Double Data Rate SDRAM

DDR-SDRAM is again an evolution, this time of the SDRAM specification. As the speed of computer processors has increased in leaps and bounds, the amount of data they are able to process in a set amount of time has also increased vastly. The recent families of processors from Intel and AMD such as the Pentium 4 and Athlon XP are capable of several billion operations per second. This is wonderful from a performance standpoint if you are looking at the speed of the chip alone, but it presents somewhat of a problem for the system as a whole, since it is limited by the bandwidth of the memory.

The bandwidth of the memory is how much data it can potentially transfer in a set period of time. Essentially, the faster a processor can go, the faster the memory system supporting it needs to be able to carry data.

To increase its bandwidth, DDR-SDRAM transfers data twice on each clock cycle, achieving twice the theoretical maximum bandwidth of SDRAM running at the same speed. This does not translate to twice the memory or system performance, since the efficiency of the memory (expressed as a percentage where 100% efficiency equals one data transfer performed every clock cycle) suffers as the speed it attempts to perform operations in increases.

Despite this, it is still capable of feeding and receiving considerably more data than SDRAM, and is a suitable memory platform for modern processors like the AMD Athlon XP and the Intel Pentium 4, both of which rely chiefly on various speeds of DDR-SDRAM to provide memory support.

DDR-SDRAM has been in development since the late 90's, and was first introduced to the desktop PC market in the Geforce video card by Nvidia, followed by AMD in late 2000 with their 760 chipset for the Athlon processor. It has since completely supplanted SDRAM as the memory of choice for the home and small business PCs using both Intel and AMD processors.

In the form of 184-pin DIMM modules, DDR-SDRAM is currently available in a few speeds: PC1600 (200Mhz) PC2100 (266Mhz), PC2700 (333Mhz), PC3200 (400Mhz), PC3500 (433MHz), PC3700 (466MHz), PC4000 (500MHz), PC4200 (533MHz) and PC4400 (566MHz). The first number, for example 'PC2100' represents the maximum memory bandwidth the module can provide in Megabytes per second. The Mhz value is the clock speed it is certified to operate at. DDR-SDRAM is commonly available in 64MB-2GB sizes.

Note that some newer chipsets such as Nvidia's nForce and nForce 2 and the Intel I865 use dual-channel memory, essentially accessing two separate DDR memory modules at the same time to double the maximum bandwidth. Dual-channel requires that memory modules be added in identical pairs to the board. Regular DDR-SDRAM can still be used in this case, just be sure to purchase identical modules.