Double Data Rate DRAM: The successor to SDRAM. DDR is SDRAM capable of passing data at twice the data rate of first generation SDRAM by sending data on the trailing as well as the rising edge of the clock. DDR DRAMs are generally built as 184 pin modules with a form factor similar to 168 pin SDRAMs. At least one other form factor is approved. Two types were initially available -- PC1600 and PC2100. PC1600 operates at 200MHz (100MHz x2). PC2100 Operates at 266MHz (133MHz x2). The number nnnn in PCnnnn is roughly the number of million Bytes per second transferred during continuous data flow. As with SDRAM, DDR modules transfer 64 bits at a time.

DDR DRAMs are not physically interchangeable with previous generation SDRAMs. Neither are they interchangeable with Rambus memory modules. Indeed, while PC1600 DRAMs appear to be largely interchangeable, substantial compatibility problems have been reported between PC2100 modules from different vendors. It is expected that the compatibility problems will decrease with time, but possible incompatibility will be an issue to keep in mind when reusing older modules.

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