DDR SDRAM

From TheBestLinks.com

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. This effectively doubles the transfer rate without increasing the frequency of the front side bus. Thus a 100MHz DDR system, termed PC1600 or DDR200, has an effective clock rate of 200MHz.

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 - 100MHz actual clock rate, 200MHz effective clock rate, 1.6GB/s bandwidth per channel.

PC2100 or DDR266 - 133MHz actual clock rate, 266MHz effective clock rate, 2.1GB/s bandwidth per channel.

PC2700 or DDR333 - 166MHz actual clock rate, 333MHz effective clock rate, 2.7GB/s bandwidth per channel.

PC3200 or DDR400 - 200MHz actual clock rate, 400MHz effective clock rate, 3.2GB/s bandwidth per channel.

PC3700 or DDR466 - 233MHz actual clock rate, 466MHz effective clock rate, 3.7GB/s bandwidth per channel. (Partially approved by JEDEC, but does not as yet have ECC support)

PC4200 or DDR533 - 266MHz actual clock rate, 533MHz effective clock rate, 4.2GB/s bandwidth per channel. (Unlikely to be approved by JEDEC, as it would interfere with the introduction of DDR-II)

PC4800 or DDR600 - 300MHz actual clock rate, 600MHz effective clock rate, 4.8GB/s bandwidth per channel.

Some new chipsets use these memory types in dual (and in some rare cases, quad) channel configurations, which double (or quadruple) the effective bandwidth. In the dual-channel configuration it is recommended to use a matched pair of memory modules to optimize performance. The modules in a pair have the same size, speed and CAS latency, enabling the chipset to interleave accesses with maximum efficiency.

DDR SDRAM

DDR is slowly being 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.

Memory manufacturers have stated that it is impractical to mass-produce DDR-I memory with effective clock rates in excess of 400MHz. DDR-II picks up where DDR-I leaves off, and is available at clock rates of 400MHz and higher.

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