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3. OPERATING SYSTEM

From above discussions it is obvious that we would be writing instructions (source code) for same sets of operations again and again, for example, to fetch a program from secondary memory; to interact with other hardware, e.g., printer and thousands of other components; to ask processor to perform various operations like addition, subtraction, etc; to specify the location of intermediate program data; to interact with other systems around the globe. We could write a set of programs that help us do all these. These sets of programs are a program in turn, and constitute an operating system. Considering the domain of this set, we could know that this program is enormous. Hence, although making this program itself is a daunting task, this considerably simplifies our work and reduces time drastically. Time is money in business and hence this set of programs is of utmost importance. This set of program is called an Operating System. Now, as all this boiler plate instruction set is made once, we could concentrate on the main part (logic) of our program. Microsoft Windows, Linux are examples of operating systems. If you want you may write (make) one, but, chances are that you may not need to write one ever in your life and use existing operating systems.

We have not yet discussed few more aspects of an operating system. The first one is multiprogramming, the ability to ask a processor to execute multiple instructions (each of different program) concurrently. A processor can execute instructions one-at-a-time, so, how can it execute multiple instructions at a time? It can’t. But as processor executes an instruction in very short span of time, we can execute an instruction from first program, then an instruction from second program, and then an instruction of another and so on. Then we come back again to the first program and execute the next instruction in the list, and similarly execute next instructions for the rest of all the programs, one by one.