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matching you CPU FSB speed with RAM speed

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April 5, 2007 1:43:56 AM

Hi can anyone help clarify how DDR2 RAM speeds work with CPU / motherboard FSB Speeds.

First of all i am from the background of matching your CPU FSB speed with your RAM speed.
So I knew if bought an AMD motherboard that supported for example 266MHZ fsb, included AMD CPU with an FSb of 266MHZ and DDR 266MHZ.
Same for the AMD boards / CPU that supported 400MHZ FSB, I knew i matched all speeds the same.

I alsi knew Intel boards / CPU that for example I had an an FSB of 800MHZ , you used a CPU with 800MHZ FSB and DDR400 since ram ,
This worked because the true CPU FSB was 200 x 4 = 800MHZ and the true speed of DDR 400 was 200 x2!
Because the base frequency of both the RAM & FSB before multiplication was 200MHz sync / speed matching was possible.
I also have an intel based laptop which follows the same theory , FSB of 533 and uses DDR266, again following the same rule that DDRR266 is 2 x 133MHZ and 533 fsb is 4 x 133.

Ok with DDR2 memory i have seen intel cpu / motherboards specifying an FSB of 800 to be used with DDR2 533!
now the equal multipuls / sync dosen't work?
by using the old formula now we have a true FSB before multiplication of 200 with ram speed of 266 before multiplication.

So does that mean the ram is being under clocked?? also some boards state to use DDR2 667 with an 800MHZ fsb.

I've also seen intel fsb of 1066 used with 533 ram which does seen to fit with true sync/ the old formular.

With AMD am2 boards im seeing FSB of 1000MHZ used with DDR2 800 or 667 or 533,
none of these ddr speeds match the 1000MHZ fsb , so how is syn acheived and more importantly which ram do you use ??

I know the AMD hyper transport is not a true FSB anymore but does that mean you no longer have to sync the ram with HT speed??

So you could have an HT FSB of 1000MHZ with DDR2 533? or DDr2800

Can anyone help clarify how these new FSB / ddr configurations work?? for both AMD an intel.

I always knew where i stood with the old FSB / DDR speeds but the new dont always follow the same rules.
April 5, 2007 2:11:54 AM

Quote:
Hi can anyone help clarify how DDR2 RAM speeds work with CPU / motherboard FSB Speeds.

First of all i am from the background of matching your CPU FSB speed with your RAM speed.
So I knew if bought an AMD motherboard that supported for example 266MHZ fsb, included AMD CPU with an FSb of 266MHZ and DDR 266MHZ.
Same for the AMD boards / CPU that supported 400MHZ FSB, I knew i matched all speeds the same.

actually, DDR is double data rate, which is 2x the speed of your FSB. so on socket 939, all AMD CPU runs at 200Mhz x various multiplier, and you use DDR 400 to sync with that (200 Mhz x 2 = 400Mhz)

Quote:

I alsi knew Intel boards / CPU that for example I had an an FSB of 800MHZ , you used a CPU with 800MHZ FSB and DDR400 since ram ,
This worked because the true CPU FSB was 200 x 4 = 800MHZ and the true speed of DDR 400 was 200 x2!
Because the base frequency of both the RAM & FSB before multiplication was 200MHz sync / speed matching was possible.
I also have an intel based laptop which follows the same theory , FSB of 533 and uses DDR266, again following the same rule that DDRR266 is 2 x 133MHZ and 533 fsb is 4 x 133.

this is correct
Quote:

Ok with DDR2 memory i have seen intel cpu / motherboards specifying an FSB of 800 to be used with DDR2 533!
now the equal multipuls / sync dosen't work?

FSB 800 is only the specification, which means the maximum safe FSB speed is 800Mhz. you can get higher than that, but that would be overclocking...

