Memory modules buyers guide

Latencies

Apart from actual clock speed latencies play an important part in the performance of DDR2 memory. Latency is basically the time between a request for data and the actual availability of the requested data. Latencies are shown as a row of numbers, for instance 4-4-4-12, indicating CAS latency, RAS to CAS latency, RAS precharge and RAS cycle time (see frame). Obviously the lower the latency, the better the performance of the modules in question. The most common latencies of DDR2 memory are 3-3-3-9, 4-4-4-12 and 5-5-5-15. All modules we have tested are tested at these speeds, as shown in the table further down.

Speeds and latencies are adjustable in a computer's BIOS screen. Only overclockers or experienced users will change these manually, other users will use the memory's SPD module to set the timings and speed. A SPD chip tells the motherboard of its timings; the SPD values can be easily checked with a program like CPU-Z (freeware). Figure 1 shows the SPD timings of GEIL memory modules we have tested, as you can see this specific memory can operate at 667MHz with 5-5-5-15 timings, 533MHz with 4-4-4-12 or 400MHz with 3-3-3-9. In our product database the specifications of DDR2 modules will also show these SPD values.

You can check the SPD of your modules with cpu-z