An operating system (OS) is a collection of
system programs that together control the
operation of a computer system.
An operating system controls the way in which
the computer system functions .in order to do
this ,the operating system includes programs
that :
1- initialize the hardware of the computer
system.
2- provide basic routines for device control.
3- provide for the management , scheduling
and interaction of tasks.
4- maintain system integrity and handle errors.
There are many types of operating systems,
the complexity of which varies depending
upon what type of functions are provided ,
and what the system is being used for .
Some systems are responsible for managing
many users on a network . Other operating
systems do not manage user programs at all .
These are typically found in hardware devices
like petrol pumps ,airplanes, video
recorders,washing machines and car engines.
Windows NT Workstation is know as a
general-purpose operating system. This is
because it provides the ability to run a
number of different programs , such as
games ,word processing ,business
applications and program development
tools.
An operating system for a security control
system (such as a home alarm system)
would consist of a number of programs .
One of these programs would gain control
of the computer system when it is powered
on , and initialize the system.
The first task of this initialize program would
be to reset(and probably test) the
hardware sensors and alarms . Once the
hardware initialization was complete, the
operating system would enter a continual
monitoring routine of all the input sensors. If
the state of any input sensor changed , it
would branch to alarm generation routine.
Initialize the
hardware
Enter monitoring
sequence
Any
sensor
changed
Process alarm
Input and output devices are components
that form part of the computer system.
These devices are controlled by the
operating system.
Input devices provide input
signals such as commands to
the operating system .these
commands received from
input devices instruct the
operating system to perform
some task or control its
behavior.
Typical input devices are a
keyboard, mouse, temperature
sensor, air-flow valve or door
switch . In the previous example
of our simple security control
system, the input devices could
be door switches, alarm
keyboard panel and smoke
detector units.
Output devices are instruments
that receive commands or
information from the operating
system . Typical output devices
are monitor screen, printers,
speakers , alarm bells, fans,
pumps, control valves, light
bulbs and sirens.
We are all familiar with the concept of
sitting down at a computer system and
writing documents or performing some task
such as writing a letter . In this instance
there is one keyboard and one monitor
that you interact with .
Operating systems such as windows 95 ,
windows NT workstation and windows 2000
professional are essentially single user
operating systems . They provide you the
capability to perform tasks on the
computer system such as writing programs
and documents , printing and accessing
files .
In essence , a single-user operating system
provide access to the computer system by
a single user at a time . If another user
needs access to the computer system ,
they must wait till the current user finishes
what they are doing and leaves .
Students in computer labs at colleges or
university often experience this . You might
also have experienced this at home ,
where you want to use the computer but
someone else is currently using it . You
have to wait for them to finish before you
can use the computer system .
A multi-user operating system lets more
than one user access the computer system
at on time . Access to the computer
system is normally provided via a network ,
so that users access the computer remotly
using a terminal or other computer .
In the early days of large multi-user
computers , multiple terminals (keyboards
and associated monitors) were provided .
These terminals sent their commands to the
main multi-user computer for proccessing ,
and the results were then displayed on the
associated terminal monitor screen .
Today , these terminals are generally
personal computers and use a network to
send and receive information to the multiuser computer system . Examples of multiuser operating system are UNIX , Linux and
mainframes such as the IBM
The operating system for a large multi-user
computer system with many terminals is
much more complex than a single-user
operating system. It must manage and run
all user requests, ensuring they do not
interfere with each other. Devices that are
serial in nature (devices which can only be
used by one user at a time, like printers
and disks) must be shared amongst all
those requesting them (so that all the
output documents are not jumbled up).
If each user tried to send their document
to the printer at the same time, the end
result would be garbage. Instead,
documents are sent to a queue, and each
document is printed in its entirety before
the next document to be printed is
retrieved from the queue.
The operating system consists of hundreds
of thousands of lines of program code and
stored on hard disk. Portions of the
operating system are loaded into
computer system memory (RAM) when
needed. Utilities are provided for
Managing Files and Documents 
Development of Programs and Software 
Communicating between people and 
with other computer systems
Managing user requirements for 
programs, storage space and priority
In addition, the operating system provides
each user with an interface that accepts,
interprets and executes user commands or
programs. This interface is commonly called a
SHELL or command line interpreter (CLI). In
some systems this might be a simple text
mode line-by-line entry using keywords (such
as MSDOS or UNIX), in other systems it might
be highly graphical using windows and a
pointing device such as a mouse (such as XWindows).
The advantage of having a multi-user
operating system is that normally the
hardware is very expensive, and it lets a
number of users share this expensive
resource. This means the cost is divided
amongst the users. It also makes better use
of the resources. Since the resources are
shared, they are more likely to be in use
than sitting idle being unproductive.
One problem with multi-user computer
systems is that as more users access it, the
performance becomes slower and slower.
Another disadvantage is the cost of
hardware, as a multi-user operating system
requires a lot of disk space and memory. In
addition, the actual software for multi-user
operating systems tend to cost more than
single-user operating systems.
A multi-tasking operating system provides
the ability to run more than one program
at once. For example, a user could be
running a word processing package,
printing a document, copying files to the
floppy disk and backing up selected files
to a tape unit. Each of these tasks the user
is doing appears to be running at the same
time.
A multi-tasking operating system has the
advantage of letting the user run more
than one task at once, so this leads to
increased productivity. The disadvantage
is that more programs that are run by the
user, the more memory that is required.