
Unfortunately not everyone can afford a 400fsb XP3.2+ , so maybe they choose a 333fsb Barton XP2.5+ with the same motherboard and memory, it just means that the motherboard and ram will by running at 333fsb and 166 instead of 200/200.
So I suppose you could argue given the above, the user should have bought PC2700 memory and a slightly older mainboard that only goes upto 333fsb but I am of the opinion that for the few pounds/dollars saved it just isn't worth it and of course you are generally buying into a slightly faster platform anyway with the potential of a small upgrade at some future point.
Martin


Replying helps further our knowledge, without comment leaves us wondering.

 

[cdogg]

It's also worth pointing out that the memory bus does NOT have to run in sync with the FSB. Modern BIOS's have settings to allow you to set the memory bus speed to a ratio of the FSB. For example, the FSB could run at 200MHz while using PC2700 DDR running at 166MHz.

Although it is possible to offset the memory from the FSB, it is typically not recommended due to increased latency. For best overall performance, you want the two to match.

Just remember that when it comes to DDR, it is running at a double multiplier of the memory bus, not the frontside bus. Since most systems have the two busses running at a ratio of 1:1, it is often misunderstood that memory speeds directly depend on the FSB.


~cdogg
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