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Changing state consists in shifting the dial's pointer from one labelled position to another.
This device functions as a simple memory; for example, a dial with three positions can be used to record whether the square that the scanner has just vacated contained '0' or '1', or was blank.
The basic principle of the modern computer—the idea of controlling the machine's operations by means of a program of coded instructions stored in the computer's memory—was conceived by Alan Turing.
Turing's abstract 'universal computing machine' of 1936, soon known simply as the universal Turing machine, consists of a limitless memory, in which both data and instructions are stored, and a scanner that moves back and forth through the memory, symbol by symbol, reading what it finds and writing further symbols.2 By inserting different programs into the memory, the machine is made to carry out different computations.
He gave the following simple example.7 A machine—call it M—begins work with an endless blank tape and with the scanner positioned over any square of the tape.M has four states, labelled 'a', 'b', 'c', and 'd', and is in state a when it starts work.In the table shown, 'R' is an abbreviation of the instruction 'move right one square', 'P' is an abbreviation of 'print 0 on the scanned square', and analogously 'P'.Nowadays, when so many people possess a physical realization of the universal Turing machine, Turing's idea of a one-stop-shop computing machine might seem as obvious as the wheel.But in 1936, when engineers thought in terms of building different machines for different purposes, Turing's concept was revolutionary.