DDR Ram Types?

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I'm building a new system and want good DDR ram for the PC. I've seen different types of DDR RAM and would appreciate someone letting me know what the best to get is.

Currently I've got my eye on these lot below.

1: Crucial 2x1GB 184Pin DIMM PC3200 Non-ECC Unbuffered - £117.98

I've seen other types that are CL2, EEC and buffered but don't know what it means. They're more expensive so they must perform better then the CRucial ones but why?

Thanks :)

AdrianT

you should check your computer CPU and mobo first. My athlon64 doesn't like pairs of double sided dimms and falls back to 333mhz even though the ram is 400mhz. I need single sided or a mix of double and single in the right slots to get 400mhz.

other than that, go for the smallest CL you can find at the right speed. Crucial are a good brand. Cheaper good ram is likely to be CL2.5 or 3.

FlameDuck

ECC is Error Correcting Chip/Code. It drasticly reduces the chance of program failure due to memory corruption. Since the chance for the is already pretty slim (one in ten million or so) it's not normally used outside server or mainframe type computers.

ECC Ram is not normally compatible with non-ECC RAM and requires different sockets.

Grey Alien

CL is Cache Latency. 2.5 is normal.

flamey: is it actually true that ECC requires a different socket? Nothing I've read indicates this. There is even an option in my BIOS to select whether or not I've got ECC installed.

Gabriel

I don't think it's a specific ECC-only socket, if that's what you mean. I think it's just that certain sockets ( 939 and 754 spring to mind, but don't quote me as I haven't checked it ) don't support it.

Or maybe it's the chipset rather than the socket itself. I do know that most Amd64 939's and 754's don't support ECC anyway.

Yan

CL stands for Column Address/Access Strobe (CAS) Latency - It's the delay between the column being addressed and the data being available on the output. Lower is faster, especially with the A64.

As for ECC, that'd be down to the memory controller. This is on-die for A64s and supports unbuffered ECC (registered for FX). ECC RAM is slightly slower than non-parity RAM.

xlsior

flamey: is it actually true that ECC requires a different socket? Nothing I've read indicates this. There is even an option in my BIOS to select whether or not I've got ECC installed.

No, the sockets are identical. Some motherboards support both, so you can choose what you want.
some even allow you to mix & match ECC and non-ECC dimm's, although in that case the ECC functionality will get turned off so there really is no point in doing so.

ECC is more reliable (since it can detect & correct some mistakes, where a bit falls over due to environmental interference), but also more expensive and slightly slower (due to the extra step required to verify that the info is indeed correct).
you pretty much only see ECC used in servers and high-end workstations / critical applications. Pretty much all normal desktop PC's use non-ECC.

FlameDuck

No, the sockets are identical.

If you'll remove and examine your ECC DIMM you'll notice that the notch on it, is different from the notch on your non ECC DIMM. While your motherboard sockets might support both, they are not interchangable, as you seem to suggest.

Mustang

#10

CAS - RAS latency

Latency is the time it takes for the memory devices, after getting a command and address to produce its first data word. CAS latency is the clock cycles between the issuance of the read command and when the data comes out. This is a critical element of speed for PC100/133 memory. DIMMs with CAS latency 2 are faster than DIMMs with CAS latency 3. An 8ns DIMM with CAS latency 2 is faster than a 6ns DIMM with CAS latency 3. A 100 MHz DIMM operating at CAS latency 2 operates at 125 MHz enhancing system performance. A CAS 2 DIMM operates at 15-25% faster depending on system application. It is recommended that P-II & P-III and Athlon chips use CAS 2 DIMMS running at 100 or 133MHZ for extra speed, and crash free operation.

What are RAS and CAS?

They stand for Row Access Strobe (RAS) and Column Access Strobe (CAS). Each describes how long it takes to read a row or column of memory cells, known as the CAS/RAS Latency. Each is described with a rating number, where lower numbers are better. The rating is also dependant on the front side bus speed of your motherboard so the rating may rise on higher speeds.

A common marketing term attached to SDRAM modules is either "CAS2" or "CAS3". Unfortunately, this is a bit misleading, as they should be referred to as CL2 or CL3, since they refer to CAS Latency timings (2 clocks vs. 3 clocks). As you have seen from the discussion above, the CAS Latency of a chip is determined by the column access time (tCAC). This is the time it takes to transfer the data to the output buffers from the time the /CAS line is activated.

The "rule" for determining CAS Latency timing is based on this equation:

tCL = tCAC / tCLK

In lay terms, CAS Latency times the system clock cycle length must be greater than or equal to the column access time (tCAC). In other words, if tCLK is 10ns (100 MHz system clock) and tCAC is 20ns, the CAS Latency (CL) can be 2. But if tCAC is 25ns, then CAS Latency (CL) must be 3. The SDRAM specification permits CAS Latency values of 1, 2 or 3.

http://bugclub.org/eric/memory/computermemory.html

#11

Flame: I thought that was true of SDRAM packages, where one notch placement denoted the features of the RAM and another denoted the operating voltage.

I am a bit behind the times though :)

Grey Alien

#12

CL stands for Column Address/Access Strobe (CAS) Latency

Oh OK, I was misinformed by my PC parts rep, nice...

Amon

#13

Thanks for all the info guys. :)