History of Computers - Von Neumann Architecture

Introduction

Von Neumann architecture was developed by computer scientists John von Neumann, John William Mauchly and J. Presper Eckert in 1945. It describes a stored-program computer, and in contrast to the more modern Harvard architecture, it uses the same storage to hold both the instructions needed for a computation and the data needed or generated by the computation. (Harvard architecture, on the other hand, uses different sets of data and address buses to read/write data and fetch instructions.)

Overview

In previous, more special-purpose computers, the instruction set was programmed into the hardware, and only the data set and the immediate computational results were stored, and thus these computers were restricted to performing only the one computational procedure that they were built for. With the development of Von Neumann architecture, computers became reprogrammable and thus more general-purpose since instructions could be changed or replaced easily.

A basic Von Neumann computer is composed of five parts:memory/storage for both the data set and the program (nowadays, this memory is RAM), a control unit (CPU), an arithmetic logic unit (ALU), input structure, and output structure. The control unit/program counter executes the program instructions sequentially, one at a time, and has a register (sometimes called an accumulator) to store immediate results.

A drawback of Von Neumann architecture is that computers built with this architecture can only do either an instruction fetch or a data operation at a time, and they spend a lot of time moving data to and from memory, so performance slows down and reaches a bottleneck (i.e. the Von Neumann Bottleneck). However, this issue has been corrected with the development of parallel computers, which are based upon Von Neumann processors, but in which the CPUs are connected in parallel.

Significance

Von Neumann architecture allowed computers to become truly reprogrammable for the first time, for it allowed for instructions to be changed easily since only the working storage (and not the permanent storage) had to be changed. This set the stage for future, more general-purpose computers, which could be used to perform a variety of different computational procedures since new instruction sets could be inputted. Another reason that Von Neumann architecture is significant to computer history is because computers today are still stored-program, albeit using the two-bus system of Harvard architecture. In fact, Von Neumann architecture laid the foundation for the development of Harvard architecture.