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DDR SDRAM

DDR SDRAM or double-data-rate synchronous dynamic random access memory is a type of memory integrated circuit used in computers. It achieves greater bandwidth than ordinary SDRAM by transferring data on both the rising and falling edges of the clock signal.

DDR SDRAM DIMMs have 184 pins (as opposed to 168 on SDRAM), and are physically the same apart from the extra pins. The package sizes in which DDR SDRAM is manufactured are standardised by JEDEC.

The speed standards approved by JEDEC are as follows:

PC1600 or DDR200 - 200MHz clock rate, 1.6GB/s bandwidth per channel.

PC2100 or DDR266 - 266MHz clock rate, 2.1GB/s bandwidth per channel.

PC2700 or DDR333 - 333MHz clock rate, 2.7GB/s bandwidth per channel.

PC3200 or DDR400 - 400MHz clock rate, 3.2GB/s bandwidth per channel. (Note that PC3200 has only been partially approved by JEDEC - currently it is used in recent desktop chipsets, but not server chipsets due to the lack of ECC)

PC3500 or DDR433 - 433MHz clock rate, 3.5GB/s bandwidth per channel. (Note that PC3500 has yet to be approved by JEDEC, although it is used by several vendors)

PC3700 or DDR466 - 466MHz clock rate, 3.7GB/s bandwidth per channel. (Note that PC3700 has yet to be approved by JEDEC, although it is used by several vendors)

Some new chipsets use these memory types in dual (and in some rare cases, quad) channel configurations, which double (or quadruple) the effective bandwidth.

At some point in the upcoming months, DDR will be replaced by DDR-II, which has some modifications to allow higher clock frequency, but operates identically to DDR. Competing with DDR-II will be Quad Data Rate (QDR) and Quad Band Memory (QBM) SDRAM. It is expected that DDR-II will become the standard, since QDR is too complex to implement cheaply, while QBM is lacking support.

RDRAM is an alternative to DDR SDRAM, but most manufacturers have dropped support from their chipsets.