DDR SDRAM

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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 nearly doubles the transfer rate without increasing the frequency of the front side bus. Thus a 100 MHz DDR system has an effective clock rate of 200 MHz when compared to equivalent SDR SDRAM, the “SDR” being a retrospective designation.

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 100 MHz, DDR-SDRAM gives a max transfer rate of 1600 MB/s.

JEDEC has set standards for speeds of DDR SDRAM, divided into two parts: The first specification is for memory chips and the second is for memory modules.

Chip specification

DDR-200: DDR-SDRAM memory chips specified to operate at 100 MHz
DDR-266: DDR-SDRAM memory chips specified to operate at 133 MHz
DDR-333: DDR-SDRAM memory chips specified to operate at 166 MHz
DDR-400: DDR-SDRAM memory chips specified to operate at 200 MHz

Stick/module specification

PC-1600: DDR-SDRAM memory module specified to operate at 100 MHz using DDR-200 chips, 1.600 GByte/s bandwidth
PC-2100: DDR-SDRAM memory module specified to operate at 133 MHz using DDR-266 chips, 2.133 GByte/s bandwidth
PC-2700: DDR-SDRAM memory module specified to operate at 166 MHz using DDR-333 chips, 2.667 GByte/s bandwidth
PC-3200: DDR-SDRAM memory module specified to operate at 200 MHz using DDR-400 chips, 3.200 GByte/s bandwidth

Note: All RAM speeds in-between or above these listed specifications are not standardized by JEDEC — most often they are simply manufacturer optimizations using higher-tolerance or overvolted chips.

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. PC-1600 (designed to run at 100 MHz) and PC-2100 (designed to run at 133 MHz). 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).

DDR SDRAM DIMMs have 184 pins (as opposed to 168 pins on SDR SDRAM, or, 240 pins on DDR-2), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDR SDRAM has two). DDR operates at a voltage of 2.5 V, compared to 3.3 V for SDR SDRAM. This can significantly reduce power usage.

Many new chipsets use these memory types in dual-channel or even quad channel configurations, which doubles or quadruples the effective bandwidth.

Alternatives

DDR is being replaced by DDR2 SDRAM, which has some modifications to allow higher clock frequency, but operates on the same principle as DDR. Competing with DDR2 will be Rambus XDR-DRAM. It is expected that DDR2 will become the standard, since QDR (Quad Data Rate) is too complex to implement, while XDR is lacking support.

DDR Prefetch buffer width is 2 bits, DDR2 uses 4 bits.

Memory manufacturers have stated that it is impractical to mass-produce DDR1 memory with effective clock rates in excess of 400 MHz. DDR2 picks up where DDR-1 leaves off, and is available at clock rates of 400 MHz and higher.

RDRAM is a particularly expensive alternative to DDR SDRAM, and most manufacturers have dropped its support from their chipsets.

External links

Official JEDEC website
http://www.ddrmemoryram.com/ddrsdram_and_sdram.html
http://www.pcguide.com/ref/ram/tech_SDRAM.htm

Computer memory

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Chip specification

Stick/module specification

Alternatives

See also

External links