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Double Data Rate SDRAM (DDR SDRAM)
Only a few years ago, "regular" SDRAM was introduced as a proposed replacement for the older FPM and EDO asynchronous DRAM technologies. This was due to the limitations the older memory has when working with systems using higher bus speeds (over 75 MHz). In the next couple of years, as system bus speeds increase further, the bell will soon toll on SDRAM itself. One of the proposed new standards to replace SDRAM is Double Data Rate SDRAM or DDR SDRAM.
DDR SDRAM is similar in function to regular SDRAM, but doubles the bandwidth of the memory by transfering data twice per cycle--on both the rising and falling edges of the clock signal. The clock signal transitions from "0" to "1" and back to "0" each cycle; the first is called the "rising edge" and the second the "falling edge". Normally only one of these is used to trigger a data transfer; with DDR SDRAM both are used. Does this technique sounds familiar? It is also used by the new AGP technology to double performance over the older PCI bus technology.