DDR2 533 is also double data rate, which is 533 /2 = 266 Mhz. Intel's Core 2 speed is 266Mhz x various multiplier. this means that the computer is still synchornized. FSB is 1066Mhz, since its quad pumped (which means 4x 266Mhz)

Quote:

by using the old formula now we have a true FSB before multiplication of 200 with ram speed of 266 before multiplication.

things changed a little after Intel switched to DDR 2. However, they are still basic multiplications. you just need to get the concept right, and the rest is fairly easy
Quote:

So does that mean the ram is being under clocked?? also some boards state to use DDR2 667 with an 800MHZ fsb.

same thing. DDR 2 667 means the corresponding FSB should be 667/2 = 333 Mhz. 333 Mhz * 4 will equal 1333Mhz, which is Penryn's FSB.
Quote:

I've also seen intel fsb of 1066 used with 533 ram which does seen to fit with true sync/ the old formular.

look above :p 
Quote:

With AMD am2 boards im seeing FSB of 1000MHZ used with DDR2 800 or 667 or 533,
none of these ddr speeds match the 1000MHZ fsb , so how is syn acheived and more importantly which ram do you use ??

I know the AMD hyper transport is not a true FSB anymore but does that mean you no longer have to sync the ram with HT speed??

So you could have an HT FSB of 1000MHZ with DDR2 533? or DDr2800

basically, AMD CPUs are still running at 200Mhz base speed, with various multiplier. HT is running at 5x the speed of base speed, 1000Mhz.

AMD implemented RAM speed adjuster (i don't think its called adjuster. but it basically means the CPU will automatically adjust the RAM to run asynchornized. for example, using DDR2 800, CPU will then run at FSB : RAM = 5:4. using DDR2 667, CPU will then run at 5:3.3. although it will still work, but there will be latency penalties.

i know my articulation sux. maybe someone like jack or turpit may explain better than i do. :) 
April 5, 2007 3:24:39 AM

so basically if your AMD 2000 HT(1000MHZ) which is 200MHZ x 5 running with DDR 800MHZ (400 x2 )the memory controller will use a ratio between the ram and HT speed to sync.

or does it under clock the ram ?? since there is no DDR2 with a bus base speed of 200MHZ

In regards to intel i can see there is some examples that uses the same sync methods of the FSB to RAM

but what about the odd ones like intel FSB 0f 800MHZ used with DDR2 ram like 533.............800MHZ (200 x 4) & 533(2x 266)

in this istance is the ram being under clocked??
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April 5, 2007 4:49:53 AM

Quote:

in this istance is the ram being under clocked??


Yes, is that a problem? This is one reason why I tell people who are buying C2D setups that they don't need to worry about buying DDR2-800 ram. Unless your doing some monster overclocking, DDR2-667 still allows for a healthy bump in frequency. (333MHz instead of 266.) If your running stock, and a 1:1 ratio, there is little reason to buy the more expensive 667/800 ram.

As for running the FSB and RAM in sync, I don't think that's as big a deal as it used to be.
April 5, 2007 5:48:29 AM

Quote:
so basically if your AMD 2000 HT(1000MHZ) which is 200MHZ x 5 running with DDR 800MHZ (400 x2 )the memory controller will use a ratio between the ram and HT speed to sync.

yep
Quote:

or does it under clock the ram ?? since there is no DDR2 with a bus base speed of 200MHZ

CPU will underclock the RAM only if it cannot find a suitable ratio between them. for example, if you pitch Core 2 stock with DDR 2 800, the actual speed of RAM will be 799 (1:3)
Quote:

In regards to intel i can see there is some examples that uses the same sync methods of the FSB to RAM

but what about the odd ones like intel FSB 0f 800MHZ used with DDR2 ram like 533.............800MHZ (200 x 4) & 533(2x 266)
in this istance is the ram being under clocked??

currently the only processor supporting FSB 800 is Prescott and Core 2 Allendale (E4xxx). i believe CPU will automatically find a 1:1.3 ratio. if not, it'll find the closest ratio and readjust the speed of RAM. but i'm not entirely sure. you can ask people like jack or turpit. they're a lot more knowledgeable than i am :oops: 
April 5, 2007 6:17:03 AM

so I have an AMD system using a 1000MHZ HT bus(2000mhz using both directions).
I've got DDR 800MHZ ,

So since my HT bus is 200MHZ before XX and my ram is 400MHZ before xx
what ratio is the HT running to the RAM speed??

Whats the point of a mobo supporting DDR 800 if no CPU external can actually clock at that speed or rather 400MHZ.

I know you can over clock but not eveyone wants to do that.
!