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==Digital circuits==
The conceptual design above could be implemented using a variety of different technologies. As previously mentioned, a stored program computer could be designed entirely of mechanical components like Babbage's. However, [[digital circuits]] allow [[Boolean logic]] and [[binary arithmetic|arithmetic using binary numerals]] to be implemented using [[relays]] - essentially, electrically controlled switches. [[Shannon's]] famous thesis showed how relays could be arranged to form units called [[logic gates]], implementing simple [[Boolean]] operations. Others soon figured out the [[vacuum tubes]] - electronic devices, could be used instead. Vacuum tubes were originally used as a signal [[amplifier]] for radio and other applications, but were used in digital electronics as a very fast switch; when electricity is provided to one of the pins, current can flow through between the other two.
Through arrangements of logic gates, one can build digital circuits to do more complex tasks, for instance, an [[adder (electronics)|adder]], which implements in electronics the same method - in computer terminology, an [[algorithm]] - to add two numbers together that children are taught - add one column at a time, and carry what's left over. Eventually, through combining circuits together, a complete ALU and control system can be built up. This does require a considerable number of components. [[CSIRAC]], one of the earliest stored-program computers, is probably close to the smallest practically useful design. It had about 2,000 valves, Some of which were "dual components", so this represented somewhere between 2 and 4,000 logic components.
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