Hardware components of the PC
Main Memory
DDR RAM

Double Data rate (DDR) RAM has recently become available. DDR RAM is twice as fast as SDRAM. as it is able to transfer data on both the rising and falling edge of the clock signal, whereas Single Data (SDR) RAM only transfers data on the rising of the cycle. So, DDR RAM transfers twice the amount of data per clock cycle.

DDR RAM uses a larger 184 pin socket and so is not a direct replacement for the 168 pin DIM.