VON NEUMANN ARCHITECTURE

Most computers today operate according to the “von Neumann architecture.” The main idea of the von
Neumann architecture is that the program to be executed resides in the computer’s memory, along with the
program’s data. John von Neumann published this idea in 1945.
Today this concept is so familiar it seems self-evident, but earlier computers were usually wired for a certain
function. In effect, the program was built into the construction of the computer. Think of an early calculator; for
example, imagine an old hand-cranked mechanical calculator. The machine was built to do one well-defined
thing. In the case of an old hand-cranked calculator, it was built only to add. Put a number in; crank it; get the
new sum.
To subtract, the operator needed to know how to do complementary subtraction, which uses addition to
accomplish subtraction. Instead of offering a subtract function, the old calculator required the operator to add
the “ten’s complement” of the number to be subtracted. You can search for “ten’s complement” on Google to
learn more, but the point for now is that early computing devices were built for certain functions only. One could
never, for instance, use an old adding machine to maintain a list of neighbors’ phone numbers!
The von Neumann architecture is also called the “stored program computer.” The program steps are stored
in the computer’s memory, and the computation cycle of the machine retrieves the next step (instruction to be
executed) from memory, completes that computation, and then retrieves the next step. This cycle simply repeats
until the computer retrieves an instruction to “halt.”
There are three primary units in the von Neumann computer. Memory is where both programs and data are
stored. The central processing unit (CPU) accesses the program and data in memory and performs the calculations.
The I/O unit provides access to devices for data input and output.