A few FSB/Clock timing questions

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ThirtySixBelow

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Dec 4, 2004
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I'm starting to get confused over all this cpu/memory clock speed stuff. Currently I have an XP 3200+ which runs at 400Mhz FSB. I have to sticks of corsair xms running at 400Mhz. When I am in my bios and look at the fsb speed, it says 200, which i guess is because DDR 400Mhz really is 200Mhz x 2. If i'm wrong there , that might be a start. But, now say I get an AMD 64 3500+ or something with 1Ghz fsb. How does that break down, 250x9? Is it divided 4 times for a bus speed of 250? I'm not seeing how I go from CPU fsb, to what I see in my bios as 200Mhz. With this being the case, memory is causing the same confustion. Say i want to get DDR2 533. Will that be 266 in bios, meaning i will only have 16Mhz of a ceiling to OC with on the fsb? Even so, the timings seem to be low, in this case:
http://www.newegg.com/Product/Product.asp?Item=N82E16820146428

And with this, are the very high ceilings of say the DDR2 1000, only for overclocking purposes. Because DDR2 1000 would lead me to beleive that I could forseeably OC my 250mhz althon all the freakin way up to 500 before the memory would shit. This being the case if you kept the memory speed down to your native cpu of 250, could you lower the timings a whole lot then instead? If i'm totally off someone save me :p
 
First off, you have the bus speed/memory speed correct for the older xp processors.

Second, just toss all your xp knowledge out the window for now and dont try to make comparisons between bartons and a64's. Initially it will just confuse you.


heres a starting point:

venice +3000 is 200x9 = 1.8ghz
+3200 is 200x10 = 2 ghz

Obviously thats basically the same way cpu speed is calculated for the xp series.

the K8 generation throws one more multiplier in the mix that is independant of cpu/ram speed. Thats what you are confusing as "1 ghz front side bus" in you previous question.

The HT link multiplier, at its stock setting is x5. The 200 from above x the HT link multiplier gives you the 1000 figure (hypertransport link) thats confusing you. Just a simple rule of thumb, keep that number under 1000...generally as close as possible to 1000, but dont be that concerned with it.

as you increase your overclock (changing a stock venice +3000 from 9x200 to 9x250) ~ you need to keep dialing the HT link multiplier back a bit to keep it under 1000 total or close to. (250x5 (ht link multiplier) is 1250. Over 1000. So you'll want to knock that multi down to x4 to bring it back to 1000



Does that make sence so far?
 
Yes, thank you, that does make sense. So is their a cpu multiplier like normal for the 200x10 and then in the bios there will be a "fsb" multiplier? And just keep these in tandum under 1Ghz? Also, with the ram, does that mean if I get that DDR 533 I would be able to push the 64 up to a 266 bus speed before maxing out the memories frequency? In which case I would have to back the fsb mult. down. Even though only going to 250 seems best because then you can use the multiplier to get an even 1Ghz, whereas if you went to 266, you would need a multiplier of 3.5 to stay under 1ghz which is only 900, assuming they go in +-0.5 increments... It seems to make sense as long as the above is correct :p

Ram frequency is double the bus speed, and cpu is 4x bus speed with an extra multiplier?
 
I would suggest reading Eclipse's guide to OC'ing A64s (its a sticky in the OC'ing and Cooling forums), it should answer most of your questions. And by the way, K8 processors (A64s, Opterons, FX, X2) only support DDR. DDR2 support will come with Socket M2.
 
ThirtySixBelow said:
Yes, thank you, that does make sense. So is their a cpu multiplier like normal for the 200x10 and then in the bios there will be a "fsb" multiplier? And just keep these in tandum under 1Ghz?
Click to expand...
Essentially yes.


ThirtySixBelow said:
Also, with the ram, does that mean if I get that DDR 533 I would be able to push the 64 up to a 266 bus speed before maxing out the memories frequency? In which case I would have to back the fsb mult. down. Even though only going to 250 seems best because then you can use the multiplier to get an even 1Ghz, whereas if you went to 266, you would need a multiplier of 3.5 to stay under 1ghz which is only 900, assuming they go in +-0.5 increments... It seems to make sense as long as the above is correct :p
Click to expand...
You are on the right track. Dont be worried about making your Hypertransport Link exactly 1000 (and stop calling it FSB :) ) As long as it's at least 700 or more you will be fine. I've been told the reason for this is the HT link bandwidth isnt something any of us are really anywhere near using all of yet.

As far as the memory goes, thats another thing thats very different from the xp series. The memory controller being built into the A64 allows for dividers to be used to manipulate ram speeds with little to no performance drawbacks. Don't be that worried about trying to get high priced high speed ram, as anything over DDR400 is mostly just for bragging rights and benchmarking. (but yes a higher ram clock will give increased overall performance, just not that much to split hairs over) Overclocking with an a64 is all about overall CPU mhz.


ThirtySixBelow said:
Ram frequency is double the bus speed, and cpu is 4x bus speed with an extra multiplier?
Click to expand...


cpu speed is bus speed x cpu muliplier.

It will be alot easier when you have a board infront of you to play with.
 
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