DDR, DDR2, and DDR3 memories are classified according to the maximum speed at which they can work, as well as their timings. Timings are numbers such as 3-4-4-8, 5-5-5-15, 7-7-7-21, or 9-9-9-24, the lower the better. In this tutorial, we will explain exactly what each one of these numbers mean.

DDR, DDR2, and DDR3 memories follow the DDRxxx/PCyyyy classification. By the way, if you are interested in knowing the difference between DDR, DDR2, and DDR3 memories, read our tutorial on this subject.

The first number, xxx, indicates the maximum clock speed that the memory chips support. For instance, DDR400 memories work at 400 MHz at the most, DDR2-800 can work up to 800 MHz, and DDR3-1333 can work up to 1,333 MHz. It is important to note that this is not the real clock speed of the memory. The real clock of the DDR, DDR2, and DDR3 memories is half of the labeled clock speed. Therefore DDR400 memories work at 200 MHz, DDR2-800 memories work at 400 MHz, and DDR3-1333 memories work at 666 MHz.

The second number indicates the maximum transfer rate that the memory reaches, in MB/s. DDR400 memories transfer data at 3,200 MB/s at the most, and hence they are labeled as PC3200. DDR2-800 memories transfer data at 6,400 MB/s and they are labeled as PC2-6400. And DDR3-1333 memories can transfer data at 10,664 MB/s and they are labeled as PC3-10600 or PC3-10666. As you can see, we use the number “2” or “3” after “DDR” or “PC” to indicate that we are talking about a DDR2 or DDR3 memory, not DDR.

The first classification, DDRxxx, is the standard used to classify memory chips, while the second classification, PCyyyy, is the standard used to classify memory modules. In Figure 1, you can see a PC3-10666 memory module, which uses DDR3-1333 memory chips. Pay attention to the timings (7-7-7-18) and voltage (1.5 V).

click to enlarge
Figure 1: A DDR3-1333/PC3-10666 memory module

The maximum transfer rate for a memory module can be calculated through the following formula:

Maximum Theoretical Transfer Rate = clock x number of bits / 8

Since DIMM modules transfer 64 bits at a time, “number of bits” will be 64. As 64 / 8 equals 8, we can simplify this formula to:

Maximum Theoretical Transfer Rate = clock x 8

If the memory module is installed on a system where the memory bus is running at a lower clock rate, the maximum transfer rate the memory module will achieve will be lower than its theoretical maximum transfer rate. Actually, this is a very common misjudgment.

For example, let’s say that you bought a pair of DDR3-2133/PC3-17000 memories. Even though they are labeled as DDR3-2133, they won’t run at 2,133 MHz automatically on your system. This is the maximum clock rate they support, not the clock rate at which they will be running. If you install it on a regular PC system supporting DDR3 memories, they will probably run at 1,333 MHz (DDR3-1333) – which the maximum DDR3 standard speed –, achieving a maximum transfer rate of 10,664 MB/s (or 21,328 MB/s if they are running under dual channel mode, read our tutorial on dual channel to understand more about this subject). So, they won’t automatically run at 2,133 MHz nor automatically achieve the 17,000 MB/s transfer rate.

So, why would someone buy these modules? Someone would buy them for overclocking. Since the manufacturer guarantees that these modules will run up to 2,133 MHz, you know that you can raise the memory bus clock up to 1,066 MHz to achieve a higher performance with your system. However, your motherboard must support this kind of overclocking (read our tutorial on memory overclocking for more details). Thus buying a memory module with a labeled clock rate higher than what your system supports is useless if you are not going to overclock your system.

For the average user, that is everything you need to know about DDR, DDR2, and DDR3 memories. For the advanced user, there is yet another characteristic: the temporization of the memory, a.k.a. timings or latency. Let’s talk about it.