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RAM and FSB question

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March 30, 2007 1:28:04 PM

Scratching my head over this one now.

What is the point of running your RAM at 400MHz and your FSB at 266MHz? Because that is how my system is running currently. I just put it together...still using stock defaults.

CPU: E6600
Mobo: Asus P5B-Deluxe
RAM: 1GBx2 Crucial Ballistix PC2-6400 4-4-4-12

More about : ram fsb question

March 30, 2007 2:28:07 PM

First your memory can run at 4-4-4-12. Change that in the bios. It is supposed to default to 5-5-5-18 like you see. If you run your RAM at 1:1 with the FSB (266MHz) then your Ram would be operating at a measly 533MHz. If you clock up the RAM to 400MHz, (which is more than the FSB) then you get DDR2-800, which is (just a little) faster than DDR2-533. So you run the memory faster because 1:1 with the stock FSB is slow. (compared to the RAM's max speed which is DDR2-800)
March 30, 2007 2:58:19 PM

RAM Breakdown

That will explain, rather show you some of the benefits of using DDR2 800 over DDR2 533.

The thrust of the article is, 1:1 is optimal (esp when overclocking) and 5:4 sucks because of the increased latency from running your RAM and FSB asynchronously, 3:2 is finally where the speed of the RAM starts making up for the increased latency form asynchronous mode.

So generally faster RAM = faster computer (depending on the app)
Related resources
March 31, 2007 1:40:47 AM

So....let me ask my question in another way since I may not have gotten the point across with the original post.

Is the RAM sitting around twiddling it's thumbs waiting for the FSB to get stuff done? Or is it actually worth something to have the RAM clocked higher than the FSB?
March 31, 2007 1:50:08 AM

Quote:
Scratching my head over this one now.

What is the point of running your RAM at 400MHz and your FSB at 266MHz? Because that is how my system is running currently. I just put it together...still using stock defaults.


Maybe your not understanding 400mhz and the 266mhz speeds for the CPU?

266x9=2394mhz or 2.4ghz

So if you bump the FSB up:

400x9=3600mhz or 3.6ghz (so your rated FSB is 1600, not 1066)

Does that help?
March 31, 2007 2:09:59 AM

Yea I understand how the calculations work.

But what I'm trying to understand is how it is better to have your RAM operating at 400MHz when your FSB is at 266MHz. It seems to me that the FSB wouldn't be able to feed the RAM fast enough. The RAM would just be sitting there waiting for the FSB to catch up, right?

So wouldn't it be better to make the RAM:FSB a 1:1 ratio? That is, what benefit is there to having a 3:2 ratio? What benefit is there to having the RAM operating at a higher frequency than the FSB?
March 31, 2007 3:33:19 AM

I was never that great at explaining the memory ratios, but to understand why a 5:4 or 3:2 can benefit, your able to reach higher speeds in OC'ing. At 1:1 you be stuck at max speed that the ram can do. I did find this that explains it better:

Quote:
Intel does not like or allow this so that's where the RAM ratios come into play. Since you can't change the multiplier, each step change of the CPU FSB results in a given linear change of the RAM freq. The MOBO makers have made it where the OCing user can select a ratio that will allow higher OCs. Thus, the choices of 1:1 which is default, and 5:4 and 3:2. The 5:4 and 3:2 ratios make each step increase of the FSB effect the RAM less. This allows higher FSB with existing RAM.

The ratios are basic math. Obviously a 1:1 ratio will yield a 200FSB and a RAM frequency of 200, 205 = 205 etc. However, since Dual Data Rate RAM transfers data on the rise and fall of each cycle (each half of the sine wave for those more technically inclined) you actually get data X 2 or DDR400 at FSB200.

A 5:4 ratio is figured like this. Example: FSB 250 divided by the first number of the ratio, the CPU factor of 5.....250/5 = 50. You then multiply the memory factor of 4 by 50.....4 X 50 = 200 or DDR400. A 3:2 is for example FSB200/3 = 66.6 X 2 = 133.2 or DDR266.

