DDR SDRAM

DDR SDRAM
Double Data Rate Synchronous Dynamic Random-Access Memory

Comparison of DDR modules for desktop PCs (DIMM)
Front and back of a 1GB DDR-400 RAM module for desktop PCs (DIMM)
Developer	Samsung[1][2][3] JEDEC
Type	Synchronous dynamic random-access memory
Generations	DDR2 DDR3 DDR4 DDR5
Release date	DDR: 1998; 26 years ago (1998) DDR2: 2003 DDR3: 2007 DDR4: 2014 DDR5: 2020
Specifications
Voltage	DDR: 2.5/2.6 DDR2: 1.8 DDR3: 1.5/1.35 DDR4: 1.2/1.05 DDR5: 1.1

Double Data Rate Synchronous Dynamic Random-Access Memory (DDR SDRAM) is a double data rate (DDR) synchronous dynamic random-access memory (SDRAM) class of memory integrated circuits used in computers. DDR SDRAM, also retroactively called DDR1 SDRAM, has been superseded by DDR2 SDRAM, DDR3 SDRAM, DDR4 SDRAM and DDR5 SDRAM. None of its successors are forward or backward compatible with DDR1 SDRAM, meaning DDR2, DDR3, DDR4 and DDR5 memory modules will not work on DDR1-equipped motherboards, and vice versa.

Compared to single data rate (SDR) SDRAM, the DDR SDRAM interface makes higher transfer rates possible through more strict control of the timing of the electrical data and clock signals. Implementations often have to use schemes such as phase-locked loops and self-calibration to reach the required timing accuracy.^[4][5] The interface uses double pumping (transferring data on both the rising and falling edges of the clock signal) to double data bus bandwidth without a corresponding increase in clock frequency. One advantage of keeping the clock frequency low is that it reduces the signal integrity requirements on the circuit board connecting the memory to the controller. The name "double data rate" refers to the fact that a DDR SDRAM with a certain clock frequency achieves nearly twice the bandwidth of a SDR SDRAM running at the same clock frequency, due to this double pumping.

With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate (in bytes/s) of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus, with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.