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"Theoretically DDR-SDRAM provides double the bandwidth of normal SDRAM. At 100 MHz memory clock it's able to supply a data bandwidth of 1600 MB/s, at 133 MHz it goes up to 2100 MB/s. That is why 'Team DDR' decided to call the specs of those two DDR-memory types 'PC1600' and 'PC2100'. However, I have to disappoint you, if you should think that DDR-SDRAM is simply twice as fast as SDRAM at single data rate. The basic reason is rather simple. While DDR-SDRAM is indeed able to provide double the peak bandwidth of normal SDRAM, it has still the same (CL2 DDR-SDRAM) or even a slightly worse latency (CL 2.5 DDR-SDRAM) than its older sibling. The result is that in worst-case situations DDR-SDRAM can be as slow or even slightly slower than normal SDRAM. At the same time DDR-SDRAM can easily outperform single data rate SDRAM under ideal conditions. In average, systems with DDR-SDRAM are most certainly faster than systems with normal SDRAM, but the gain depends on the application and can hardly ever reach more than 20%."
According to Tom's Hardware Guide

DDR-RAM [[i]Double Data Rate Synchronous Dynamic Random Access Memory[/i]] is bascially SDRAM [[i]Synchronous Dynamic Random Access Memory[/i]], which has been modified so as to make it more advanced, and ultimately faster.

I hope this helps,
- Stack Overflow

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