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architecture, computability A computer architecture

conceived by mathematician John von Neumann, which forms the

core of nearly every computer system in use today (regardless

of size). In contrast to a Turing machine, a von Neumann

machine has a random-access memory (RAM) which means that

each successive operation can read or write any memory

location, independent of the location accessed by the previous

operation.

A von Neumann machine also has a central processing unit

(CPU) with one or more registers that hold data that are

being operated on. The CPU has a set of built-in operations

(its instruction set) that is far richer than with the

Turing machine, e.g. adding two binary integers, or

branching to another part of a program if the binary integer

in some register is equal to zero (conditional branch).

The CPU can interpret the contents of memory either as

instructions or as data according to the fetch-executecycle.

Von Neumann considered parallel computers but recognized the

problems of construction and hence settled for a sequential

system. For this reason, parallel computers are sometimes

referred to as non-von Neumann architectures.

A von Neumann machine can compute the same class of functions

as a universal Turing machine.

[Reference? Was von Neumann's design, unlike Turing's,

originally intended for physical implementation? How did they

influence each other?]

(2003-05-16)