programming / C++ / sailing / nerd stuff
DDR Memory
© 2002 / Mario Konrad

(DDR = double data rate)

When you like to buy some RAM for your computer, you have to decide which one you like or need. Then you see specifiers in the form of:


But what do those numbers mean? Read here the explanation:


The computer system of today are very fast, therefore timing is an important issue. The numbers on the RAM sticks describe the timing the memory can do.

The timing description is written in the form:


What do the numbers mean:

Part Time Description Range
A t_CL CAS (column address select) latency. This is the time between the chipset is requesting the data and the memory is getting ready, measured in clock cycles. 2.5 is available only with DDR. 2 / 2.5 / 3
B t_RCD RAS-to-CAS delay. This is the delay between the appearance of CAS and RAS. The RAM is has not infinite speed, so there is a little time to wait between those signals. 2 / 3
C t_RP RAS (row address select) precharge. This is the time which the signal RAS may pushed before the data is read from the memory. 2 / 2.5 / 3
D t_RAS Active to precharge
E Command Rate 1 / 2

Most modern mainboards/BIOS are configurable in some way. Often is OK to let the BIOS detect the type of memory you have. Sometimes it is better to provide the information manually.

Note: the RAM voltage has to be as specified by the RAM stick manufacturer. The voltage may differ from your mainboards default voltage!