A form of computer memory called SDRAM has a variant called DDR. DDR can perform two read/write operations per clock cycle, one on the rising edge of the electrical signal and the second on the falling edge, in contrast to SDR (single data rate) SDRAM, which can only perform one operation per clock cycle. DDR is also more power-efficient than SDR because it runs at a lower level (2.6 v as opposed to 3.3 v).

DDR RAM is offered in SO-DIMM and DIMM units, respectively, for notebooks and desktop PCs. A DDR DIMM cannot be put into an SDR DIMM space or vice versa because it has more wires than an SDR DIMM module and a different notch location.

DDR RAM units are ranked according to how much data they can move per second, which is determined by their processing speed. A PC-1600 processor, for instance, runs at a 100 MHz frequency rate. It has a total transmission speed of 1,600 MB/s and transmits data at a rate of 64 bits, or 8 bytes, twice every clock cycle (100 MHz x 2 transfers per cycle x 8 bytes). Instead, a quicker PC-3200 processor runs at a frequency rate of 200 MHz, resulting in a total transmission pace of 3,200 MB/s (200 MHz x 2 transfers per cycle x 8 bytes).