DDR2 memory is a type of SDRAM. SDRAM stands for Synchronous Dynamic Random Access Memory. The memory is organized like a matrix or chart, with data arranged in rows and columns. The data is stored in blocks whose location are found by the coordinates of the specific rows and columns. Latencies come from the memory looking for the data in these series of rows and columns. The four most common latencies are:
- Column Address Strobe Latency (tCAS / CAS / tCL). This is the number of clock cycles needed to access a specific column of data.
- Row Address Strobe (tRCD, RAS). This is the number of clock cycles that it takes for the memory to actually start reading or writing from the time the coordinates of the data are defined.
- Row Precharge time (tRP) and is the number of clock cycles needed to end access to one row of memory and open access to the next row of memory.
- Active to Precharge Delay (tRAS) and is the number of clock cycles needed to access a specific row of data in the memory between the data request and the pre-charge command.
So what you have are 4 series of latencies. If you didn’t get much of the above paragraph, get this. The lower the latencies the better for system performance. However, lower latencies mean less stability at any given voltage. Common value of latencies are 3-3-3-X, 4-4-4-X, 5-5-5-X. The reason I put X in the last spot is because the latencies in this sport vary greatly, but are most commonly between 4 and 18 clock cycles.
Simply comparing memory latencies with considering the speed at which the memory is running those latencies is silly. This is because the overall latencies in nano-seconds is derived from dividing your total latencies in cycles by how many cycles your RAM can complete in one second. This gives you latencies per operation in seconds.
For example:
DDR2-800 does 800,000,000 cycles per second. Latencies of 4-4-4-12 add up to 24 cycles per operation of latency. Divide 24 cycles of latencies by 800,000,000 cycles and you get 30 nano-seconds worth of latencies per operation. However, DDR2-1000 with latencies of 5-5-5-15 also net you the same 30 nano-seconds of latencies per operation (30 / 1,000,000,000).
However, even though both settings have the same latencies. DDR2-1000 @ 5-5-5-15 is better than DDR2-800 @ 4-4-4-12, this is because DDR2-1000 has more data throughput when compared to DDR2-800.
see:
Intel Overclocking Redefined: Guide to Successful Overclocking with NB Straps in Mind - Overclock.net - Overclocking.net