This is important since right now our CPU speeds are sort of "outrunning" our RAM. What I mean by this is that we have P4s out there but there that are approaching a FSB of 300 when OCd! That is DDR600 People that are running their FSB that high are using a 5:4 or a 3:2 ratio. 300/5 = 60 X 4 = 240 or DDR480. 3:2...300/3 = 100 X 2 = 200 or DDR400.


A Noob's Guide to Selecting RAM and a CPU written by a Noob

Thought the guy did a good job explaining. I know it's better then I could explain. :lol: 

Hope that helps.
March 31, 2007 3:48:56 AM

Quote:
The 5:4 and 3:2 ratios make each step increase of the FSB effect the RAM less.


That's wrong at least from my understanding.

5:4 and 3:2 make each step increase of the FSB increase the RAM more, not less. Correct?
March 31, 2007 4:27:40 AM

Not sure if your taking it in the right context. The effect is the amount of stress the CPU puts on the RAM to make it run faster.

The CPU can go faster then the RAM. In other words the RAM is effected more when running it at 1:1 ratio, then 5:4 or 3:2. It just helps allow higher OC's on the CPU.

I may have it wrong, though... I'm ready to pass out, and go to bed. :oops: 
March 31, 2007 4:48:46 AM

OK, i get this more or less... so what would be the best OC for a C2D E6400? iv got DDR2 800Mhz memory and its running on a Asus P5N32-E SLI. Thats 266 x 4 = 1066FSB right? and 266 x 8 = 2100MHZ? The memory runs at 800Mhz so in dual channel you get run up to 1600MHZ FSB am i correct? Should you drop the memory frq and better the latency or what?
March 31, 2007 4:54:31 AM

DDR2 running at 266 is running at 533 for what you would be able to see. Double data rate is the operative part in DDR. The 2, well, I'm not really sure what that's for.
March 31, 2007 5:00:09 AM

Ok so lets see if i get this right, if you run a CPU at 7 x 400 thats 2.8Ghz and your memory will be running at 400 x 2 = 800 Mhz is that right?
March 31, 2007 5:46:18 AM

Quote:
Ok so lets see if i get this right, if you run a CPU at 7 x 400 thats 2.8Ghz and your memory will be running at 400 x 2 = 800 Mhz is that right?


Close, very close (if I understand the implications of your statement). The effective speed would be 800MHz, yes, but the actual clock speed would be 400MHz. In the aforementioned case you are assuming a 1:1 on the FSB:MEM ratio, so 400 x 1 = 400MHz (actual rate) * 2 = 800MHz (effective rate).

Also, in the above case your FSB would be 4 x 400MHz (actual) = 1600MHz (effective).

The reason we multiply by 2 on the RAM is because DDR2 is able to do 2 operations per clock cycle, whereas SDRAM is only able to do 1 operation per clock cycle. Thus, we multiply by 2 to account for that added advantage of DDR/DDR2 (the 2 denotes second generation, not twice as fast as DDR).
March 31, 2007 5:55:10 AM

Quote:
Ok so lets see if i get this right, if you run a CPU at 7 x 400 thats 2.8Ghz and your memory will be running at 400 x 2 = 800 Mhz is that right?


Looks right to me. If you want to keep the stock CPU speed, you can set the multiplier to 6 and you will be at 2.4GHz.
March 31, 2007 6:02:33 AM

Quote:
Ok so lets see if i get this right, if you run a CPU at 7 x 400 thats 2.8Ghz and your memory will be running at 400 x 2 = 800 Mhz is that right?


Looks right to me. If you want to keep the stock CPU speed, you can set the multiplier to 6 and you will be at 2.4GHz.

Not a great idea, esp when boards support 800 out of the box with out OC'ing. Changing the multi like that on a 965 chipset actually OC's the NB by a factor of (Stock Multi)/(Set Multi), so in your case 9/6 = (1.5-1) * FSB = 200, which would result in a 600MHz NB clock, not good (nor possible on all boards).
March 31, 2007 6:06:49 AM

Quote:
Scratching my head over this one now.

