What is Turing Machine XNx2?

Turing Machine XNx2 is one of the example Turing Machines that Sir Roger Penrose described on page 61 of his book The Emperor's New Mind (Oxford University Press, 1989).

It is also one of the sample Turing Machines that's built into both of the Excel files that you can download from the Home page of this website.

What the XNx2 Turing Machine does is read the number on the tape (in Expanded Binary Notation), double it, and display the answer on the tape (again in Expanded Binary Notation).