Help me understand intel dividers!

[FoxEye]

The problem I have is related to the "quad pumping" of C2D chips. They can apparently make 4x (FSB speed) reads/sec from the FSB.

DDR memory on the other hand can only be accessed 2x (FSB speed) times per second.

There is talk of the best overclock for an E5200 being on a 1:1 divider and setting FSB speed to 400. However doing this would give you a chip capable of 1600 FSB reads/sec paired with memory only capable of 800 FSB reads/sec.

I don't understand how this can give any bandwidth improvement over the standard 1:2 ratio, where the ram is run at twice the FSB as the chip? Don't you need the RAM to be running twice as fast to compensate for having 1/2 as many reads/cycle?

Thus for a proper E5200 oc, wouldn't you need DDR 1600?

Why am I reading that I should find a mobo with a 1:1 divider and run some DDR2 800 RAM?

Don't get it :/

 

[byteagig]

i found this post helpful -

CPU Bus is quad pumped x 64bits, so Q6600 for example is 266.5x4 (1066)
Most motherboards use dual channel ddr memories (ddr1/2/3 makes no difference).

Dual channel = 128bits, so DDR2 PC5300 for example is 266.5 x 2 (533), but you use two sticks in parallel to achieve a 128bit wide bus.

So bandwidth...
CPU 1066 x 64bits = 68224
Memory 533 x 128 bits = 68224

If you want to take the calculations all the way, remember its 1066mhz, not 1066hz, so multiply by 1,000,000 to get bandwidth in bits/second. But the example is simpler by ignoreing that



Quadpumped V Double pumped only tells half the story, the deciding factor is the width of the busses. (at least P4, and Core processors, AMD's chips and Nehalem have integrated memory controllers which do away with needing to know about ratios between FSB bandwidth and memory bandwidth).

so in your case it would be
cpu 800fsb x 64bits=51200
memory 400 x 128bits =51200

or with the fsb set at 400 and 1:1 with ram its

1600fsb x 64
800 x 128

 

[Big.Wayne]

Don't get it :/

I think you got it right, in theory anyway!

Bandwidth Balance

I will be trying to get some DDR2 running at 666MHz pretty soon!

 

[TheRedZealot]

D9 chips?

 

[FoxEye]

Ach. Simple when you know how. Ta!

I'd previously been reading a guide which said nothing about dual channel architecture, and didn't include it in the calculations.