What is CAS latency and timings

[deano3041]

ok im a newbie at this RAM stuff so fill me in what is cas latency and what r the timings i kno that the lower they r the better but im building a new rig and i am gettin a gig of ocz performance cas lantency 3 timings 3-3-3-7 and im usin this rig for gaming and i want great performance what the heck does this stuff mean and do i need better?

 

[Spike]

http://en.wikipedia.org/wiki/CAS_Latency

CAS is an acronym for column address strobe, or sometimes column address select, both referring to the column of the physical memory location in an array of capacitors (a grid comprised of columns and rows) used in dynamic random access memory modules. Latency refers to the amount of time, or lag, that is experienced in traversing a system. Taken together, the CAS Latency (CL) refers to the length of time, in clock cycles, it takes for a request sent from the memory controller to read a memory location and send it to the module's output pins.

Data is stored in individual memory cells, each uniquely identified by a memory bank, row, and column reference. In accessing DRAM, controllers first select a memory bank, then a row location (using the RAS, row address strobe), then a column location (using the CAS). Because the column selection is the last step before requesting to read the data from the physical location of the memory cell, the CAS Latency measures the number of clock cycles between the time the request for data is sent to the actual memory location and the time the data is transmitted out from the module.

When selecting a RAM card the lower the CAS latency rating the better your system will perform. Current DDR2 RAM should have a CAS latency of about four, or optimally three.

Standard DDR CAS Latency should these days be 3, 2.5 or even 2 if possible, instead of the 4 or 3 that the article states for DDR2. Of course, that does largely depend on whether you'll see any benefit to it - it depends on how you use your PC.

In simple terms, it's the number of clock cycles it takes for the memory to respond when it's told to do something. A CAS Latency of 3 means that the memory responds to requests after 3 clock cycles.

Now I'm not an overclocker so I may well be wrong, but as far as I'm aware, the CAS Latency is something that isnot really all that alterable without buying memory with a lower CAS Latency.

 

[Justin]

DDR2 RAM will have a higher CAS latency than DDR. 4 is quite common for DDR2.

 

[Spike]

yes - worth noting also that DDR2 is better than standard DDR - even if it has a higher latency (it's a trade-off between latency and speed).

Thought I'd mention that, purely because I think I may have muddied the subject a little.

I have no experience of DDR2 myself in real life. Does anybody use it that can tel me if it's all it's cracked up to be?

 

[deano3041]

no idea i havnt used it either but hey guys what r the timings on ram and how important r they?

 

[Justin]

I am using 1024mb DDR2-533 in this machine with a Dual Core P4 830+


It is extremely fast, however my Athlon64 system with 1024mb of DDR-400 actually OUTPERFORMS it in many areas.

However, keep in mind that DDR2 can go all the way up to 667 and even 800 (a whopping 400mhz clock). I am sure that DDR2-800 would smoke DDR400.

This board supports DDR2-800 but I cannot afford it.

 

[Justin]

Timings are very important - Although in many cases adjusting timings will not net a noticeable gain or loss in performance, incorrect timings can cause your system to freeze, crash, or even fail to boot.


IF you are not overclocking or a performance nut, you don't ever need to adjust timings - Letting the motherboard auto-detect the timings via SPD is more than enough.

When overclocking, they are crucial. IF you want a detailed explanation we can provide it

 

[deano3041]

i suppose i know the basics of OC ing but u can fill me in if u please

 

[Spike]

but be prepared for a long read. lol

Come to think of it, I know a little, but I wouldn't mind betting that there are people here that know ALL about it. If anybody can be bothered for sake of my curiosity I guess (depending on whether deano may or may wnot want to know), I wouldn't mind a little bit of an explanation. No rush in my case though. I'm just interested in getting to grips with it properly. (the CAS Lat. that is - lol. asking someone to write ALL about OCing would just be plain rediculous. lol!)

Thanks for the DDR2 inf Soul - much appreciated. Might give it a try myself when I next upgrade.

Edit: I've just found this from google (via the SysOpt forums)...

http://corsairmicro.com/memory_basics/153707/index.html

It's probably teaching your grandmother to suck eggs for many, but for those that don't know but would like to, it's probably an interesting watch. Points 19 - 24 in the presentation deal with the settings you mentioned in your original post, deano.

 

[deano3041]

aww icic well should i over clock my 3500+ when i git it?

 

[Spike]

The basics of overclocking are the easy bit. To overclock properly though, it tends to take a lot of knowledge of how differend pieces of hardware interact (eg, how the CPU interacts with the physical memory.) To know that, you have to have an understanding of how these parts work, in terms of what effect they have on each other (eg, the internal and external clocks of the CPU and the controller clock of the memory).

ramping up the FSB is the easy bit. The hard bit is knowing how that will effect the speed of the PCI slots, the AGP slot, and the memory in particular. Then there's voltages etc.

For an example of how complicated it can all be to a person new to OC'ing (It's why I don't overclock to any real extent lol. I don't have the time to learn it all rght now) watch that link I posted in the edit at the bottom of my last post in this thread from beginning to end. You don't need to know all of it, but when you've never OC'd, you don't know which parts of it you need.

My advice to you would have to be not to overclock that system. 3500+ is plenty fast enough - for a little while at least.

 

[deano3041]

hey thanx a lot for your info man i really appreciate it
im new to this forum so is there ne way i can add u to my friends list or contacts or something?