John Neumann

Joh Neumann

John Neumann is named after the mathematician and early computer scientist John von Neumann. Such computers implement a universal Turing machine and have a sequential architecture.
A stored-program digital computer is one that keeps its programmed instructions, as well as its data, in read-write, random access memory (RAM).

Stored-program computers were an advancement over the program-controlled computers of the 1940s, such as the Colossus and the ENIAC, which were programmed by setting switches and inserting patch leads to route data and to control signals between various functional units. In the vast majority of modern computers, the same memory is used for both data and program instructions.

The terms "von Neumann architecture" and "stored-program computer" are generally used interchangeably, and that usage is followed in this article. In contrast, the Harvard architecture stores a program in a modifiable form, but without using the same physical storage or format for general data.

The earliest computing machines had fixed programs. Some very simple computers still use this design, either for simplicity or training purposes. For example, a desk calculator (in principle) is a fixed program computer. It can do basic mathematics, but it cannot be used as a word processor or to play games with. Changing the program of a fixed-program machine requires re-wiring, re-structuring, or re-designing the machine. The earliest computers were not so much "programmed" as they were "designed". "Reprogramming", when it was possible at all, was a laborious process, starting with flow charts and paper notes, followed by detailed engineering designs, and then the often-arduous process of physically re-wiring and re-building the machine. It could take up to three weeks to set up a program on ENIAC and get it working.

The idea of the stored-program computer changed all that. A computer that by design includes an instruction set architecture and can store in memory a set of instructions (a program) that details the computation.

A stored-program design also lets programs modify themselves while running. One early motivation for such a facility was the need for a program to increment or otherwise modify the address portion of instructions, which had to be done manually in early designs. This became less important when index registers and indirect addressing became usual features of machine architecture. Self-modifying code has largely fallen out of favor, since it is usually hard to understand and debug, as well as being inefficient under modern processor pipelining and caching schemes.