motherboard/CPU FSB speed sync with DDR2

Posted on 2007-04-02
Last Modified: 2013-12-09
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 would buy a motherboard that supported for example 266MHZ fsb,  an AMD CPU with an FSb of 266MHZ and  DDR 266MHZ.
Same for the AMD boards / CPU that supported 400MHZ FSB.

Now with Intel I know they were slightly different to AMD in that they if you had 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 ddr2 667

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 which ram do you use ??

I was also told that because of AMD hyper transport bus that speed match of FSB / RAM was required anymore because the ram would work at its rated speed?? is this true ?
So you could have an FSB of 1000MHZ with DDR2 533??

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


Question by:softley
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Expert Comment

ID: 18841410
I don't have that much time to get into it tonight, but the main thing is that AMD's HyperTransport bus isn't really a FSB.
It's the cpu/memory controller bus, which is on the cpu die. Intel still uses the FSB for everything to talk to each other.

I will see if I can find a better explainion for you, and check back tomorrow. Maybe one of the other people will have posted something by then.

Author Comment

ID: 18841737
Thanks for the reply!

Yes i have read somewhere that hyper transport uses a differn't method to  FSB!
so does that mean you no longer have to sync ram speed to the hyper transport for the cpu??
In my above example , if you have hyper transport of 1000MHZ can you either DDR2 677, 533 or 800 without having to sync to the HT frequency??

What about the intel chips that are using 800MHZ FSB with DDR2 533 how does that work or in my above example is the ram being under clocked??

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Accepted Solution

_ earned 250 total points
ID: 18848473
Yes I put that badly. In my defense, it was 1:30am.  : )

>>  in my above example is the ram being under clocked??

That's pretty much it.
Intel's cpu bus speed was faster than the DDR2 ram at that time was capable of running. So they ran it at a ratio of the cpu bus.
So a cpu of 800 (200) ran memory at 667 (333 - 166) or 553 (266 - 133) (depending on how you figure it).
Then the 800MHz ram came out, but the cpus went to 1066MHz.

With AMD it wasn't as much of a problem as they just went to DDR2 about a year ago, and mostly still use a 200 max cpu bus.
The HyperTransport bus is still based on the cpu clock

The new DDR2 coming out the last couple of months is now out pacing the cpus. I have seen some that are rated at 1142MHz and one that was 12 something. But that is for the OC'ers, until Intel releases the 1333MHz cpus (the nVidia 680i chipset for Intel mobo's already officially supports this setting.
So running 1:1 ratios is possible again. Which is still considered to be the sweet spot.

These might help you with the HyperTransport Bus:
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ID: 18848482
>> The HyperTransport bus is still based on the cpu clock

Scratch that. That's not really correct, either.

Assisted Solution

AndrewCink earned 250 total points
ID: 18848689
As Coral more stated, the thing these days is, the speeds are no longer sync'd up like they used to be. I miss those good old days of AMD. I had to defect to a Core2Duo since they are the better CPU right now. :( In the "old days" a couple years ago, people typically sync'd the FSB and memory and overclocked both simultaneously and to the same extent. Typically either the RAM or CPU would cap out at some point, where both would then have to stop, regardless of if one had more bandwidth to give.

Nowadays, almost everyone "unlocks" the processor and memory frequencies so they do not rise in tandem. The benefits of overclocking the memory now pale in comparison to overclocking the CPU, especially with RAM speeds coming up so much in recent years. Most anyone who overclocks now un-syncs the memory and CPU, and then overclocks the CPU much more than the RAM (especially in the case of intel processors, as the core2duo chips overclock ridiculously well)

The honest answer is, it sounds to me like you're planning to buy a new rig. I recently bought a new rig myself about a month ago and replaced my aging 3800+ Venice with a 2.67ghz core2duo chip. All of the research I did indicates that while faster RAM does of course contribute to overall performance, that your spending dollars are much more wisely spent on the CPU side than the RAM side.

For example, if you were to spend 600 dollars, the 400 dollar CPU with 200 dollar RAM would whip tailgate all over the 300 dollar CPU with 300 dollar RAM. Common sense? Maybe, maybe not :) Of course, you want good RAM, and plenty of it (2 gigs minimum in my opinion). The sweet spot now is to get the fastest CPU you can swing, and some good DDR800 to go with it. The days of RAM glory have passed somewhat by the wayside, much to my chagrin of trying to get sickeningly low CAS latencies on my RAM to make my AMD scream.

