CPU/RAM/FSB speeds

[telecomguy9]

Okay, I want to make sure that I understand things correctly. Let's say I get a mobo that has a 800/533 MHz FSB. Let's say it supports a Pentium 4 CPU 533 MHz FSB and dual channel DDR2 667 MHz RAM. So I put the 533 MHz FSB CPU in the mobo and I put a dual channel memory kit (DDR2 667 MHz 2x512) into the memory slots. For optimal performance do I need to have the memory clock back to 533 MHz? Also, does the mobo FSB usually clock back to what the processor will support, or might I need to force it to run at 533 MHz as well? Is having all of the FSB options and memory speed running at the same speeds (in this case 533 MHz) the optimal solution?

[Peachy]

If you leave the BIOS settings on automatic for the CPU and memory timings they will use what the component is designed for. So if the CPU is a 533MHz bus processor the FSB will automatically set itself for that. You can manually override those settings and try and push the FSB as high as the CPU can handle. Same thing for the memory. However, a good motherboard designed with overclocking in mind should let you adjust both the FSB and memory frequencies. That being said whether you should run the memory bus synchronously with the CPU or asynchronously depends on the quality of the memory. Ideally, the CPU FSB should be synchronous with the memory bus so that you don't introduce any latency in the overall system because of one component slowing down to match the other. So, you would have to manually lower the memory's bus frequency to match the FSB.Now, some people suggest that a faster memory frequency with very low memory latencies can overcome the disadvantage of have such asynchronicity. This is something you can only test with your system and running a few applications to see if you notice any difference. The other factor that would mitigate this setup is that memory modules tend to increase latencies at higher frequencies for stability.Tom's Hardware published an interesting article last week that may help to illustrate some of these issues.Tight Timings vs High Clock Frequencies

[telecomguy9]

Thanks for the info Peachy. This makes sense, and it is what I thought I had heard or read a while ago. I'm looking at upgrading my PC and this is why I wanted to know. It looks like I'll be getting a P4 that has a FSB of 800 MHz, a mobo that has a FSB of 800 MHz and that supports DDR2 667 MHz memory dual channel config. I got confused because of the difference in the memory speed versus the FSB of the mobo and processor.

[Peachy]

No problem, telecomguy9!Just FYI, I have a Pentium D 920 (2.8GHz / 800MHz FSB) that I am currently running overclocked. I upped the FSB frequency to 1064GHz (or 266MHz quad-pumped). With it's 14x multiplier this gives me a nicely stable 3.7GHz CPU. The system has DDR2 memory rated for PC5400 speeds (667MHz or 333MHz memory bus).

[telecomguy9]

Let me show you what I'm looking at getting. All of this will likely be bought from newegg.com. I still have to look around and see if I can get it all anywhere for less $$$.CPU P4 3.0 GHz (Cedar Mill)mobo ECS C19-A SLIRAM Patriot DDR2 667MHz 2x1GBThe processor has a 800 MHz FSB, the memory runs at 667 MHz, and the FSB of the mobo is 1066/800. I do not plan to OC, and I actually believe this mobo does not support it (officially or unofficially, I don't know). And that's okay because I don't plan to ever OC anyway.

[Shamrock]

I'm surprised at the suggestion to underclock the ram to synchronize with the CPU. I had always thought that running the ram at the highest frequency possible would maximize performance. It seemed to me if the CPU is the bottleneck (by comparison only) slowing down the ram will not help. I don't mean to contradict what you say. It's just that it hadn't occurred to me. Can I get a more detailed explanation of how this might be?

[Shamrock]

Thanks. That much makes sense. I guess it just didn't occur to me that there would never be a time that it wouldn't help. It certainly stands to reason that there's not much sense in pushing the performance of RAM if nothing else can keep up.

[telecomguy9]

Wow, I think I'm a little bit lost. Looking at the hardware I said I was looking at getting, am I right to conclude that the entire system will only run at the speed of the slowest component listed? The RAM is slowest, running at 667 MHz, so everythin else will run that "slow", right?

[Peachy]

No, no, no. The FSB and memory bus are two separate buses. Rereading my original posting I realised I should have been more clear. It has been accepted wisdom that the base clock frequencies of the FSB and memory should match, or at least the memory should be 1 level faster. A CPU with a FSB of 800MHz has quad-clocking (4x) of its base bus speed which would be 200MHz. A matching DDR2 memory base clock would be 2x the base or 200MHz x 2 = 400MHz. Your memory is 667MHz, which is 2 x from the base clock of 333MHz. So the memory base bus clock is running faster than the CPU base clock. The CPU communicates with the motherboard chipset at 800MHz while the memory communicates with the motherboard at 667MHz. If the memory and the CPU communicate directly with each and bypass the motherboard chipset then they would want to communicate at the base 200MHz speed without incurring any wait states. Since your memory is running 2x faster than the CPU base clock it won't be that much of an improvement until you get the memory bus up to 800MHz.It's kinda convoluted but that's how I think it's supposed to work.You will be fine.

[telecomguy9]

Thanks for the reply Temmu. It appears to be as I thought. Between those three components the system will only run as fast as the "slowest" component. In my case the 667 MHz RAM, which I realize isn't slow but that word works in this case.Peachy, I think that makes sense. I appreciate your helping me understand this stuff.

Edited April 18, 2006 by telecomguy9