Date: Jun 24, 2001
Category: General Hardware
Author(s): Andy Hui, Andy Hui

DDR - Double Data Rate
SDR - Single Data Rate
SDRAM - Synchronous Dynamic Random Access Memory


DDR memory was developed to increase memory bandwidth while maintaining near the same latency and cost. The JEDEC (Joint Electron Device Engineering Council) Solid State Technology Association oversaw the development of the DDR memory specifications and standard.

DDR memory is currently based on SDRAM. As the acronym implies, DDR memory delivers twice the data that SDR SDRAM does. SDR memory transfers 64 bits of data once each clock cycle. DDR memory transfers 64 bits of data twice each clock cycle (once on rising edge and once on falling edge), effectively giving - a theoretical - 128 bits of data per clock cycle. In theory that is how it works, but DDR memory has a slight overhead, resulting in less real-world bandwidth. Hazarding a guess, it would probably be 90-95% efficient.

DDR memory uses a different speed classification than does SDR memory. SDR SDRAM memory uses such ratings as PC66, PC100, PC133, etc. They just add PC in front of the clock speed (in MHz) the memory is running at. For DDR RAM, however, it is based on bandwidth rather than clock speed. They add PC in front of the bandwidth (in MBps). Therefore DDR SDRAM that runs at 200MHz would have a classification of PC1600 because the bandwidth is 1.6 gigabytes or 1600 megabytes per second. PC2100 DDR SDRAM is running at 266MHz and provides 2.1 gigabytes per second or 2100 megabytes per second.

You can find the bandwidth of memory by multiplying the clock speed by the bus width.

i.e.

PC100 SDR SDRAM = 64 bits x 100MHz = 8bytes x 100000000Hz = 800 megabytes per second (MBps)

PC1600 DDR SDRAM = 64 bits x 2 x 100MHz = 8bytes x 2 x 100000000Hz = 1600 megabytes per second (MBps) or 1.6 gigabytes per second (GBps)

DDR SDRAM comes on 184-pin DIMMs (as opposed to 168-pin DIMMs on regular SDRAM modules) and do not fit in SDR SDRAM slots.

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