Nowadays, CPU power is king. The only time it really makes sense to buy super high end RAM, is if you got the absolute best CPU money could buy, and you want to be even faster still. Otherwise just put that money into the CPU.

Author Comment

ID: 18849523
so basically some intel FSB speed like the 1066 FSB (266 bus before multiplication) still follows the old sync theory when using 533 ram(266 before multiplication).
But for the intel fsb of say 800(200 before multiplication) running with 667(333 before multiplication) the ram is being under clocked but leaves head room for over clocking the cpu?

with AMD its a similar princple the 100oMHZ HT or bus actually runs at 200MHZ before multiplication and all DDR2 is underclocked with headroom for over clocking??

so without over clocking your not getting the max out of your DDR2?

I already have an AMD system with a 1000MHZ HT (or 2000MHZ if you count both directions)
I have soom 800MHZ ram! so does that mean 800MHZ ram isn't running at full speed?
is my ram being under clocked?

Author Comment

ID: 18849539
by the way the intel core duo used in the intel apple mac uses 1333MHZ bus with DDR 667 which again seems to follow the sync methods.

Expert Comment

ID: 18853575
I must not have explained things right. I was trying to say, RAM speed is sufficient that it is not the bottleneck in the system these days. Increasing RAM speeds only lends to minor increases in performance, because there is already sufficient bandwidth available.

The place real performance gains are seen is by overclocking the CPU, as the RAM really doesn't need it or gain that noticably by it. The RAM speed to FSB speed ratio seems to be 1:2 to ensure the CPU isn't starved. So a 1333 mhz FSB should have at least DDR 666 to make sure it's not starved. There are gains from faster RAM, but in the application, the RAM speed gains pale in comparison to gains from a faster CPU or a overclocked CPU.

Author Comment

ID: 18854830
Ok, I might not have asked my question clearly,

I guess what im asking is what Ram do you use in accordance to FSB speeds.

I knew in the old days with an AMD board if I used a CPU with a 400MHZ FSB I used DDR400
If I used a CPU with an FSB of 333 I used DDR333

If I had a an intel FSB of 800MHZ I used DDR400
If I had an intel FSB of 533 I used DDR 266

So for example now if I have an intel FSB of 800MHZ with ram options of DDR2 533, 667 & 800
Which ram do you use?

Or intel 0f 1066MHZ FSB do you DDR2 553 to follow the original method old method of sync or can you use DDR2 800
Without under clocking it because the CPU FSB works to a ratio to ram to enable sync?

If you have an AMD 1000MHZ HT speed with ram options of DDR2 533, 667 & DDR800 which do you use?

Or again can you any option due to sync not being required
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ID: 18855648
>> So for example now if I have an intel FSB of 800MHZ with ram options of DDR2 533, 667 & 800. Which ram do you use?

800 is preferred. . You still need to make sure that the mobo chipset allows that Option. (most of the mobo in the last 2 years should)

>> Or intel 0f 1066MHZ FSB do you DDR2 553 to follow the original method old method of sync or can you use DDR2 800

800 is preferred. Same warning applies.
Luckly they are getting past this Async bus stuff. <knock on wood>. It's just a headache.

Most of the 1066MHz Intel cpus are using 800MHz ram and a stock bios setting of AUTO, which runs the cpu at 266 base, and the ram at 200 base. Part of this is due to the price of the 533 and 667 chips
But the OC'ers were getting something like Corsairs 6400C3 (with 3-4-3-9 timings, stock), and loosen them to 5-5-5-15, bump the voltage up a notch, and set them to 1:1 with the cpu to run everything at 1066MHz.
But like I said earlier, the ram makers are catching up, so this raito stuff should be mostly behind us. (see the Corsair Dominator. The Twin2X2048-8500C5D is 1066MHz rated, and the Twin2X2048-9136C5D is rated at 1142MHz. And have been OC'ed to 1250MHz)

AMD didn't really have this problem when they finally went DDR2 on the Socket 939/AM2, the ram speeds were up to 200 base (same as the cpus) and there was finally enough ram hitting the market, for the price to be worth it.
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ID: 18862324
Thank you much.  : )

I saw I messed up part of a sentence in my last post.

>>  Part of this is due to the price of the 533 and 667 chips <<
should be..

Part of "the reason they are still using the slower ram" is due to the price of the 533 and 667 chips.

I am going to shutup now.  ; )

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