Stored Program Concept

ENIAC, the first machine that was manufactured in 1946 as the world’s first general purpose electronic computer had a lot of drawbacks. Worst of all was that the system was quite inflexible. Every time a program was changed, the machine needed to be rewired,re-structured, or re-designed.

Let’s try to illustrate this further. In computers, there is a set of basic logic components. By combining these components in different ways, different operations could be performed. In fact, it was necessary to design and configure these logic components in order to enable the machine to perform the required computation. One such configuration caters for one specific computation, and could be considered as a program or a hardwired programe. How ever, hardwired programming was
complicated, time-consuming and inflexible.

To overcome this problem, a mathematician called Dr. Von Neumann proposed a new concept called the ‘Stored Program Concept’. The idea was to keep the general purpose configuration of logic circuits (without customization for each program), and provide operational commands as a set of control signals. The set of hardware will function according to the control signals applied to hardware. This eliminates the need to rewire hardware for each ‘program’, and supply a new set of control signals in place of the ‘program’.

A program consists of a sequence of steps, where each step corresponds with a certain arithmetic or logic operation. The set of control signals required to complete one step will be given a unique code. The general-purpose hardware will contain a segment which accepts the code, and function accordingly. This code is called an instruction.Once the particular hardware receives the instruction, it will interpret the instruction and generate the set of control signals. The sequence of codes/ instructions is called software.

The improved machine manufactured based on this concept was called EDVAC (Electronic Discrete Variable Automatic Computer). EDVAC was more flexible than ENIAC, and it had been further improved over many generations. Almost all the computers today are based on the basic architecture proposed by Dr. von Neumann.

Von Neumann Model

The machine stores all program instructions inside the computer; therefore to switch to a different program, re-wiring was not necessary. Instead, the machine reads the instruction from computer storage for switching to a new program. This took less than a second to do the switching. The introduction of the system concept is a significant milestone in the development of the digital computer.