CAS Latencies/RAM Timings...

[chazdraves]

Well, I'm sick of not knowing... Everday I read reviews on RAM and such and look at the timings/various other info. I can tell what's faster than what, but I don't have a bloody clue what it means... Let's start here, if yah don't mind. #1 What are CAS Latencies? What does CAS stand for? What affects CAS Latencies/what benefits do lower CAS latencies provide? (big question)

Okay, here's part two for anyone with time enough to answer. In the reviews they will often list like "2-2-3-4" or "2-2-2-3-5". Why are there sometimes 3 numbers, sometimes 4, or even 5? Shouldn't this be "x" number of numbers? And what do these numbers mean?/How do I find my timings/adjust them...?

One last thing I just thought of... I was just reading the review of the Buffalo HotStix (or whatever the bloody 'eck) and I noticed that some of the Quake III benchmarks spread by about 70FPS with the only difference being the RAM... 70FPS? Eh? Can RAM make that big of vid performance difference? I'm running cheap Best Buy RAM 1x 512 PC3200 and 2x512 PC2700... last I looked the CAS was 2.5...

Phew... sorry, I know that's way too much for one post, but help me out here... I'm so confused... To whomever is daring enough to tackle this multi-part behemoth: my most heartfelt thanks, cause there's a lot there.

- Chaz

 

[CTho9305]

First question:
http://ctho.ath.cx/pics/funny/answer.gif i googled "cas latency" without quotes, and hit i'm feeling lucky:
http://www.corsairmemory.com/main/trg-cas.html

Second question:
I don't know. Hopefully someone will enlighten me too .

Third question:
Where did you see that review? I'd have a hard time believing you'd get 70FPS more from a RAM change. I thought you usually gained only a few percent unless it was something major (dual channel vs single channel gives something like a 20-30% gain I think).

 

[chazdraves]

That 70FPS gain was in Anandtech's review of the Buffalo Hotstix... That didn't make a bit of sense to me, so I thought I'd bring it up...

- Chaz

 

[Cattlegod]

ill give you a quick rundown, nothing complex

ram is set up as a grid

XXXXXX|
XXXXXX|
XXXXXX|
XXXXXX|
_______
each X represents a bit.

the cas timings are the time it takes for the charge travel from its location in the grid to the bar on the right, and also the time it takes it to discharge to the bottom line.

thats really all you need to know, what does this mean? well ill tell you. this means you can O/C the ram and lower the cas timings without any race conditions. or if you leave the cas timings regular the data just travels a bit faster on the card.

 

[Matthias99]

Originally posted by: chazdraves
Okay, here's part two for anyone with time enough to answer. In the reviews they will often list like "2-2-3-4" or "2-2-2-3-5". Why are there sometimes 3 numbers, sometimes 4, or even 5? Shouldn't this be "x" number of numbers? And what do these numbers mean?/How do I find my timings/adjust them...?

They're a bunch of different latency timings, expressed in clock cycles. It has to do with the way that DRAM is addressed, and I don't have time to go into it in depth right now. Suffice it to say it's complicated, and only an electrical engineer would really care about it.

You *usually* see four timings -- CAS Latency, then two other latencies I can never remember, then the tRAS. Sometimes they also throw in the command rate (1 or 2, sometimes listed as 1T or 2T). So the usual thing you'd see is something like 2-2-2-5-1T or 3-4-4-8 (implies 2T, usually). I have no clue what 2-2-2-3-5 is, unless they're listing things out of order (which some manufacturers do). Note that CAS Latency (the first number) is by FAR the most important, and that most Athlon chipsets actually run fastest with a tRAS (the highest number there) of 10 or 11, which any RAM will do just fine. Take a look at some of the Anandtech memory articles and they should go into this in detail.

Your timings can be accessed in your BIOS (the thing you get to by hitting DEL or F1 or something as your computer boots). The memory timings are usually in an "Advanced" menu, along with your FSB adjustment. You may have to set it to "Manual" mode to be able to change them independently -- normally, it reads the timings that are programmed into the RAM itself (the so-called "SPD Timings"). In *some* versions of the Award BIOS (I think), you have to hit a key combination (uh, CTRL-F1, maybe?) to get at the advanced menu.

One last thing I just thought of... I was just reading the review of the Buffalo HotStix (or whatever the bloody 'eck) and I noticed that some of the Quake III benchmarks spread by about 70FPS with the only difference being the RAM... 70FPS? Eh? Can RAM make that big of vid performance difference? I'm running cheap Best Buy RAM 1x 512 PC3200 and 2x512 PC2700... last I looked the CAS was 2.5...

Quake3 is *extremely* CPU and system-limited at lower resolution settings with today's processors and video cards.

I take it you're talking about the results here and here? One's at DDR400, and one's at DDR500. That's a pretty big step up in memory bandwidth, and if they're using a multiplier-locked processor (ie, a Pentium 4 that's not an engineering sample), they're also upping the CPU frequency proportionally. You'd see the same results in 3DMark01 and other old DX7/8 games (horribly system limited), but the results in 3DMark03 (and newer DX8.1/9 games) would be pretty flat.

 

[chazdraves]

I see. Actually... I was referring to these here as there's a much wider gap. I did manage to find a pretty good article explaining the timings... I just need now to know which timings are the 4 they reference... knowing the first one is CAS and the 4th one is tRAS really helps, now for the other 2... heh. As far as that 2-2-2-3-5 timing, I was just making up numbers...

Thanks for all the info... anybody know the other 2 timings?

- Chaz

 

[Matthias99]

this looks like it could be a good resource. That was the top hit for "DRAM timings" on google.

 

[Tiamat]

Originally posted by: chazdraves
I see. Actually... I was referring to these here as there's a much wider gap. I did manage to find a pretty good article explaining the timings... I just need now to know which timings are the 4 they reference... knowing the first one is CAS and the 4th one is tRAS really helps, now for the other 2... heh. As far as that 2-2-2-3-5 timing, I was just making up numbers...

Thanks for all the info... anybody know the other 2 timings?

- Chaz

I think that 2-2-2-3-5 is misordered from 2-3-2-5-2.

Where, 2-3-2-5 is the four common ram latency timings, and the last 2 is the ram timing that can also be 4. I forget what it is, but many motherbaords dont even let your access it. INcluding the NF7-S ver2

 

[sao123]

the last value is the T value. It can be 1 or 2. 1 is faster.
as in 2-3-2-6-1.