Double Data Rate
DDR is a technique where two data are transmitted per clock cycle, instead of only one. With this technique the device achieves the double of the performance of a similar device running at the same clock rate but transferring just one data per clock cycle.
This technique is used by DDR-SDRAM memories and by the external bus of AMD CPUs (Duron, Athlon, Athlon XP, etc).
Since the device achieves a ”doubled“ performance usually the clock rate reported by the manufacturers isn’t the device’s real clock but its ”performance“ instead. For example, ”400 MHz“ DDR-SDRAM memories work at 200 MHz. Another example: Athlon XP’s external bus is said to be of 266 MHz, 333 MHz or 400 MHz while in fact it works at 133 MHz, 166 MHz and 200 MHz, respectively.
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Figure 1: How DDR technique works.