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View Full Version : How do memory dividers work?


sillyeagle
04-18-06, 02:03 AM
I can't find memory divier info or charts anywhere on the net, not in any overclocking guide, so I have had to attempt to figure it my self. My main concern is that I run my "FSB", and therefore my CPU, at the speed that will run my memory at its rated speed, since it has little to no OC headroom.

So in order to do that, is this right?

200MHz FSB @ 2x = 400MHz Mem STOCK

200MHz FSB @ 1.66x = 333MHz Mem Mem divider dropped one notch

240MHz FSB @ 1.66x = 400MHz Mem The FSB speed I need in order to run my mem at its rated speed.

I've been running at 245MHz, which if I have figured it correctly I have been running my mem at its full potential, but I jsut want to make sure I've got it right.

Other than my mathematical hack job here, I would be very appreaciative if somebody can tell me the actual formula or whatever is used for calculating memory speeds.

ViN86
04-18-06, 02:22 AM
with the onboard controller on athlon processors, the memory divider you refer to, actually is a divider for the clock speed. (if you want to see it in action checkout CPUZ, which will give "CPU/n" where n is some integer for the FSB : DRAM ratio)

ill break it down for you. first off, the dividers you listed (2x and 1.66x) usually read on most boards as 200 and 166. figuring out what they do is very simple, and can be done with your head, a piece of paper and a pencil, or a simple calculator.

first, you want to find the ratio of your divider with respect to 200 (the norm for athlon64's). lets take your 166 for example. first off, lets remember that our 166 value is actually a 166.666(repeating 6) value, and that 133 is also 133.33(repeating 3). now, if we divide 200 by 6, we get 33.333333... then we multiply this value by 5 and we get 166.66666...
(a simpler way to do this is divide 200 by 166.6666 which yields 1.2)

this shows that 200 (our normal speed) is 6/5ths of 166. so to find the FSB value at which our memory will still run at 200, we multiply 200 by 6/5ths and get 240, which is the speed you found.

one more example should help:
say we have a 150 option for our DRAM speed. we divide 200 by 150 to get 1.3333333 (which is actually 4/3). we then multiply this by 200 to get 266.666. if we wanted to OC to 2.666 GHz, we would run a 266 FSB with a 10x CPU multiplier and 150 FSB speed.

get it? ;)

or you could be lazy like me, buy DDR500 memory, run your FSB at 250 and leave everything else the same lol.

sillyeagle
04-18-06, 05:26 PM
Wow thanks for the awesome explanation! :)

ViN86
04-18-06, 07:21 PM
np (nana2)

sillyeagle
04-27-06, 01:19 AM
this shows that 200 (our normal speed) is 6/5ths of 166. so to find the FSB value at which our memory will still run at 200, we multiply 200 by 6/5ths and get 240, which is the speed you found.


I found something interesting when I ran Everest.

It actually shows I need to run my FSB at 250 in order to run my memory at 400 when using the 333 setting.

When I use the 333(166) setting its gives me CPU/14, rather that CPU/11 which I get using the stock 400(200).

So when I overclock to 240x11 I get 2640/14 = 377(188)MHz memory.

SLippe
04-30-06, 12:34 AM
Hell if I know. I know I'm running my divider at 180 with DDR400, but I have no clue what that makes it run at. I'll look at CPUZ and check out what you said about the ratio thing.

EDIT: My CPUZ shows CPU/15. I'm running 270x10 with 180 divider. What does that make my memory running at?