Quick question for OC'ing my ram..

QuietIce said:

For AMD DDR2 systems (AM2/AM2+) this is correct. However, BIOS's use different ways to express the MemLimit - some are "400" for full speed (1:1), some are "800" for full speed just as DDR (939/940) BIOS's use both "200" and "400" to mean 1:1. Either way the 1:1 setting is running DDR2-800 on AM2 regardless of the BIOS representation (just as 1:1 is DDR-400 on 939/940).

But unlike DDR, the DDR2 memory clock is double the "reference" clock (or "FSB"), 400 MHz instead of 200 MHz at stock, which is why the divider is only half the CPU multiplier instead of the whole CPU multiplier as it is with DDR. The extra doubling of both types of memory (to DDR2-800 and DDR-400, respectively) comes from the "DDR" designation, which means "Double Data Rate". Both DDR2 and DDR use the "top" and "bottom" of the memory clock cycle - they send two packets of data per clock cycle instead of just one. So DDR2, with a stock MemClock of 400 MHz, is DDR2-800 and DDR, with a stock MemClock of 200 MHz, is DDR-400.


One more note - the DDR2 MemClock runs slightly less than 400 (at stock) on odd CPU multipliers. The reason is the formula for calculating the actual MemClock, which is (CPU speed/MemDivider). The MemDivider for DDR2 (at 1:1) is half the CPU multiplier but must always be an integer and rounded up. So the MemDivider for an x13 CPU is 13/2=7, therefore a CPU running 2600 MHz (200x13) will only have MemClock of (2600/7) ~371 MHz, which doubles to DDR2-743 instead of DDR2-800 ...

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majinwar said:

I have DDR2 PC6400 ram and in the explanation it says to set the speed to 200mhz (1:1). So I set my ram to DDR 400 in the bios to obtain the 1:1 ratio.

This is the part im stuck on... let's say i can get it to load with 250 FSB, does this mean that when I overclock everything, the max my ram can go is 500mhz? (aka 1000mhz) I feel noob that I do not remember this, but help would be appreciated!

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shirker said:

huh??? sounds like a random tangent to me. if you've got a 9x multiplier running at a 260MHz fsb speed, nowhere in that equation will you see the numbers 486 or 1.8, unless you have a wack memory divider setup..

so with a 260MHz FSB:
your cpu will be running at 2.34GHz (260MHz * 9 = 2340MHz)
your memory will be running at 260MHz, same as the FSB, but since DDR2 is "double data rate", your memory would be running at a DDR2-520 rate (2 * 260MHz)

your memory is rated DDR2-800, which is a 400MHz fsb speed, so you're underclocking your memory quite a bit, like i said

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meionm said:

486 probably is on a divder.

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AlabamaCajun said:

Clock speed x (LDT or K8-HT) Multi = HT speed (IO Bus) 1000 (1000 x 2 = 2000 ).
200 x 5 = 1000 x 2 = 2000

Clock speed x CPU Multi = CPU speed 1900Mhz
200 x 9.5 = 1900

RAM Speed = CPU speed / RAMDiv = RAM speed X 2 = DDR2 speed. RAMDiv = a calculated number your BIOS looks up in a table to match your CPU speed for stock DDR2 speeds. It runs at 200 but is quad pumped to provide an effective speed of 800. Actual speed onboard the Dimm is 200Mhz, the Dimm to CPU connection runs at 400 and the DDR rate doubles dataspeed making it 800.
1900/10 = 190 * 4 = 760 10 is the resulf of RAM not having a half divider so it's rounded up from 9.5.

More in this thread. http://www.ocforums.com/showthread.php?t=521670

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shirker said:

I stand very corrected. In theory you're overclocking it correctly, and those cpu-z screens are accurate and are what you are running. What really threw me off from the get-go was the 486MHz. I didn't check your math and so I completely missed that, and I tried to get 486MHz by doing various calculations and wasn't getting anywhere. Secondly, I've been looking at nothing but intel cpu overclocking lately, and the fact that AMD cpus have integrated memory controllers was completely lost on me. It wasn't until I saw your cpu-z screens and the 468MHz memory speed that I realized what was going on. Apologies for confusing the heck out of you

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