Digital computers are built from hardware of great simplicity. First, they are built from devices with two states: on or off, one or zero, high voltage or low voltage, or logical TRUE or FALSE. Second, the devices are connected with extremely fine connections, currently on the order of size of a large virus. Their utility, value, and perceived extreme complexity lie in the software controlling them. Different devices have been used to build computers: relays, vacuum tubes, transistors, and integrated circuits. Theoretically, all can run the same software, only slower or faster. More exotic technologies have not proved commercially viable. Digital computer hardware has increased in power by roughly a factor of 2 every 2 years for five decades, an observation called Moore’s Law. Engineering problems with very small devices, such as quantum effects, heat, and difficulty of fabrication, are increasing and may soon end Moore’s Law.
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