A Super User’s Guide to Memory (QotW #40)

Memory.  Every Super User knows that they need it, and if you’re like me, you can never get enough.  I know I somehow find ways to use up all 16 Gb of my memory on my desktop.  In fact, back in Nov 2010 Tom’s Hardware suggested that the minimum system RAM should be around 8Gb!

But what’s really frustrating is buying memory.  There are so many factors to consider that it can get overwhelming.  This is the same issue that Super User nathpilland had:

What am I looking for in RAM?

This is the one area of computer building that still has me in the dark, and I think a lot of people are with me… There are many different types of RAM, with each company having high and low end sticks. What is the difference between the high and low end? Also, what do the numbers in the latency mean? What is the speed rating (I know 1600 MHz is about normal) and how much is too much? What’s the difference between dual channel and single channel? Can you overclock/overvolt? Is there even a point to do so if this was possible? As you can see, I’m thoroughly confused here. I tried doing some research, but I can’t find this information anywhere on the internet. I’m not actually buying RAM, I’m just trying to get a better picture as to how all this works so I can be more educated on the hardware in computers.

So here we go.  This blog post is a stab at defining the basics you need to know about RAM.

A quick look over at NewEgg there were a few major categories that advertising threw at me:

• Type (DDR, DDR2, DDR3)
• Speed (typically in terms of 1066, 1333, 1600, etc)
• Timings
• CAS Latency

Memory Type: DDR (1 vs 2 vs 3 vs …)

What does it mean?

Kyle Brandt (SF valued associate #1) over as blog.ServerFault gave an awesome brief definition of what DDR stands for and what the major difference between the different versions are:

DDR stands for Double Data Rate. In the case of DDR memory the data transfer event actually manages to carry two chunks of data. With DDR2, you get 4, and with DDR3 you get 8. This does not include the rate of operations, but rather the amount of data chunks carried in each operation.

I’m assuming as time goes on, DDR versions will continue to double in “chunks” of data they’re capable of transferring.

What should you look for when purchasing?

Compatibility is going to be the key when selecting memory type.  Memory type is specific to motherboards.  Meaning that a motherboard running DDR2 will only use DDR2.  So if you’re looking to upgrade your memory, then make sure to keep the same type as what you had previously.  If you’re looking to build, the higher the version, the more data that the memory can carry.

Speed

What does it mean?

This is typically measured in Megatransfers per sec (MT/s).  From wikipedia:

1 MT/s is 10⁶ or one million transfers per second.

What should you look for when purchasing?

The higher the number, the faster the transfer rate.  This means the higher the performance.

Timing (CL-tRCD-tRP)

What does it mean?

The three front number of a timing (i.e. 9–9–9-27) represent three different things:

1. CL – Clock cycles between sending a column address to the memory and the beginning of the data in response
2. tRCD – Clock cycles between row activate and reads/writes
3. tRP – Clock cycles between row precharge and activate

What Should you look for when purchasing?

TM has a great answer as to what you should look for:

Generally, lower is better, however, it’s not always true, as the separate numbers have an effect on each other. You can think of this as basically a measure of how “quick” the memory is, as opposed to how “fast” it is. It’s basically the delay before actions can get started. Because the numbers measure clock cycles, remember that lower timings don’t necessarily mean faster access times. Memory running at 100mhz with a timing of 2-2-2-5 will have effectively slower timings than memory running at 400mhz with a timing of 5-5-5-15 for example (even though it takes 5 clocks rather than 2 clocks, it cycles 4x faster).

Column Address Strobe (CAS) Latency

What does it mean?

From wikipedia:

the delay time between the moment a memory controller tells the memory module to access a particular memory column on a RAM memory module, and the moment the data from given array location is available on the module’s output pins.

What should you look for when purchasing?

It’s a measure of latency, which means that, in general, the lower the number the better the performance.

Putting it all together

All in all, the higher the DDR version and speeds, and the lower the timings and CAS Latency the better the memory.  When comparing memory modules, make sure that you compare similar.  For example DDR3 ONLY with other DDR3 modules, and 1066 speeds ONLY with 1066 speeds.  Getting outside of those realms can cause a misunderstanding of what the real difference is, as noted in TM’s answer.