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fsb:ram ratios have me confused

Last response: in Motherboards
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November 16, 2007 1:52:35 AM

i just need a confirmation on this:
from what i understand, you will not always have a 1:1 ratio when you buy a pc (prior to overclocking).
here's an example:
q6600 runs at 2.4ghz with multiplier of 9 means it runs at 266mhz. ddr2800 is 800mhz effective but clock is 200mhz.
this is a 266:200 ratio.
so you will not always be getting a 1:1 ratio. am i right?
November 16, 2007 2:19:04 AM

I would say...somewhat right :) 

The DDR is Double Date Rate, so actually that DDR2-800 /2 = 400mhz, but your point remains about the ratio. However, as far as I know, having a CPU:RAM where CPU<RAM doesn't hurt your performance at all, it just means you bought memory that you aren't making full use of.

If you wanted that Q6600 to run 1:1 at stock settings, you could buy DDR2-533 and be fine (533/2 = 266) unless you wanted to overclock or tighten the memory timings (5-5-5-18 to say 4-4-4-12).

DDR2-800 would let you OC the Q6600 all the way up to 3.6ghz, at which point you'd need some pretty serious cooling :)  I like DDR2-667 as it will get the Q6600 to the 'free' 3.0ghz OC if you're using a P35 mothboard (1333fsb). Though if you carefully comb through prices you'll probably find DDR2-800 or only like $10 more than 667.

Anyway.. I hope that made sense, I think I strayed a little there :) 
November 16, 2007 2:20:22 AM

Effective clock of DDR2 800 is 400mhz... but you are correct, you wont always have a 1:1 ratio. Just alot of people believe they get best OCing results off of 1:1 (which seems in general to be true)

EDIT: Seems Jevon got there before me
Related resources
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a b K Overclocking
November 16, 2007 11:18:51 AM

By way of added: The FSB runs at 4x the base speed. Your RAM runs at 2x. So a 1066 FSB has a baseline of 266 Mhz, 266 x 2 gets you to DDR2 533 memory for 1:1 at stock speeds. So if you decide your overclock target in advance you can do the math in your head and figure out what speed memory you need to buy.

Now, 1:1 is an 'ideal' that may or may not be fastest for your system, depending on the parts you have assembled. If I understand it properly, it's more important for an AMD setup than an Intel. But it's not necessary, since you can set the memory divider to run at the ratio you like - An easy example might be running a stock clocked CPU on a 1066 bus with DDR2 1066 memory. At 1:1 that'd only equal DDR2 533. But your RAM is twice as fast at that, so you can set the memory divider to 1:2 and run it at that speed.
November 16, 2007 6:14:26 PM

thanks for the replies guys , im glad i understand it the right way. i follow you completely, but i have a question:
jevon said:

I like DDR2-667 as it will get the Q6600 to the 'free' 3.0ghz OC if you're using a P35 mothboard (1333fsb).

i understand what you mean here - 333mhz base memory @ stock, q6600 @ stock 2.4ghz or 266fsb overclocked to 366mhz fsb will get you a 1:1 with ddr2 667@333mhz and q6600@333mhz fsb. assuming everything is fine.
but what if you decide to overclock further? are you limited by the motherboards max fsb of 1333mhz?
i dont think so, because otherwise how are people overclocking it past 3.0ghz, which i know some are?


btw thanks for the exact respones - i always get people who beat around the bush on other forums and never anwer
January 18, 2010 8:40:17 PM

Ok, help needed, my RAM is currently running at an FSB:D RAM ratio of 3:1.....is this good bad or ugly............I dont know as this is the first PC ive ever built
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