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Double data rate synchronous dynamic random access memory (DDR SDRAM) is a type of random-access memory module that allows for higher transfer rates and faster performance compared to earlier RAM modules. DDR SDRAM transfers memory on both the rising edge and falling edge of a clock cycle, doubling the transfer rate. This is where the name “double data rate” comes from.
DDR SDRAM works on the principle of transferring data on both the rising edge of a clock cycle and the falling edge. While most people might perceive a computer’s clock cycle as a discrete event in time, electronically it is a wave, typically a square wave. This wave has a rising edge and a falling edge. Typical RAM only transmits data on the rising edge, but SDRAM transfers data on the falling edge, which allows it to transfer more data in a clock cycle, doubling the performance of a chip. This is called “double pumping.” DDR can transfer data at a rate of up to 1600 MB/s.
DDR SDRAM modules have complex circuitry to maintain this timing, using phase-locked loops to keep the timing accurate. DDR has been supplanted by DDR2, DDR3 and DDR4. None of these standards are forward or backward compatible, so motherboards with one type of DDR can only use RAM modules that support that standard.