unbuffered / registered / cas / ecc ddr ram

[wizdum]

ok here's the deal:

what does unbuffered / registered mean? and what is the difference? i was looking at ram today to buy for my PC and it was either unbuffered or registered. i have an abit kx7-333r and i was just wondering what would be the best ram to purchase. also, what is cas 2.5 etc. mean? thanx for the help.

 

[boi]

"In unbuffered memory, the chipset controller deals directly with the memory. There is nothing between the chipset and the memory as they communicate. Buffered modules contain a buffer to help the chipset cope with the large electrical load required when the system has a lot of memory. Registered modules are unbuffered modules that contain a register that delays all information transferred to the module by one clock cycle. Buffered and registered modules are typically used only in servers and other mission-critical systems where it is extremely important that the data is properly handled."

"CAS latency is the amount of time it takes for your memory to respond to a command. It only affects the initial burst of data. Once data starts flowing, latency has no effect.

Latency is measured in terms of clock cycles. For example, a CL=2 part requires two clock cycles to respond, while a CL=3 part requires three clock cycles. Thus, CL=2 parts complete the initial data access a little more quickly than CL=3 parts. However, a clock cycle for a systems with a 100MHz front side bus is only 10 nanoseconds (10 billionths of a second), so you probably won't be able to tell the difference between a CL=2 and a CL=3 part. "

 

[John]

What are ECC, Registered and Buffered SDRAM?

What does CAS2 and CAS3 RAM mean?

 

[AnAndAustin]

Here's my take on RAM latency:

CAS is Column Address Strobe, RAS is Row Address Strobe. The RAS, CAS and the RAS-to-CAS latency are basically the time spent waiting to locate, read or write a given area of memory but if a consecutive block of memory is needed the significance of any latency is pretty minimal, so latency is of greatest importance when using memory randomly.

You should get quoted CL2.0 as being 2-2-2-5 and CL2.5 as being 2.5-3-3-7. The 1st number is Cas Latency, 2nd RAS Precharge, 3rd RAS-CAS, 4th Act Precharge (IIRC).

As always there are many factors, CL2.0 is NOT 25% faster in real world RAM performance. When o/c, you can raise some of these 'latency' settings in order to run at a higher mhz. So you may prefer 2-3-3-7 333mhz over the default 2-2-2-5 266mhz for example.

At the same clockspeed 2-2-2-5 is rougly 1.5% faster in Sandra's RAM Benchmark than 2.5-3-3-7. It does depend on what you do with the RAM as to the exact perf diff. Sysmark2000 shows a 4% improvement. Quake3 & 3Dmark2001 show 3% improvement. 2-3-3-7 gives pretty much the same perf as running 2.5-2-2-5. I would generally choose PC2700 CL2.5 over PC2100 CL2.0 even on SktA systems, however, see what the price differences are!

If adding a new DIMM to an existing DIMM rem that the limit and timings are dictated by the slower DIMM. 256MB DDR333 CL2.0 coupled with 256MB DDR266 CL2.5 will both effectively be limited to DDR266 CL2.5. Cas Latency should be selectable in the BIOS, SPD is automatic, if yu don't find many settings for latency then try the latest BIOS.

I'm not saying any of the above is 100% definitive by any means, just what I've gathered from notes I've made coupled with experience.