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Old 10-01-2006, 11:40 AM   #1 (permalink)
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Default Whats the timing on RAM mean?

For instance,

The timing on the RAM im interested in is 5-5-5-12


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Old 10-01-2006, 12:42 PM   #2 (permalink)
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RAM timings are time delays (or latencies) of the memory. They're measured in cycles. For a particular type of RAM, the lower the timings the better, since the RAM delays less to locate and access its contents. However, better (lower) timings make the RAM much more expensive and usually require more voltage to operate.

We usually use the first timing number, called CAS Latency (Column Access Strobe), to compare RAM speed. The RAM that you posted takes 5 cycles to perform the CAS operation. That RAM runs at 400Mhz (800Mhz effective due to DDR), so it goes through 400 millions cycles per second.

For comparison with other timings and other RAM clock speeds, it's useful to convert timings in cycles to timings in nanoseconds. I'll show you how to do it (other people can correct me if I'm wrong).

Example: Corsair 800Mhz DDR2 5-5-5-12 RAM

1) Get "real" clock speed.
Since the memory is DDR2 (or DDR), the real clock speed is always the advertised clock speed divided by 2. So 800Mhz/2 = 400Mhz for this module.

2) Calculate the duration of one cycle.
To do this, just divide 1000 by the clock speed in Mhz. The result is in nanoseconds (ns). So for instance, you'd do 1000/400 = 2.5ns

3) Multiply the duration of one cycle by the latency.
For CAS latency 5, this would be 2.5ns x 5 = 12.5ns.

So this memory module has a CAS latency of 12.5ns.

If you run the same calculation with 400Mhz (200Mhz real) DDR with CAS 2, you'll see that this older RAM is actually a bit faster. DDR2 usually have worse timings than DDR.

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