It was a fabulous idea—a single machine of fixed structure which, by making use of coded instructions stored in memory, could change itself, chameleon-like, from a machine dedicated to one task into a machine dedicated to a quite different one.
Turing showed that his universal machine is able to accomplish any task that can be carried out by means of a rote method (hence the characterization 'universal').
Right from the start there was a mismatch of visions. 'In working on the ACE', he said, 'I am more interested in the possibility of producing models of the action of the brain than in the practical applications to computing'.
Turing's employers, on the other hand, thought the ACE would be Britain's national computer: a single machine that could satisfy the computing needs of 'the whole country'. Woolly-minded administrators wasted the brilliant technological achievements of Turing and his group.
His technical report 'Proposed Electronic Calculator', dating from the end of 1945 and containing his design for the Automatic Computing Engine (ACE), was the first relatively complete specification of an electronic stored-program digital computer.
In the United States the Hungarian-American mathematician John von Neumann shared Turing's dream of building a universal stored-program computing machine.
The scanner moves back and forth through the memory, examining one square at a time (the 'scanned square').
It reads the symbols on the tape and writes further symbols.
A Turing machine has a small repertoire of basic operations: move left one square, move right one square, print, and change state. The scanner can print a symbol on the scanned square (after erasing any existing symbol).
There is a happy ending—but by that time Turing had turned his back on the ACE forever.
Much of Part II is in the words of the original protagonists, drawn from documents of the time.
The operation of the machine is governed by (what Turing called) a table of instructions.
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.