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-execute cycle.
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?]
(http://salem.mass.edu/~tevans/VonNeuma.htm).
(2003-05-16)