Fast as an arrow

An arrow is really fast, about 100 m/s fast when it leaves the bow, that’s about 360 km/h. A microprocessor is running at perhaps 3 GHz, so how far does an arrow move during one CPU cycle? 3 GHz means 3 thousand million cycles per second, and therefore an arrow moves in one cycle 30 nanometres. That’s the order of magnitude of structure elements on microchips, and is about the length of 30 atoms of silicon end-to-end.

Light, the fastest thing in the universe, itself moves only 10 cm in one CPU cycle, that’s for most people about the width of their hand. Since electric signals travel at most at the speed of light, this is the total length that impulses can travel within the CPU to accomplish whatever needs to be done in one cycle. If your memory module is 10 cm away on the mainboard from your CPU, just sending a signal out and back takes at least two cycles. This explains one of the speed limits computers had when they were large like cupboards, built from discrete components. If the cycles were too short, it would be difficult to keep distant parts of the computer in sync, because signals back and forth took longer than a cycle. This also gives a good reason why in Apple’s M processors the memory is right next to the processing units in the encapsulation. That’s bad for adding memory later and modularity, but signal paths are so much shorter. As processing speed is reaching limits of physics, it seems that such tight packages of processing and memory are going to be the common form of future processors, as they already are in processing units for mobile phones and tablets.

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