Hypothetical computing device proposed by Alan M. Turing (1936). Not actually a machine, it is an idealized mathematical model that reduces the logical structure of any computing device to its essentials. It consists of an infinitely extensible tape, a tape head that is capable of performing various operations on the tape, and a modifiable control mechanism in the head that can store instructions. As envisaged by Turing, it performs its functions in a sequence of discrete steps. His extrapolation of the essential features of information processing was instrumental in the development of modern digital computers, which share his basic scheme of an input/output device (tape and tape reader), central processing unit (CPU, or control mechanism), and stored memory.