Page 22 - DCAP507_SYSTEM_SOFTWARE

System Software

Notes 2.1 Evolution of Operating System

Operating systems are so ubiquitous in computer operations that one hardly realizes its presence.
Most likely you must have already interacted with one or more different operating systems. The
names like DOS, UNIX, etc. should not be unknown to you. These are the names of very popular
operating systems.
Try to recall when you switch on a computer what all happens before you start operating on it.
In a typical personal computer scenario, this is what happens. Some information appears on the
screen. This is followed by memory counting activity. Keyboard, disk drives, printers and other
similar devices are verified for proper operation. These activities always occur whenever the
computer is switched on or reset. There may be some additional activities on some machine
also. These activities are called power-on routines. Why do these activities always happen? You
will learn about it elsewhere in this unit.
You know a computer does not do anything without properly instructed. Thus, for each one of
the above power-on activities also, the computer must have instructions. These instructions are
stored in a non-volatile memory, usually in a ROM. The CPU of the computer takes one instruction
from this ROM and executes it before taking next instruction. ROMs are of finite size. They can
store only a few kilobytes of instructions. One by one the CPU executes these instructions. Once,
these instructions are over, the CPU must obtain instructions from somewhere. Where are these
instructions stored and what are their functions?
Usually, these instructions are stored on a secondary storage device like hard disk, floppy disk
or CD-ROM disk. These instructions are collectively known as operating system and their
primary function is to provide an environment in which users may execute their own instructions.
Once the operating system is loaded into the main memory, the CPU starts executing it. Operating
systems run in an infinite loop, each time taking instructions in the form of commands or
programs from the users and executing them. This loop continues until the user terminates the
loop when the computer shuts down. In order to exploit the most from a computer, therefore, a
deep understanding of operating system is a must.
Previously operating systems have been strongly associated to the computer architecture, it is
good thought to learn the history of operating systems from the architecture of the computers
on which they execute.
Operating systems have evolved via number of distinct stages or generations which matches
approximately to the decades.

The 1940's – First Generation

The most primitive electronic digital computers had no operating systems. Machines of the time
were so prehistoric that programs were frequently entered one bit at time on rows of mechanical
switches (plug boards). Programming languages were unidentified (not even assembly
languages). Operating systems were unheard of.

The 1950's – Second Generation

By the early 1950's, the routine had enhanced to some extent with the introduction of punch
cards. The General Motors Research Laboratories executed the first operating systems in early
1950 are for their IBM 701. The system of the 50's usually ran one job at a time. These were known
as single-stream batch processing systems since programs and data were submitted in groups or
batches.

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