What is DDR400?
DDR 400 is a type of computer memory. DDR stands for Double Data Rate, and 400 indicates the effective clock rate of the memory module. DDR is usually only seen in older computers, as it has been superseded by DDR 2 and 3.
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Double Data Rate Memory
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A DDR module comes in a standard DIMM (Dual Inline Memory Module) package, and can be distinguished from other types by the location of the notch in the contact edge. Unlike earlier memory modules, it can send two signals per clock cycle, effectively doubling the rate at which the memory runs.
Clock Speed
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With DDR, the apparent clock speed is twice that of the actual clock speed. For instance, a DDR 400 module does not run at 400 megahertz; rather, it runs at 200 MHz, but sends out a signal on both the rising and falling sides of the clock sine wave. A DDR module that actually ran at 400 MHz would have an apparent speed of 800 MHz. Because of heat issues, no DDR memory module ever ran that fast--DDR 2 was the first memory standard that supported such high speeds.
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Uses
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DDR 400 was used between 2000 and 2005 as the standard memory speed for high-end computers. Many older computers can still be found in operation that use DDR memory, and because of that, DDR is still being sold.
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