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 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
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.
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.