Memory timings
Memory timings or RAM timings measure the performance of DRAM memory using four parameters: CL, TRCD, TRP, and TRAS in units of clock cycles; they are commonly written as four numbers separated with dashes, e.g. 7-8-8-24. The fourth (tRAS) is often omitted, and a fifth, the Command rate, sometimes added (normally 2T or 1T - also 2N, 1N). These parameters specify the latencies (time delays) that affect speed of random access memory. Lower numbers usually imply faster performance. What determines absolute system performance is actual latency time, usually measured in nanoseconds.
When translating memory timings into actual latency, it is important to note that they are in units of clock cycles, which for double data rate memory is half the speed of the commonly-quoted transfer rate.
For example, DDR3-2000 memory has a 1000 MHz clock frequency, which yields a 1 ns clock cycle. With this 1 ns clock, CL=7 gives an absolute latency of 7 ns. Faster DDR3-2666 (with a 1333 MHz clock, or ~0.75 ns per cycle), even with a longer CL=9, gives a shorter absolute latency of ~6.75 ns.
