Computer Hardware DDR1 and SDRAM

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DDR2 SDRAM

By early 2003, the original DDR-SDRAM technology was fast approaching its limits. As AMD and Intel transitioned to higher FSB speeds, DDR-SDRAM has been hard pressed to keep pace. Mainstream DDR-SDRAM tops out at PC3200. Dual-channel DDR chipsets (which combine the bandwidth of paired memory modules) using PC3200 memory limit peak bandwidth to 6,400 MB/s. That matches the bandwidth requirements of a processor with a 64-bit (8-byte) wide memory channel operating with an 800 MHz FSB, such as mainstream Pentium 4 models, but as new processors are introduced, even dual-channel DDR-SDRAM will be unable to keep up with increases in processor bandwidth.

The long-term solution is DDR2 SDRAM. DDR2 incorporates a series of evolutionary improvements on standard DDR technology, including increased bandwidth, lower voltage (1.8V versus the 2.5V of DDR), lower power consumption, and improved packaging. Just as DDR-SDRAM doubled bandwidth over SDR-SDRAM when running at the same clock rate, DDR2-SDRAM doubles bandwidth over DDR-SDRAM by doubling the speed of the electrical interface. DDR2 DIMMs use a new 240-pin connector that is incompatible with the 184pin DDR-SDRAM and earlier connectors. lists the important characteristics of DDR2-SDRAM, with PC3200 (DDR400) DDR-SDRAM shown for comparison.

Table 6-1. DDR2 characteristics

Chip

Chip clock

I/O clock

Module name

Module bandwidth

DDR400

200 MHz

400 MHz

PC3200

3,200 MB/s

DDR2-400

100 MHz

200 MHz

PC2 3200

3,200 MB/s

DDR2-533

133 MHz

266 MHz

PC2 4200

4,200 MB/s

DDR2-667

166 MHz

333 MHz

PC2 5300

5,300 MB/s

DDR2-800

200 MHz

400 MHz

PC2 6400

6,400 MB/s

DDR2-1000

250 MHz

500 MHz

PC2 8000

8,000 MB/s


Although Intel has pushed DDR2 hard since its introduction, initial uptake was slow for two reasons. First, DDR2 memory originally sold at a very high premium over DDR memory, sometimes as much as 200% to 300%. By late 2005, that differential had dropped to 15% or 20%, making DDR2 a more reasonable choice. Second, although DDR2 offers much higher bandwidth than DDR, it also suffers from much higher latency. In effect, that means that although DDR2 can deliver data at higher speed than DDR, it takes longer to start delivering the data. For applications that use primarily sequential memory access, such as video editing, the bandwidth advantage of DDR2 offers noticeably higher memory performance. For applications that access memory randomly, including many personal productivity programs, the latency advantage of DDR gives it the edge. DDR2-SDRAM DIMMs use 240 pins and can be discriminated from 168-pin SDR-SDRAM and 184-pin DDR-SDRAM memory modules by noting the number of pins and the position of the keying notch.


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