DDR SDRAM (Double Data Ram) evolved from SDRAM when an alternative was sought.
DDR SDRAM is the internal pipelined Double Data Rate (DDR) which occurs when there are two data accesses per clock cycle, as opposed to the Single Data access per clock cycle (SD) during the process of information in system memory.
In layman terms, DDR is basically the result of designing the data to be processed on both the upward and downward signal slopes, whereas SDR is just a single process per signal slope. Imagine an oscilloscope which displays a series of curves which goes up and down. So DDR is sort of a 2-for 1 deal, (faster) if you will.
Currently, the JDEC standards available for DDR-SDRAM are PC1600, PC2100, and PC2700. This new labeling refers to the total bandwidth of the memory, as opposed to the old standard which listed the speed rating (in MHz) of the SDRAM memory, in this case, the PC66, PC100, and the PC133.
Another telltale difference between SDRAM and DDRAM is
the amount of pins used. SDRAM consists of only 168 pins, whereas DDRAM has 184
pins. The two types of memory are not interchangeable, and the change in pin numbers ensures memory is not inserted in boards which do not support it.
One of the largest major memory manufacturers in the
world is Micron. They were one of the first to announce the availability and
prices for DDRAM (through their online store Crucial.com ) when little else was known about this newly adopted type of memory platform.
Initially the first crop of chipsets which support DDR
memory would outperform the venerable SDRAM PC100/ PC133 in most applications and benchmarks by up about 10+ percent. Now a days, with the release of DDR333 supporting chipsets everywhere, the performance gap is substantially higher.