Wikipedia
Double Data Rate 2 Synchronous Dynamic Random-Access Memory (DDR2 SDRAM) is a double data rate (DDR) synchronous dynamic random-access memory (SDRAM) interface It superseded the original DDR SDRAM specification, and was itself superseded by DDR3 SDRAM (launched in 2007) DDR2 DIMMs are neither forward compatible with DDR3 nor backward compatible with DDR In addition to double pumping the data bus as in DDR SDRAM (transferring data on the rising and falling edges of the bus clock signal), DDR2 allows higher bus speed and requires lower power by running the internal clock at half the speed of the data bus The two factors combine to produce a total of four data transfers per internal clock cycle
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