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.

  • PC3700 or DDR466 - 466MHz clock rate, 3.7GB/s bandwidth per channel. (Partially approved by JEDEC, but does not as yet have ECC support)

  • PC4200 or DDR533 - 533MHz clock rate, 4.2GB/s bandwidth per channel. (Unlikely to be approved by JEDEC, as it would interfere with the introduction of DDR-II)

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 years, DDR will be replaced by DDR-II, which has some modifications to allow higher clock frequency, but operates on the same principle as DDR. Competing with DDR-II will be Rambus XDR, 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 and XDR are lacking support.

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

See also: SDR SDRAM

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