Page "Universal Turing machine" Paragraph 2
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Every Turing machine computes a certain fixed partial computable function from the input strings over its alphabet.
In that sense it behaves like a computer with a fixed program.
However, we can encode the action table of any Turing machine in a string.
Thus we can construct a Turing machine that expects on its tape a string describing an action table followed by a string describing the input tape, and computes the tape that the encoded Turing machine would have computed.
Turing described such a construction in complete detail in his 1936 paper:
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