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 (double pumped). This effectively doubles the transfer rate without increasing the frequency of the front side bus. Thus a 100MHz DDR system has an effective clock rate of 200MHz.
With data being transferred 8 bytes at a time DDR RAM gives a transfer rate of (memory bus clock rate) * 2 (for dual rate) * 8 (number of bytes transferred). Thus with a bus frequency of 100MHz, DDR RAM gives a transfer rate of 1600MB/s. This leads to each DDR RAM speed having two different names, one being its effective clock rate (e.g DDR200) and the other being its transfer rate (e.g PC1600).
JEDEC has set standards for speeds of DDR SDRAM, they 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.
The package sizes in which DDR SDRAM is manufactured are also standardised by JEDEC.
There is no architectural difference between DDR SDRAM designed for different clock frequencies, e.g DDR200 (designed to run at 100MHz) and DDR266 (designed to run at 133MHz). The number simply designates the speed that the chip is guaranteed to run at. Hence you can run DDR SDRAM at lower clock speeds than it was made for (underclocking) or higher clock speeds than it was made for (overclocking). Note that overclocking should only be done with high quality memory and by those that know what they are doing (see overclocking for details on why).
DDR SDRAM DIMMs have 184 pins (as opposed to 168 on SDRAM), but are physically the same apart from the extra pins. DDR operates at a voltage of 2.5V, compared to 3.3V for SDRAM. This can significantly reduce power usage.
Some new chipsets use these memory types in dual or even quad channel configurations, which doubles or quadruples 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 latency timings, enabling the chipset to interleave accesses with maximum efficiency.
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-DRAM. It is expected that DDR-II will become the standard, since QDR (Quad Data Rate) is too complex to implement, while XDR is 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.
- Tom's Hardware Guide: DDR-SDRAM Has Finally Arrived (http://www6.tomshardware.com/mainboard/00q4/001030/)
- Comparison of DDR RAM and RDRAM from Crucial (http://www.crucial.com/library/ddr_vs_rdram.asp)