Sign in with
Sign up | Sign in
Your question

Memory question Cas. Latency and timings

Last response: in Systems
Share
July 26, 2006 1:03:12 AM

I keep hearing that lower latency and timings are better. What is really the big difference? The reason I ask is because I'm going with DDR2 800 and between latency of 4 and 5 is $100 difference.
July 27, 2006 1:39:29 AM

Ok to explain the whole thing the reason lower latencies are better is because there are less wasted clock cycles and thus better performance. CAS means Column Address Strobe. A CAS latency of 4 means that it takes four clock cycles for the data to appear to the system whereas a latency of 5 means that it takes 5 cycles for the data to appear. Now think about it, the less time the system waits for the data the better right? The only problem is that at higher speeds latencies need to be more laxed, or higher because of stability problems. I'm sure they have DDR2 800 at CAS 3 and that would give great performance. Hope this answers your question! :D 
July 27, 2006 1:41:03 AM

Quote:
Ok to explain the whole thing the reason lower latencies are better is because there are less wasted clock cycles and thus better performance. CAS means Column Address Strobe. A CAS latency of 4 means that it takes four clock cycles for the data to appear to the system whereas a latency of 5 means that it takes 5 cycles for the data to appear. Now think about it, the less time the system waits for the data the better right? The only problem is that at higher speeds latencies need to be more laxed, or higher because of stability problems. I'm sure they have DDR2 800 at CAS 3 and that would give great performance. Hope this answers your question! :D 


Thanks for the great description.

Yes CAS3 DDR2-800 RAM is on the market. It costs $450 too for a 2GB kit.
Related resources
July 27, 2006 1:43:22 AM

Yeah I forgot to mention that. The reason lower latency memory is more expensive is because they have to invest a little more time and money into the stability of the module. Lower latencies sometimes cause instability due to an overflow of memory or a misread.
July 27, 2006 3:22:28 AM

Thanks for the explanation. Can a difference be seen in realtime use? Reason I ask is because I'm building a rig for gaming. Would I actually notice a big jump in performance between CAS 4 and 5?
July 27, 2006 3:25:08 AM

For gaming? It depends on how resource-intensive the game is. If it's a faster paced game you might notice a little but that's a little more dependant on your graphics card.
July 27, 2006 3:38:19 AM

That's what I wanted to figure out. I'm going with either a 7900gt or 1800XT. So I'd be fine with CAS 5 to get the job done, right?
July 27, 2006 3:47:15 AM

Hmm... well those are both great graphics cards but for gaming I'd still go with the CAS 4 although CAS 3 provides the best performance but it's hardly noticable. Also if you go with CAS 5 you'll probably notice your high fps going down at high resolutions just in case your frame buffer runs out.
July 27, 2006 5:15:27 AM

Then there's the issue of running "slower" but much higher CAS memory, like DDR667 CAS 3 or DDR400 CAS 2(true 2 - 2- 2). For a lot less money.

It gets confusing fast, :)  I think memory speed (DDR xxx) is overrated as it is compared to low latency - the computer won't necessarily use the higher bandwidth, but WILL get an advantage out of lower latency.

(like an online game - slow computer but low latency is often better than a rocket with 100ms lag)
July 27, 2006 5:50:11 AM

This is true... it's really up to you. But always try to keep the memory speed and HT or FSB in a nice even ratio. Mixing up in a 2:3 or odd ratio can slow you down too.
July 27, 2006 8:52:34 AM

Quote:
Then there's the issue of running "slower" but much higher CAS memory, like DDR667 CAS 3 or DDR400 CAS 2(true 2 - 2- 2). For a lot less money.

It gets confusing fast, :)  I think memory speed (DDR xxx) is overrated as it is compared to low latency - the computer won't necessarily use the higher bandwidth, but WILL get an advantage out of lower latency.

(like an online game - slow computer but low latency is often better than a rocket with 100ms lag)


Already there :p 

For me it all comes down to my budget. As of now cas 3 is out of the question. Just trying to figure out where I should cut costs. Going with an AMD2 system, so my options are DDR2 667, or 800.
July 27, 2006 12:45:35 PM

Quote:
That's what I wanted to figure out. I'm going with either a 7900gt or 1800XT. So I'd be fine with CAS 5 to get the job done, right?


I built an,
AMD Athlon 64 3800+
ASUS M2N-E Socket AM2 Motherboard
XMS2 Corsair 2GB (2 x 1GB), 5-5-5-12
ATI Radeon X1800XT

That PC runs Counter-Strike: Source at 1280x1024, with all the highest settings otherwise. Such as 16X Anisotrophic Filtering, 8X Anti-Aliasing, HDR, etc. FPS averages around 100 (usually much higher), which is much better than my rig (see sig) that runs CS:S at 1280x960, 2X Anti-Aliasing, Trilinear Filtering, no HDR, medium/low settings otherwise.

That RAM will pull you through gaming seamlessly. After I built that box, I gave it a "personal stress test" and played CS:S for about 3 hours straight without a single slow down, hiccup or anything.

I also read somewhere that DDR2-800 in comparison to DDR-400, you can almost divide the timings in half due to the speed being two times faster. So, according to a guy on another forum, DDR2-800 with 5-5-5-12 timings would be equivalent to 2.5-2.5-2.5-6 DDR-400 memory.
This guy's probably the savviest computer tech I've come across, but I don't know how accurate this conjecture really is. Interesting to think about though.
!