Any discussion of computer architectures, of how computers and computer systems are organized, designed, and implemented, inevitably makes reference to the "von Neumann architecture" as a basis for comparison. And of course this is so, since virtually every electronic computer ever built has been rooted in this architecture. The name applied to it comes from John von Neumann, who as author of two papers in 1945 [Goldstine and von Neumann 1963, von Neumann 1981] and coauthor of a third paper in 1946 [Burks, et al. 1963] was the first to spell out the requirements for a general purpose electronic computer. The 1946 paper, written with Arthur W. Burks and Hermann H. Goldstine, was titled "Preliminary Discussion of the Logical Design of an Electronic Computing Instrument," and the ideas in it were to have a profound impact on the subsequent development of such machines.

Von Neumann's design led eventually to the construction of the EDVAC computer in 1952. However, the first computer of this type to be actually constructed and operated was the Manchester Mark I, designed and built at Manchester University in England [Siewiorek, et al. 1982]. It ran its first program in 1948, executing it out of its 96 word memory. It executed an instruction in 1.2 milliseconds, which must have seemed phenomenal at the time. Using today's popular "MIPS" terminology (millions of instructions per second), it would be rated at .00083 MIPS. By contrast, some current supercomputers are rated at in excess of 1000 MIPS. And yet, these computers, such as the Cray systems and the Control Data Cyber 200 models, are still tied to the von Neumann architecture to a large extent.

Over the years, a number of computers have been claimed to be "non-von Neumann," and many have been at least partially so. More and more emphasis is being put on the necessity for breaking away from this traditional architecture in order to achieve more usable and more productive systems. The expectations for the fifth generation systems seem to require that substantially new architectures be evolved, and that both hardware and software be freed from the limitations of the von Neumann architecture [Sharp 1985].

We all know what the von Neumann architecture is, of course. At least we have strong intuitive feelings about it because this is what we have always used. This is "the way computers work." But to really comprehend what choices there are for computer designers, to appreciate what new choices must be found, it is necessary to have a more definitive understanding of what the von Neumann architecture is and is not and what its implications are.

Von Neumann begins his "Preliminary Discussion" with a broad description of the general-purpose computing machine containing four main "organs." These are identified as relating to arithmetic, memory, control, and connection with the human operator. In other words, the arithmetic logic unit, the control unit, the memory, and the input-output devices that we see in the classical model of what a computer "looks like."

To von Neumann, the key to building a general purpose device was in its ability to store not only its data and the intermediate results of computation, but also to store the instructions, or orders, that brought about the computation. In a special purpose machine the computational procedure could be part of the hardware. In a general purpose one the instructions must be as changeable as the numbers they acted upon. Therefore, why not encode the instructions into numeric form and store instructions and data in the same memory? This frequently is viewed as the principal contribution provided by von Neumann's insight into the nature of what a computer should be