FSB and memory multiplier

R4z0r · 7 Aug 2007 at 19:49

Hi All,

I was reading the DS3/4 guide over in the motherboards section

http://forums.overclockers.co.uk/showthread.php?t=17636047

And I came accross a bit that says set the memory multiplier to 2. I understand this lets you achieve a higher system FSB so you can clock your CPU more but surely this halves your memory bandwidth? Am I right in thinking you'd need an FSB of 400Mhz just to get back to the standard memory speed of 800Mhz for a C2D/DDR2?

Have I got this completely wrong or do the benefits of higher CPU clock speeds outweigh what you'll lose in memory bandwidth?

Thanks!

R4z0r · 8 Aug 2007 at 15:18

Anybody?

Yesam · 8 Aug 2007 at 15:39

If your FSB is 400mhz.. and you FSB : DRAM is 1:1 it would mean that your memory would be running at 800mhz.

By changing the RAM divider it allows you to run memory faster that the FSB, some would argue that this doesn't boost performance that much (on intel setups).

R4z0r · 8 Aug 2007 at 16:41

Surely changing the RAM divider will make the RAM run slower? For example, set RAM divider to 2 and FSB to 250Mhz, your memory would be running at 500Mhz rather than the stock 800Mhz?

GeForce · 8 Aug 2007 at 16:43

Yep, a divider is to make the DRAM run slower.

Usually used because the RAM can't run as fast as the FSB, so using a divider allows you to increase the FSB (for overclocking the CPU) whilst keeping the RAM at a frequency it is happy at.

Cob · 8 Aug 2007 at 17:18

The DS3/4 can't run the memory slower than the FSB anyway (P965/P35 chipset limitation).

simonnance · 8 Aug 2007 at 17:33

FSB x 2 x divider = ram speed

so 266 x 2 @ 1:1 => DDR2 533

therefore 266 x 2 @ 2:1 => DDR2 1066

most dividers are like 5:4, 4:3, 3:2, 1:1 ect and would give
266 x 2 @ 1:1 => DDR2 533
266 x 2 @ 5:4 => DDR2 667
266 x 2 @ 4:3 => DDR2 709
266 x 2 @ 3:2 => DDR2 800
266 x 2 @ 2:1 => DDR2 1066

Allowing you to utilise faster ram with CPUs running at stock.

Overclocking requires you to fiddle with the divider to get the optimum settings, or just to avoid crapping the ram out before the CPU hits its max.

str · 8 Aug 2007 at 20:23

Cob said:
The DS3/4 can't run the memory slower than the FSB anyway (P965/P35 chipset limitation).

Can you elaborate on that a bit more with some examples?

For example will a DS3/4 work with DDR667 and a E2160 overclocked to FSB 333 meaning the memory will be running at stock or will it be pushing the memory way beyond DDR667 due to the increased FSB?

Cob · 8 Aug 2007 at 20:48

I mean that the memory must be able to match the FSB.

Eg- using the lowest available memory multiplier/divider (x2.0/1:1) on the DS3/4/any other P965/P35 motherboard, the PC5300 will be running at 666mhz on a 333mhz FSB.

The memory will only run faster than the FSB if you select a higher memory multiplier (x2.5 is the default memory multiplier btw).

str · 8 Aug 2007 at 23:48

Ah I think I get you. So for a E6850 will need at least DDR667 as the CPU needs FSB 333 - slower memory just wouldn't work as there is no way to divider it below the 333 FSB?