Clock doubling
From Wikipedia, the free encyclopedia
Clock doubling is the simplest case of the clock multiplying technique used to improve the performance of computers by doubling the clock rate of the CPU compared to the bus. For instance, if the computer's main clock runs at 100 MHz, a clock-doubled CPU would run, internally, at 200 MHz. This is a simple way to improve the performance of the CPU without having to simultaneously improve the rest of the components in the system.
In theory for CPU-bound applications, that is when fetching data from memory is not the limiting factor, this technique will substantially improve the overall performance of the machine. However, in more modern processors where the multiplier is not 2, but much higher, the inverse occurs: the processor is limited by both the latency and bandwidth of the memory bus.
The most famous example of a clock-doubled CPU is the Intel 80486DX2, which ran at 50 or 66 MHz on a 25 or 33 MHz bus. Another example was the Weitek SPARC POWER µP, a clock-doubled 40 MHz version of the SPARC processor that could be dropped into the otherwise 20 MHz SPARCStation 2. In both cases the overall speed of the systems increased by about 75%.
The most modern significance of clock doubling is presented by the Intel Core Duo U2500. The U2500 runs on 0.75W of power on average, at a clock speed of 1066MHz. Its front side bus is 533MHz.
Today almost all processors run at a different speed than the underlying bus, and the terminology is no longer needed.
