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Can someone explain RAM cas latency?

S

sphfaros

Member
I'm comparing different types of RAM and wondering what the difference is in the cas latency. Mainly comparing cas3 to cas2.5.

Also what does the timing mean, I'm seeing 2.5-3-3-7 for a type of ocz RAM.

Any explanations would help 🙂
 
"Latency" can be explained as "waiting". High latency means that your CPU is slowed down due to it having to finish one task before another is started.

So, the lower the latency the better (faster). The first number is the one that gives the biggest difference, and that's why you will see often only that number listed. But all RAM has 4 numbers associated with latency.

 
Originally posted by: sphfaros
I'm comparing different types of RAM and wondering what the difference is in the cas latency. Mainly comparing cas3 to cas2.5.
Click to expand...

Technical explanations aside, all you really need to know is that lower latency is always better. So, CAS 2.5 memory is going to be faster (better) than CAS 3.0. Note that CAS latency is sometimes abbreviated "CL."

Also what does the timing mean, I'm seeing 2.5-3-3-7 for a type of ocz RAM.
Click to expand...

CAS latency is a part of the timings. Specifically, the first number in the string of four is the CAS latency. So, your 2.5-3-3-7 has a CAS latency of 2.5, because that is the first number. Ideally, you want the other numbers to be low, too. 2-2-2-5 memory timings are considered some of the best on the market. However, most people agree that the first number of the four--the CAS latency--is the most important.

Generally speaking, latency does not affect the performance as much as other factors, such as speed or capacity. How this works out in the real world is that you want to buy the lowest latency RAM you can, but make sure you're not paying too much. If CAS 2.5 memory is a lot cheaper than CAS 2.0, for instance, then you should probably go with the CAS 2.5. CAS 3.0 memory should be avoided, though, in most cases, because CAS 2.5 memory about the same price, and yields better performance.

You can pretty much ignore the other three numbers unless you're buying high-end components.
 
Actually, the CAS number does not really dictate how much performance you're going to get. The most important are the 2nd and 3rd numbers. The RAS to CAS delay (TRCD) and the RAS Precharge (TRP). As long as you can keep those 2 numbers down, you'll get pretty good performance.
 
What about the mhz that the ram runs at? I've seen the regular value pack kind run at 400mhz whereas the higher end ones run at 400+, does this have any noticable effect in real life computing?
 
Originally posted by: HorseShoe
What about the mhz that the ram runs at? I've seen the regular value pack kind run at 400mhz whereas the higher end ones run at 400+, does this have any noticable effect in real life computing?
Click to expand...

Please use the search function. There are NUMEROUS excellent memory reviews right here at Anandtech that will answer all your performance questions and then some. There's also an excellent thread stickied in the CPU/Overclocking forum about RAM performance differences.

Short answer: RAM speed does not affect performance very much at all on the Athlon64 platform (although low timings provide a slight boost in performance). Pentium 4 CPUs see a decent benefit from faster (MHz) RAM, but little benefit from lower timings.
 
Originally posted by: OCZ John
Actually, the CAS number does not really dictate how much performance you're going to get. The most important are the 2nd and 3rd numbers. The RAS to CAS delay (TRCD) and the RAS Precharge (TRP). As long as you can keep those 2 numbers down, you'll get pretty good performance.
Click to expand...

This is only for AMD 64bit CPU's, right?

Thanks,

Fern
 
Remember when buying RAM don't waste your money with expensive low latency RAM. There will be no real benefit. So just get some Corsair Value Select.
 
I read somewhere that going from CAS 3 to 2.5 may make a noticeable difference, but the difference between CAS 2.5 and 2 isn't that much.

True?
 
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