The Von Neumann Computer
In 1945, mathematician John von Neumann undertook a study of computation that demonstrated that a computer could have a simple, fixed structure, yet be able to execute any kind of computation given properly programmed control without the need for hardware modification. Von Neumann contributed a new understanding of how practical fast computers should be organized and built; these ideas, often referred to as the stored-program technique, became fundamental for future generations of high-speed digital computers and were universally adopted. The primary advance was the provision of a special type of machine instruction called conditional control transfer--which permitted the program sequence to be interrupted and reinitiated at any point, similar to the system suggested by Babbage for his analytical engine--and by storing all instruction programs together with data in the same memory unit, so that, when desired, instructions could be arithmetically modified in the same way as data. Thus, data was the same as program.
There is more to a von Neumann computer than the mere fact that instructions and data are both stored in the same medium. According to the original papers proposing the new architecture, a von Neumann computer has five parts: * CPU (Control Processing Unit) * RAM (Random Access Memory) or Working Storage * Permanent Storage * Input Device * Output Device
MEMORY (RAM)
(Both holding data and program)
MEMORY (RAM)
(Both holding data and program)

ARITHMETIC / LOGIC UNIT
(ACCUMULATOR)
ARITHMETIC / LOGIC UNIT
(ACCUMULATOR)
CONTROL PROCESSING UNIT
CONTROL PROCESSING UNIT

OUTPUT
OUTPUT
INPUT

INPUT

CPU: The “BRAIN” of the computer, responsible for doing the math, and telling the rest of the computer what to do.
Working Storage: Generally this is RAM that stores information temporarily. This information is lost when the computer is shut down.
Permanent Storage: ROM, hard disk, and optical disk fall into this category. They store data permanently even if the computer is shut down.
Input Devises: Keyboards, mouse, microphones and other similar devises that are used to input information into the computer.
Output Devises: This includes display monitors, printers, speakers, and other devises used to output information from the computer.

Von Neumann computers have some drawbacks. In particular, they carry out instructions one after another, in a single linear sequence, and they spend a lot of time moving data to and from the memory. This slows the computer -- a problem called the von Neumann bottleneck. One way to circumvent the von Neumann bottleneck is to build the computer so it performs operations in parallel (so-called parallel processing). Another common trick is to separate the bus into two or more busses, one for instructions, another for data. But such modifications don't really amount to much more than variations of the original architecture. To all intents and purposes, the PC on your desk today is a von Neumann machine.