Tuesday: 25.May.2004

Memory (RAM) Primer, Corsair XMS modules

I've been getting questions on the nuances of how to select the right memory (RAM). I admit it can be confusing, so I'll try to break it down (Radify it) for you.

Most current-generation motherboards, such as the Asus P4C800-E Deluxe, which I like, support two channels of DDR400 RAM. DDR400 is synonymous with PC-3200. These are different standards that (to you & me) mean the same thing. A dual-channel configuration implies you use two sticks of RAM (for best performance).

The XMS series (from Corsair) makes purchasing two sticks easy, because they offer what are called TwinX Matched Memory Pairs. These "matched pairs " are designed to work together in a dual-channel configuration.

Reader comments: 1. Dual channel works fine with any even number of DIMMs, not just two. Most dual channel motherboards support four sticks. If you install matching DIMMs in each pair of slots, you'll get dual channel performance.

2. TwinX is a marketing tactic, not a design. While it may be true that Corsair qualifies TwinX modules for working well together, they're no different from ordinary Corsair DIMMs (of the same specs) sold individually.

If you opt to go with Corsair's XMS series RAM (I did), you have a few questions to answer. The first is, do you want 512-MB (2 X 256-MB sticks) or 1-GB (2 X 512-MB sticks) of memory. Since you get double the memory for ~50% more ca$h, I recommend 1-Gig, But your system will likely run fine with 512-MB of memory.

••• continued •••

If you do things such as audio-editing or video-editing or graphics-editing, 1-Gig is definately the way to go. The more programs you have open concurrently, the more you'll want the 1-Gig. Note that the Asus P4C800-E Deluxe motherboard I mentioned earlier supports 4-GB of memory (4 x 1-GB sticks).

Next you need to decide what color heat-speaders you want. Since your memory is enclosed within the case, it really doesn't matter. But, since I like an ALL-BLACK system, I prefer BLACK heat-speaders. Corsair XMS memory modules that do NOT end with the letters PT are BLACK. PT stands for Platinum (the color, not the precious metal that is more expensive than gold).

Reader comment: 3. You neglected to mention that platinum-colored DIMM heat spreaders *perform better* than black heat spreaders; it's not just a cosmetic issue. This is because the lighter color helps reflect stray electrons back into the RAM chips, reducing the need for latency-inducing error correction loops. PS - Just kidding about this point. =)

Some modules, such as the TwinX1024R-3200C2, contains the letter 'R' in their name. The 'R' stands for Registered. You don't need registered RAM unless you have an AMD Athlon 64 system.

Reader comment: 4. Only Athlon 64 systems using Socket 940 require registered memory. Socket 754 and Socket 939 systems don't. (I'm not sure whether the latter two even support registered RAM.) Socket 940 is in the process of being phased out in favor of Socket 939.

Now we get into the trickier aspects: latencies (also called memory timings). If you look here, you'll see four numbers listed under the heading of Latency, such as 3-4-4-8. Lower numbers are faster, and therefore better.

At the standard PC-3200 speed, you can purchase two types of RAM that have lower latecies than the standard 3-4-4-8. The first is 'C2". The first number (of the four listed) refers to 'CAS' (Column Access Strobe). It is the most important number of the four. C2 refers to CAS-2 memory, which is a little zippier than standard CAS-3 memory. If you look at that first number for all the modules that have the letters C2 in their name, you'll notice that the first number is 2.

Reader comment: 5. For DDR memory, not CAS latency but rather RAS-to-CAS delay (tRCD) is considered to be the most important latency number by people who have done lots of testing, like MS over at Lost Circuits. It's also harder to improve tRCD than CAS, which is why those LL modules (with the tRCD of 2) are more expensive than plain CL2 modules.

You will pay a little more for CAS-2 certified memory. It is unlikely you will notice a difference in the performance of your system running it at CAS-2 timings, compared to CAS-3. But your system will run a little zippier. Personally, I prefer C2 memory, because it usually only co$ts a little more. If you look at the Black Beast, you'll see I selected 1-Gig of TwinX1024-3200C2 (Black heat-speaders) for US$280. Note that the same memory (C2) with Platinum heat-spreaders co$t a little less at $268 (at time of posting).

