DDR

Stands for "Double Data Rate."

DDR is a version of SDRAM, a type of computer memory. Unlike SDR (single data rate) SDRAM, which can perform a read or write action once per clock cycle, DDR can perform two—one on the rising edge of the electrical signal and again on the falling edge. DDR also operates at a lower voltage than SDR (2.6v compared to 3.3v), making it more power-efficient.

DDR RAM is available in both DIMM modules for desktop computers, and SO-DIMM modules for laptops. DDR DIMM modules have more pins than SDR DIMM modules, as well as a unique notch position, preventing a DDR DIMM from being inserted into a SDR DIMM slot and vice versa.

A Corsair PC-3200 DDR DIMM module
A Corsair PC-3200 DDR DIMM module

DDR RAM modules are graded based on the amount of data they can transfer per second, which is based on its clock speed. For example, a PC-1600 module operates at a clock speed of 100 MHz. It transfers 64 bits, or 8 bytes, of data twice per clock cycle for a total transfer speed of 1,600 MB/s (100 MHz x 2 transfers per cycle x 8 bytes). A faster PC-3200 module instead operates at a clock speed of 200 MHz, for a total transfer speed of 3,200 MB/s (200 MHz x 2 transfers per cycle x 8 bytes).

NOTE: DDR SDRAM was introduced in 2000, and succeeded by DDR2 SDRAM in 2003. DDR is also retroactively known as DDR1 when being compared to later generations.

Updated September 28, 2022

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