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Dividers? Still don't get it..

post #1 of 21
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Hey,

To really come to the point, I still (after a year of overclock.net) don't really understand what the dividers are. I don't need to know what they are, but I want to know how to use them.

I get it a little, but don't really understand it fully. If I want to run 1/1 and my CPU is @ 2.6 ghz -10x26- what divider should I then use and how can I get it 1/1? Also, how can your RAM be faster than FSB? Please help..
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post #2 of 21
I'll give it a shot. You know the CPU has an external frequency of 200MHz, and runs on a mulplier on top of that frequency (your 3700+ has a multi of 11, thus running at 2.2GHz). DDR RAM works the same in a way, and DDR 400 runs on an external frequency or also 200MHz. With everything at stock values, the ratio between RAM external frequency and CPU external frequency is 200:200 or 1:1, which we call the 1:1 divider. Now, I guess you knew all this.

When you overclock, you will increase the external frequency of the CPU. However, despite the fact that you set your RAM at, for example, DDR 400 speeds (or 200MHz external RAM frequency), your effective RAM still will go up when you increase the external frequency of the CPU! In this specific case, it will go up with the CPU on a 1:1 ratio. Thus, when you overclock your CPU 10%, the CPU external frequency will be 220MHz, the CPU clock speed will be 2.42GHz (11x220), and the external RAM speed 220MHz, thus resulting in DDR 440 speeds. (remember, we're now still on a 1:1 divider).

Now, obviously, RAM can take only so much, and often less than the CPU. Let's assume that 10% is the max for the RAM. The 1:1 divider would then limit the CPU clock to 2.42GHz. You want to get higher though! Thus, time for a divider. You set the divider my adjusting the RAM frequency in the BIOS. For example, we want to get a bit higher, we set the RAM frequency at DDR 333, or 167MHz external frequency (whichever option your board offers). Now, at stock, external RAM frequency is 167MHz, and the external CPU frequency is 200MHz, or a ratio of 167:200, thus a 5:6 divider.

Now, lets increase the CPU external frequency to 240MHz, hitting a CPU clock speed of 2.64GHz (11x240). RAM frequency will increase together with the CPU frequency, but since we now set a divider of 5:6, we will get the following outcome: External RAM Frequency = 240/200 x 167 = 200MHz, or DDR 400 speeds. Hey, RAM is back at stock speeds, with a good overclock on the CPU! I hope you understand the logic behind this now.

Now, some motherboards allow you simply to set the RAM frequency higher than the CPU frequency, but this is not really a helpful function. First, you'll need DDR 500 or so RAM to do this, adn second, there is not a noticable increase in performance when you simply increase RAM speed (it won't be able to handle more than the CPU delivers anyways).

Now, what you should try is to find the Max CPU speed (knock RAM down on a heavy divider, and crank the rig up as far you can), the Max RAM speed (lower the multi, and keep going on 1:1 until the RAM does not want anymore), and the Max HTT of yuor motherboard (lower both RAM frequency and multi and clock the crap out of it). Now, when you know those values, you can try to find your ideal overclock. Let's assume your CPU maxes out at 2.8GHz and the RAM at DDR 440 or so, while the FSB can take a beating higher than those values (thus excluding it as a limiting factor).

For your CPU to hit 2.8GHz on a multi of 11, the external frquency has to be 2800/11 = 255MHz. On a 1:1 divider, this would result in a RAM speed of DDR 510, exceeding the RAM limit. Thus, we need to set a divider. The highest external frequency the RAM will do is 220MHz, while the CPU external frequency is 255MHz. This is a 220:255 or 44:51 ratio. Ideally, we want to set RAM on this divider, but your board will not give this option. At stock, the desired RAM speed to set for this divider is 400/51*44 = 345MHz. Then we look for the next lower available setting, which is DDR 333 speed! Thus a divider of 5:6 will work in this case.
    
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post #3 of 21
I would go with the Swiftech they are talking about:

http://www.swiftnets.com/products/H20-220-APEX.asp

The thread above talks about the flow rates, etc.
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post #4 of 21
Errr huh? Flow rates v. RAM dividers
    
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post #5 of 21
You can check out my FAQ's...they may help you a bit
 
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post #6 of 21
Seems like I just retyped your FAQ in my post.. LOL
    
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post #7 of 21
You are a braver man than I Chozart....

On the DIE front...all the ASIC's and the CPU, etc. are running a bus protocol that allows them to speed up and slow down the data transfer rate based upon what multiplier they are currently running. So, the RAM is sending a signal saying wait while the CPU is busy asking for blocks of data. So, the CPU does something else until the RAM is ready to do a block transfer.

Also, the various chips have buffers. It's far more efficient to transfer whole blocks of data than to send tiny words (32 bits) one right after the other.

I think it was Compaq who first pioneered the whole double/quadruple word buffered transfer mechanism for RAM (WAY pre-DDR technology). They did it on their high end servers (Proliant 1000, 2000, etc).
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post #8 of 21
To help understand what dividers are possible depending on hardware- have a look at this.
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post #9 of 21
One thing to note: Ram speed does not always _equal_ FSB*(5/6) or *(3/4)...it's not absolute.

When you set a 5/6 divider at fsb=240 and you're running a 3700 (11x): CPU=11x240=2620. Ideal Ram speed = 240*(5/6) = 200. HOWEVER, if you look in CPU-Z, you'll notice Ram speed is given by CPU/12 or something: let me explain.

The ideal ram speed is 200, but the ram can only run at a number that divides 2620 evenly. The computer will run your ram at the fastest number that goes into 2620 without breaking the 200mhz limit. SO, the computer will look at 2620/11=240, nope not low enough. 2620/12=218, nope not low enough. 2620/13=201, not low enough. 2620/14=187MHz. Lower than 200. Therefore, your ram would be running at 187MHz, not 200MHz. Again, why? Because the ram ALWAYS, always runs at a divider of the cpu speed. 200 does not divide into 2620 evenly, but 187 does.


At stock, CPU=200x11=2200. What is ram speed? Well, the bios setting will be at 1:1 so the ideal frequency = FSB*(1/1) = 200*(1/1) = 200. The ram will run at CPU/12 = 200MHz. Not bigger than the ideal frequency, so it's good to go.

Get it? When I was learning, it was all really confusing. Just make settings in the BIOS and see what CPU-Z spits out. Draw conclusions from there...

EDIT: Please correct me if I'm wrong. I know this happens on my motherboard, but I've been surprised in the past how much stuff I know that only applies to my motherboard. Don't ask me how it deals with half multipliers (8.5, 9.5, 10.5). I really don't know. Can't figure it out...
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post #10 of 21
I assume that applies when you set RAM to AUTO in BIOS. Since if you set RAM manually in BIOS, it'll run at that speed (provided it boots). In this case, we're talkiing about manually setting the RAM speed and divider, and then you can exceed 200MHz
    
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