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The DDR SDRAM or double-data-rate synchronous dynamic random access memory, offers greater bandwidth than ordinary SDRAM. It is a type of memory-integrated circuit used in computers that transfers data on both the rising and falling edges of the clock signal. Highly efficient, it nearly doubles the transfer rate, while maintaining the frequency of the front side bus.

For example, if data is transferred 8 bytes at a time the DDR RAM transfer rate could be calculated as (memory bus clock rate) × 2 (for dual rate) × 8 (number of bytes transferred). This means with a bus frequency of 100 MHz, DDR-SDRAM gives a maximum transfer rate of 1600 MB/s. DDR Memory has 184 pins and one notch. The DDR operates at a voltage of 2.5V.

The DDR SDRAM package sizes and the standards of speed have been set by JEDEC (Joint Electron Device Engineering Council). There are two sets of specifications: one for memory chips and the other for memory modules.

There are no architectural differences between DDR SDRAM designed for different clock frequencies. The number just indicates the speed that the chip is guaranteed to run at. When you run DDR SDRAM at lower speeds than specified, it is called underclocking. Running it at higher speeds is known as overclocking. Overclocking should be tried only by pros who know what they are doing, and only on high-quality memory.

To double or even quadruple actual bandwidth, some new chipsets use these memory types in dual or even quad channel configurations. With the dual-channel configuration, a matched pair of memory modules should be used for full efficiency.

The new-gen DDR-2 with a few alterations, allows higher clock frequency and is fast replacing the DDR. It also faces competition from the Rambus XDR-DRAM. Other options like QDR (Quad Data Rate) are too complex to implement, while XDR offers no support. Today the DDR-2 is set to become the standard.