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Von Neumann Computers

Rudolf Eigenmann Purdue University

David J. Lilja University of Minnesota

January 30, 1998

1 Introduction
The term von Neumann computer has two common meanings. Its strictest de nition refers to a speci c type of computer organization, or architecture," in which instructions and data are stored together in a common memory.This type of architecture is distinguished from the Harvard" architecture in which separate memories are used to store instructions and data. The term von Neumann computer" also is used colloquially to refer in general to computers that execute a single sequence of instructions, which operate on a single stream of data values. That is, colloquially, von Neumann computers are the typical computersavailable today. There is some controversy among historians of technology about the true origins of many of the fundamental concepts in a von Neumann computer. Thus, since John von Neumann brought many of these concepts to fruition in a computer built at the Princeton Institute for Advanced Study see Figure 1, many people in the eld of computer science and engineering prefer to use the termPrinceton" computer instead of von Neumann" computer. The intention of this terminology is to acknowledge the important concepts introduced by many other individuals while not over-emphasizing von Neumann's contributions. Recognizing that many people in addition to von Neumann contributed to the fundamental ideas embodied in this widely adopted computer architecture, this article nevertheless uses thecolloquial version of the term von Neumann computer to refer to any computer with the fundamental characteristics described in Section 3. The term Princeton architecture" is then used to distinguish between computers with the split Harvard and uni ed Princeton memory organizations.

The von Neumann computer concept was developed in the 1940s when the rst electronic computers werebuilt. Nearly all modern computers are based on this stored program scheme, in which both 1

Figure 1: John von Neumann in front of the computer he built at the Institute for Advanced Study in Princeton Courtesy of the Archives of the Institute for Advanced Study. machine instructions and program data are stored in computer memory in the same manner. After the 1940s the computer industry begana rapid development with the speed and cost of computer systems improving by a factor of two every two years. Amazingly, this trend has continued, in principle, through today. Computer applications initially served the needs of the military. They soon found their way into the commercial market, however, where they revolutionized every business they encountered. The development of microprocessorsbrought the von Neumann computer onto the desks of secretaries, the counters of sales clerks, the o ce tables of homes, and into small appliances and children's games. Accompanying organizations were created to support the computer era in various ways. Notable among these are the many computer science and engineering departments established at universities and two professional societies, the IEEEComputer Society and the Association for Computing Machinery ACM.

The von Neumann Computer Architecture
The heart of the von Neumann computer architecture is the Central Processing Unit CPU, consisting of the control unit and the ALU Arithmetic and Logic Unit. The CPU interacts with a memory and an input output I O subsystem and executes a stream of instructions the computer programthat process the data stored in memory and perform I O operations. The key concept of the von Neumann architecture is that data and instructions are stored in the memory system in 2

exactly the same way. Thus, the memory content is de ned entirely by how it is interpreted. This is essential, for example, for a program compiler that translates a user-understandable programming language into...
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