Introduction

DDR and DDR2 memories are classified according to the maximum speed at which they can work. But, besides the speed, there is another information that tells you the memory performance: timings. Timings are numbers like 2-3-2-6-T1, 3-4-4-8 or 2-2-2-5, the lower the better. In this tutorial we will explain you exactly what exactly each one of these numbers mean.

DDR and DDR2 memories follow the DDRxxx/PCyyyy classification. By the way, if you are interested in knowing the difference between DDR and DDR2 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, and DDR2-667 can work up to 667 MHz. It is important to notice that this is not the real clock speed of the memory: the real clock of the DDR and DDR2 memories is half the labeled clock speed. This way, in fact, DDR400 memories work at 200 MHz and DDR2-667 memories work at 333 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, hence they are labeled as PC3200. DDR2-667 memories transfer data at 5,336 MB/s and they are labeled as PC2-5400. As you can see, we use the number “2” after “DDR” or “PC” to indicate that we are talking about DDR2 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. On Figure 1 you can see a PC2-4200 memory module from Corsair, which uses DDR2-533 memory chips.

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Figure 1: A DDR2-533/PC2-4200 memory module.

The maximum transfer rate for a memory module can be calculated thru 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 to 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 DDR500/PC4000 memories. Even though they are labeled as DDR500, they won’t run at 500 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 DDR memories, they will run at 400 MHz (DDR400) – which the maximum DDR standard speed –, achieving a maximum transfer rate of 3,200 MB/s (or 6,400 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 500 MHz nor automatically achieve the 4,000 MB/s transfer rate.

So, why someone would buy these modules? For overclocking: since the manufacturer guarantees that these modules will run up to 500 MHz, you know that you can raise the memory bus clock up to 250 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). So 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 that we have to know about DDR/DDR2 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.

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