Computer Hardware DDR1 and SDRAM
DDR2 SDRAM
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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.