What is the point of running your RAM at 400MHz and your FSB at 266MHz? Because that is how my system is running currently. I just put it together...still using stock defaults.

CPU: E6600
Mobo: Asus P5B-Deluxe
RAM: 1GBx2 Crucial Ballistix PC2-6400 4-4-4-12


A good article for you :wink:
http://www.xbitlabs.com/articles/memory/display/core2du...
March 31, 2007 6:31:22 AM

Quote:

Not a great idea, esp when boards support 800 out of the box with out OC'ing. Changing the multi like that on a 965 chipset actually OC's the NB by a factor of (Stock Multi)/(Set Multi), so in your case 9/6 = (1.5-1) * FSB = 200, which would result in a 600MHz NB clock, not good (nor possible on all boards).


I'm not sure what you are saying, because the multiplier is related to the CPU and not the Northbridge. Granted the system clock will be higher, but I'm under the impression that most decent boards can handle a system clock of 400Mhz.
March 31, 2007 6:50:58 AM



So, according to that link, which one would be better?

3:2 ratio
CPU 266 * 9= 2394
RAM 266 * 2 * 1.5 = 798

1:1 ratio
CPU 266 * 9 = 2394
RAM 266 * 2 = 532

I'm guessing the RAM = 798 is better than the RAM = 532?

Yes, but the difference in performance is small. :wink:
March 31, 2007 4:02:52 PM

Quote:

Not a great idea, esp when boards support 800 out of the box with out OC'ing. Changing the multi like that on a 965 chipset actually OC's the NB by a factor of (Stock Multi)/(Set Multi), so in your case 9/6 = (1.5-1) * FSB = 200, which would result in a 600MHz NB clock, not good (nor possible on all boards).


I'm not sure what you are saying, because the multiplier is related to the CPU and not the Northbridge. Granted the system clock will be higher, but I'm under the impression that most decent boards can handle a system clock of 400Mhz.

Lowering the multiplier on the CPU has the side effect of increasing the clock speed on the NB of a 965 chipset (possibly all intel based chipsets, unsure about this). In your setup, with the 6x multi, the NB clock would be OC'd by 200MHz just from the change in the multiplier. So, yes the FSb would be 400MHz, but the NB clock would be 9/6 * 400 = 600MHz and thats pretty darn high.

Read the article here
March 31, 2007 5:00:20 PM

I guess the original question was really is the transport can take 266MHz, putting a faster ram feeding it at 400MHz can't help right? The faster ram is waiting for many cycles for the bus to take the info away.

My guess would be that in a big sequential read, what the op says is true, but when we wait for latency cycles (often), these take much less time in a faster clock speed:

4 cycles of 533MHz: 7.5ns
5 cycles of 800MHz: 6.25ns

Can someone confirm this to be *a* speedup source and add to this?
Thanks,
Jo
April 2, 2007 7:52:57 AM

Iv got a P5N32-E SLI Mobo (680i) a C2D 6400 (8x266) and 2 GB of DDR2 800 in dual channel, so does any one have an idea how i should OC this! I dont want to go overboard but a 2.8GHZ OC will be perfect! I want to try keep my mem tyming as low as possible.... is currently @ 4,4,4,12 T2. Cooling isnt to much of a issue, got fans cooling the North B. The only problem i really have is whats a good voltage to start at, stability is always a issue!
June 12, 2008 2:44:25 AM

i just bought a new ddr ram: 1gb ddr400 pc3200
but i just realised that my cpu has fsb of 533Mhz.. my motherboard has a 865G chipset and it's stated something abt ddr400 only supported when the cpu is fsb800.

so i just wan to check here if my new ram can be used? all the calculations shown earlier are very confusing but i only want to know if it can work? will it spoil my motherboard or ram?

here are some cpu-z scan of my system







the ram shown here is my old ram.. haven't installed my new one yet. scared my whole pc will screw up

need help thx
!