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I've got some older hardware that needs a memory upgrade, and I was trying to determine what RAM would be the fastest on the platform.

My platform takes DDR2 667MHz which is (slightly) more expensive than DDR2 800MHz for example. So here are the options:

 Frequency | CL (CAS Latency) | Latency (ms)
 667MHz    | 5                | 15ms
 800MHz    | 6                | 15ms

Which is all well and good - they look like the same absolute speed.

But my question is, does the memory operate synchronously with the CPU and share the same clock speed, or does it return as soon as it can?

e.g.If it's synchronous, then when I put the 800MHz chip into my computer, then it will be running at 667MHz with a CL of 6, which would be slower.

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I personaly would use the 667MHz RAM, from memory memory is backwards compatible - the 800MHz will step down to 667MHz.

If budget is the determining factor then go with the cheapest.


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Not all memory is backwards compatible. Make sure to check before relying on this. – derobert Jul 17 '09 at 4:13
I know it should work, but will the 800MHz stick end up with 667@CL6 or 667@CL5? – Stephen Jun 24 '10 at 0:17

Some RAM suppliers, such as Crucial and MemoryC to cite two local suppliers I have used, provide a web-based tool to determine which of the memory they sell is compatible with your system(s). This gives you some confidence that any memory purchased will be compatible. Not the cheapest way to get memory but reliability is an important concern.

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