I build a lot of PCs, but I have NEVER been able to get my head around matching the CPU MB and RAM bus speeds. I have always got my supplier to do that for me, but me thinks they probably know as much as I do, so I would really love it if anyone who has a grasp of this could try to help me get it. I really need to know what factors are important and what are not. I read someplaces the FSB has to match..but others the clock speed is the only thing that has to match. For Arguments sake, there is a core2 E4500 800mhz processor, what RAM would get the best out of it....and why. And where does the MB FSB come into it?

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ok, so if the Intel cpu is 800FSB.. MB supports DDR667 dual channel.... DDR400 has the same clock speed as the cpu, but DDR667 is the MB max but has a diff clock speed. So my question is how does it affect the machine if the RAM clock does not match the CPU, or doesn't it?

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Have a look for a guide on Tomshardware.com - I was hoping ELJ or one of the other boffs on here could answer you but it doesn't look promising. I have to admit, I know my way around a PC pretty damned well but this bus speed shit fucks with my karma

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It basically depends if you're overclocking. If you have more memory bandwidth, then you can raise FSB without actually overclocking the RAM. Basically it gives you more headway.

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i could try to answer this, but whenever I go to type out an explanation my brain gets highly confused.

If the cpu has an 800MHz FSB its actually only 200MHz, but it is quad pumped which gives you the 800Mhz. Memory wise it would be DDR2 PC2-6400, DDR being double data rate giving the PC6400 800Mhz and therefore being a 1:1 ratio with the cpu. Using DDR2 PC2-5300/5400 would mean you'd be using memory running at 667MHz and would have to run the memory at a different ratio to match the cpu.

If that makes any sense to you, then you're doing better than me. I'm not really sure i've answered you question..hmm sleep!

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no, no overclocking...just want to understand the best parts etc for best performance at any given budget for a stock build only, whether it be a celeron, or quad core.

OK so lets take the PC6400 as an example. (Assuming the MB supports DDR2 800MHz PC6400), it is my understanding that this has a clock speed of 400Mhz. So does it affect performance that the CPU clock is actually only 200Mhz. Would DUAL DDR2 400MHz PC3200 with a matching clock speed of 200Mhz perform any different?

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/me dons cape and jumps onto scene


OK, I was going to type out a big long explanation, but fuck that.

HERE is a very nice description of CPU/memory bus speed calculations and their relationship to each other. Including dual channel support, etc.


Read it and enjoy.


In short answer: if you aren't gaming, then slower memory is fine for your system, however; if you are gaming or overclocking, it is important to get ram that can perform at the same speed at your CPU. And please, don't fuck up the dual channel support, it does wonders for your memory latency.

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Thanks ejo, I did read that article, but it still doesn't click with me.

..."Generally you want to keep the system clock of your memory matching with the root clock of your memory or one step above"... why? If everything is run from the CPU clock, how is having the memory clock out of sync going to be anything but bad?

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Its really not a black/white issue. there are shades of gray and even in LOLobo's case, mauve and taupe.

Memory ideally should be matched to its CPU clock, but doesn't have to be. Memory will clock down to match the bus speed of the system. (DDR2 667 will run at 400mhz for example) Its not particularly a bad thing, you are just hampering the cycles your memory does/can do and decreasing the performance you could otherwise obtain by having memory with matched bus speeds.

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I don't think there is anything to understand?

You just buy the highest RAM that your motherboard can take.

Fuck this CPU shit, its the motherboards thats slowing everything down and its so far behind in technology compared with the other components speeds.

Overclocking will not advance untill the motherboards catch up, if they ever.

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I like taupe!

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I know I can get the highest rated RAM that the mb can take... I want to try and understand in a bit more detail the theory in stock matching..maybe its not something that can be done in a forum

Ok, so it's not black and white. 667 can run at 400...one final question then...If a stock PC, not used for gaming, with a MB that supports whatever... and CPU is Intel 800FSB ... would there be a significant difference in performance if you had a/ DDR2 400 in dual channel mode or b/ DDR2 667 in dual channel mode, (even c/ single channel DDR2 800 which is 400Mhz clk to my understanding) or would it be just that the RAM bandwidth is not matched the CPU, performance to all intense and purposes very similar?

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keeping your memory in dual channel mode will always result in some performance gains.

Noticeable to you on a stock PC that you aren't working the shit out of via games or CAD or something of that nature, probably not. Neither will memory speed be that big of a noticeable factor for you.

Grab some gigs of 667, its usually cheapest, and throw them in there.

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ok..cool..so basically bus/clock won't make fuck all noticeable difference for a stock PC for general use, so long as there is enough of it. Not the number crunching I was prepared for , but hey..can't argue with that. Thanks muchly dude. I'll try to stop worrying about it

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most MB (not all) give you full access to physically determine what you want your ram to run at.
If you bought 800mhz ram and are worried its being underclocked
to match the cpu clock, you can go into your bios and force the ram to run at 800mhz at a specific timing -(ie)- 4-4-4-12 or 5-5-5-15. or leave the timing automatic.
Its a buffer where the CPU will run at its specified speed and the ram will run at what you want.
CPU Z will show you the timings and CPU:FSB ratio.

I dont game so i leave my cpu as is but i always force the ram to run at its intended speed / timing and voltage - these are actually written on the ram itself (usually).

**fuck i just realised the last post is a month old***

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you contributed something new, don't freak because it was a month late!

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Yeah, bump threads from 2002 if you think you can add to them!

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