Ive recently noticed that people have CL 2.0 or 2.5 DDR Memory! What does this mean and how would I know what i neeed for my particular mobo ? thanks
CL = CAS Latency = Column Addressing Strobe Latency. In layterm's, it means how many ns does it take to find the data the system requested on the memory module. By default, RAM and the mobo run at CAS 3, but your more enthusiast-inclined motherboards can change this, but this is one of the settings that can definitely screw with your system is the RAM isn't up to snuff.
ok, so I want this "DDR-DIMM PC2700 256MB DDR CL2.5" to run with my (theoretically) brand new nforce 2 mobo. Would it work ? or would i have to buy CL 3.0 ? I still dont understand much. i undertsand as much about the NS bit access time.
Here, let me try to explain this a tad bit better. Think of your CAS Latency like your CPU's FSB, except that as CAS decreases, speed increases. Now, say you have a i850e mobo and a P4 2.4ghz B. Both of those are guaranteed to work at both 533mhz and 400mhz FSB, so you can choose to run your proc at 400mhz (1.8ghz) or at 533mhz (2.4ghz). Getting back to RAM, by default, your motherboard should be set as CAS 3, but both the motherboard and proc are capable of running at CAS 2.5, using the RAM's full guaranteed potential, but going to CAS 2 would be pushing it for your RAM (the motherboard would probably be ok running at that since it's an nForce2, but it all depends on the RAM).
Lower cas = better memory performance, but not that you'd notice (our friend SiSoft Sandra might though ) Be aware that most PC100 or 133 based motherboard have optioned for cas 2 or 3, or you can set all the timings by SPD (sensor presence detect or something!) and basically let the board ask the memory what it best likes to run at (as all memory is set for a particular speed - e.g. a PC133 Cas 3 module when set to SPD will run at cas 3, a cas 2 module set to SPD will run at cas 3...) DDR memory is more awkward, indeed as default most DDR memory, be it PC1600 (100Mhz DDR = 200Mhz), 2100 (133Mhz DDR = 266Mhz), 2700 (166 DDR = 333Mhz), or 3200 (200 DDR = 400Mhz!), works be default at cas 2.5, and the boards often only have options for cas 2 or 2.5. However, a lot of the recent really high end memory, such as Corsair PC3200, runs by SPD at 2, and some recent boards even allow cas 1.5 operation for the memory! What some less scrupulous companies (cough *OCZ* cough) do to obtain high speeds such as PC3200, or 400Mhz memory, is to set the chips to run by SPD at cas 3, which is slightly easier on the memory and therefore easier to do at high speeds. Put a stick of cas 3 memory in some DDR motherboards, and it won't work at all, as a lot don't allow for cas 3 operation at all (I believe Intel P4 DDR chipsets are particulary harsh for this) as cas 3 was never really standard for any DDR, with cas 2.5 the norm. You may find that cas 3 rated PC2700 is little more than overclocked PC2100. I'm sure Zapwizard will tell me where I'm wrong
first off, you may disregard whypick's ramblings. CAS has nothing to do with nanoseconds. Cas is simply the number of clock cycles from the request for data arriving at the memory module and the data being ready on the output pins of the memory module. Lower is better. In SDR systems you can have CAS 1, 2, 3 etc. In DDR systems, due to the double-pumped clock, you can now go in smaller steps: 1, 1.5, 2, 2.5, 3. Lower is still better. Much of the time it may not make a lot of difference though...
Well yes and no. Each clock cycle takes time (ns) so by reducing the clock cycles you inadvertantly decreace the time (ns)
There's a lot more to it than that. Yes, the ns rating of RAM has an impact on its CAS performance, but whypick was suggesting that CAS 2.5 means that there's 2.5ns from request to data ready, and outside of core-integrated SRAMs (re cache), there is no memory that fast. Certainly no DRAMs are that fast.
Bah, Isaac's right. I think every single post that includes technical info has one minute detail wrong.
2.5 or 2 will probably both work on an Nforce2 based board, 2 will give you slightly better performance. But.. The really relevant fact that noone has mentioned is that the Nforce2 chipset has a dual-DDR controller, basically it can use 2 sticks of DDR at the same time to massively increase (double?) memory bandwidth. Taking advantage of that is going to give you much more of an increase in performance than picking CAS2 over 2.5. PC2700 is rated at 166(x2 = 333) (correct me if I wrong) so you want PC3200 instead cos its rated at 200(x2 = 400)? Then you can run it at the same speed as the bus speed - synchronous mode. In short, if you want the best performance out of an Nforce2 board, get TWO sticks of (identical) PC3200 memory. If you are just putting one stick in, you might as well get a KT400 board because you are wasting the huge advantage of dual-ddr. If I was getting one (U listening Santa?) I would put 2x 256mb Corsair XMS3500 C2. But thats just me.
And if you read another current thread, you'll see why dual-channel DDR isn't something to get desperately excited about with the athlon... http://forums.bit-tech.net/showthread.php?s=&threadid=20724
Ahh yep that makes sense, so for your doubled memory bandwidth, most of the increased bandwidth isn't actually useable. Athlons utilising less of it than P4s. I dont get the maths tho, 64 x 2 x 133000000 = 2100MB/s? Can you break that down for us maths dunces?
That sum Isaac posted comes from 64bit memory path for 'normal' DDR memory, times by 2 (DDR = double data rate ) x 133Mhz (Its 133,000,000 Hz, one megahertz = a million hertz) giving you a theoretical max of 2100MB/sec transfer....hence PC2100 memory. In the same way if you do the sums you'll see DDR400 memory provide 3200Mb/sec, hence PC3200. RAMBUS is different though, with PC800, 1066 etc not conforming to the same thing. With RAMBUS out first though, DDR companies decided to use a different scheme to give bigger numbers so people would presume its better, which isn't the case. PC2100 DDR vs PC800 RAMBUS on the P4, and mr Rambus is the winner I can't remember the way RAMBUS arrived at their numbering scheme though....tis different to DDR though IIRC...is it the effective speed of the memory in Mhz or something? I don't know.
well i still would like an nforce 2 boards because i do plan to upgrade to 2 sticks DDR, or even 3, if money lets me 2700 however, i have read too much that 3200 is not v good, so even if there is a slight performance loss, im sure the money i save is worth it
