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

With this encoding of action tables as strings, it becomes possible, in principle, for Turing machines to answer questions about the behaviour of other Turing machines. Most of these questions, however, are undecidable, meaning that the function in question

When Alan Turing came up with the idea of a universal machine he had in mind the simplest computing model powerful enough to calculate all

For those who would undertake the challenge of designing a UTM exactly as Turing specified see the article by Davies in Copeland (2004:103ff). Davies corrects the errors in the original

Davis makes a persuasive argument that Turing's conception of what is now known as "the stored-program computer", of placing the "action table"—the instructions for the machine—in the same "memory" as the input data, strongly influenced John von Neumann's

Without loss of generality, the input of Turing machine can be assumed to be in the alphabet {0, 1}; any other finite alphabet can be encoded over {0, 1}. The behavior of a Turing machine M is determined by its transition function. This function can be easily

If you allow multiple heads on the Turing machine then you can have a Turing machine with no internal states at all. The "states" are encoded as