The other option is what is called Low-Latency memory. This is the fastest memory available at the PC-3200 speed. You can identify this memory by the letters LL in the product name, such as TwinX1024-3200LL. You usually pay a hefty premium for LL memory. At time of this posting, Newegg sells 1-Gig of the LL memory (2X512-MB sticks) for US$350. That's $70 more than the C2 RAM. The difference between C2 and LL memory is that LL memory reduces the third number to 2, whereas C2 runs the third number at a timing of 3.

See reader comment #5 posted above.

Corsair also offers a PRO series, such as the TwinX1024-3200C2PRO, which sports flashing lights on the modules that illuminate in sync with memory usage. If you can see inside your case during normal opertion, this could be cool. If not, it seems like a waste, since you will pay more for these 18 "activity LEDs".

Lastly, if you want to overclock your system, you will want/need RAM designed to run faster than standard PC-3200 speeds. To this end, Corsair offers PC-3700 (DDR-466), PC-4000 (DDR-500) and PC-4400 (DDR-550) memory. Once you move above the standard PC-3200 modules, latencies/timings are fixed at 3-4-4-8, so you don't have to be concerned about either C2 or LL. Your only questions are how much and what color heat-sinks.

Reader comment: 6. You don't *need* faster-rated memory to overclock your system. As long as your motherboard lets you adjust the FSB:memory clock frequency ratio (and most current boards do), you can usually keep the memory clock at or below your RAM's rated speed while overclocking your CPU. Faster RAM is only useful if you want to run your memory at a 1:1 ratio with your overclocked FSB (which is faster than running the memory at a lower speed).

Hope that helps. To recap:

C2 = CAS 2-3-3-6
LL = Low Latency: CAS 2-3-2-6
PT = Platinum Heat Spreader
Pro = Activity LED's
R = Registered
RE = Registered with ECC
With nothing after the part# that would be a standard Black Heat spreader.
This applies to XMS only!

There's a review posted here that compares various brands of memory. It's conclusion:

"Out of the six modules tested we found that the Corsair XMS 4000 memory line was the most stable of the bunch. It had no errors from memTEST86 and RSTpro testing when ran at default/aggressive timings at DDR500 and even at high speeds at loose timings! No other memory series we tested can boast these results. As far as overclocking is concerned, the Corsair XMS 4000 has ample head room, which allowed us to reach speeds above 520MHz on our module even when run at tighter CL2.5 timings.

The fact that Corsair took a conservative approach on setting the timings even though the memory runs fine at CL2.5 actually plays in favor of the end user. Corsair makes it simple as there is only one DDR500 level of performance to pick from (XMS and PRO series are both 3-8-4-4). The Corsair modules are able to get an extra boost of performance that is available thanks to the Hynix IC’s and the great R&D work that Corsair put into the actual PCB design. With a solid price point, no errors on three of our four testing stages, and good performance it is our belief that the Corsair XMS 4000 series is the #1 memory pick on the short list of DDR500 memory modules.

From the conclusions we found on this review and working with each of these modules for over a month we feel that the 1GB Corsair XMS 4000 dual channel kit is worthy of having our editor's choice award as our pick for the best of the DDR500 memory currently on the American market or possibly even the world."

There's a good Flash presentation on memory basics posted here.

Posted by Rad at May 25, 2004 01:23 AM


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Posted by: Rad at May 25, 2004 10:37 AM

Been having problems with MT Blacklist.

Posted by: Rad at May 25, 2004 10:43 AM

I think it might be the "dash" character that is causing the "questionable content" problem.

Posted by: Rad at May 25, 2004 10:47 AM

I think - I might have - fixed the glitch with the dashes. ---

Posted by: Rad at May 25, 2004 10:58 AM

If you take the FREQUENCY the RAM operates in MHz at (in the case of DDR400 memory):


and multiply that times 2 (for Double date rate RAM), that would equal:

DDR400 (known as the "Data Rate", also called the "RAM Rating")

And if you multiply the "RAM Rating" by 8 (as in 64-bit data path = *8* bytes wide)

you get the "Module rating":

PC3200 (also referred to as the "Bandwidth", which equals 8 times the "data rate" in this case)

This might help to illustrate where those weird numbers come from.

Posted by: Mongo at May 25, 2004 03:41 PM

Frequncy | RAM Rating | Module Rating
MHz | Data Rate | Bandwidth
200 | DDR400 | PC3200
250 | DDR500 | PC4000

Posted by: RAM Guy at May 25, 2004 03:49 PM