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Memory Timings


Memory timings are the latencies (time delays) that affect RAM. They are usually measured by using four parameters: CL, tRCD, tRP, and tRAS, in units of clock cycles, which are written as four numbers separated by dashes, such as 6-7-7-24 for example, with a lower value meaning a faster speed.

A modern memory module contains a tiny ROM chip called the Serial Presence Detect (SPD) ROM chip and it contains the recommended memory timings for the module. It is sometimes possible to make an adjustment to these memory timings to increase performance via the BIOS and adjusted from the CMOS set-up page. This may be done by someone who has overclocked their computer and wants to get the maximum performance out of their RAM or to make the memory more stable after altering other settings.

CL - CAS Latency:
CL is the time it takes to send a column address to the memory.

tRCD - Row Address to Column Address Delay:
The tRCD is the number of clock cycles required between opening of a row of memory and accessing columns.

tRP - Row Precharge Time:
tRP is the number of clock cycles required between issuing the precharge command and opening the next row.

tRAS - Row Active Time:
tRAS is the number of clock cycles required internally to refresh the row.

CAS (Column Address Strobe), also called CL is the delay from when the memory controller tells the memory module to access a particular memory column on a memory module, and the RAS (Row Address Strobe) is the time taken when the memory controller tells the memory module to access a particular memory row on a memory module.

If you look on the sticker of a memory module (DIMM) then it usually tells you the CAS Latency (CL). A memory module with a CAS Latency of 7 (CL7) would be faster than an equivalent memory module that has a CAS Latency of 9 (CL9). CL7 only requires 7 clock cycles to access the RAM while CL9 requires 9 clock cycles.

Note that some motherboards will not let you alter the memory timings but those that do will let you configure the settings via the CMOS set-up page. It will possible be under the header 'DRAM Configuration' and you may see that the various memory timings are set to 'auto' which means it is using the default settings obtained from the SPD ROM chip. Changing these timings may make your computer unstable. More information can be found on the Overclocking page.

You can download the small CPU-Z utility at www.cpuid.com which will give you information on your memory including the timings.

Below is a couple of screen shots taken from CPU-Z which give the memory timings of the computer that i am using now:

Memory Timings in CPU-Z.

The Memory timings are 6-6-6-18 (CL, tRCD, tRP, and tRAS) and the memory bus is operating at 400MHz.

Memory Timings in CPU-Z.

This shows the default timings for a memory bus operating at 266MHz, 333MHz, and 400MHz and the voltage (1.8V).

This computer actually has a 266MHz FSB, and a 400MHz memory bus with a ratio of 2:3 (FSB:Memory bus), and uses PC2-6400 (DDR2-800) using four 1GB memory modules in four memory slots which have dual channels.