An operating system is a set of programs that controls how the hardware of a computer works'

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An operating system is a set of programs that
controls how the hardware of a computer works.
An operating system provides a means of
communication between the user and the computer,
deals with the loading and running of
applications programs and manages the transfer of
data and files to and from peripheral devices.

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The most widely used operating systems are called
Microsoft Windows (e.g. Windows 2000, Windows
XP), MacOS (for Apple Mac computers), Novell
Netware and UNIX. The operating system that a
computer has also determines what applications
software will run on it. Applications software
will only work on a computer that has the
operating system that it was designed to be used
with. Applications software will not run on a
computer that has a different operating system to
the one that it was designed for.

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The operating system acts as a bridge between
applications software and computer hardware.
Users need applications software to carry out
the tasks that they require.
The applications software needs the operating
system so that it can communicate with the
hardware and get it to carry out hardware related
tasks such as printing or transferring data to
and from backing storage devices. Applications
software and computer hardware cannot function
without an operating system.
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• Functions of an operating system
• It deals with input and output, which involves
• accepting data from input devices and
  transferring it to the computers memory
• making sure that any output is sent to the
  correct output device.
• It manages the transfer of data between the
  computers memory and backing storage devices.
• It manages system resources, which involves
• allocating memory space to programs and data
• keeping track of which parts of the memory have
  already been allocated and the parts that are
  still free.

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• It deals with the loading of applications
  software into memory and controls the execution,
  or running of them. It also provides a way for
  applications software to communicate with the
  computers hardware.
• It deals with any errors that occur when a
  program is being run, or when data is being
  transferred somewhere, and informs the user if
  necessary.
• It manages system security, which involves
• monitoring and restricting access to programs and
  data
• preventing unauthorised access to the system.

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• It provides a human computer interface, or HCI,
  for the user.
• It provides special facilities for
  multiprogramming.
• A multiprogramming operating system can hold more
  than one program in memory at the same time.
• There are two types of multiprogramming operating
  system multitasking, and multi-user.

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A multitasking operating system allows two or
more programs to run at the same time. The
operating system does this by swapping each
program in and out of memory in turn. When a
program is swapped out of memory it is stored
temporarily on disk until it is needed again. MS
Windows is an example of a multitasking operating
system.

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A multi-user operating system lets many users at
different terminals share processing time on a
powerful central computer. The operating system
does this by switching rapidly between the
terminals giving each one in turn a small amount
of processor time on the central computer.

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A multi-user operating system switches so quickly
between the terminals that each user appears to
have uninterrupted access to the central
computer. However if there are a large number
of users on such a system the time that it takes
the central computer to respond can become more
noticeable.

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Utility programs are usually supplied along with
an operating system. They are used to carry out
routine tasks that are often needed by a user
such as compressing a file to save space on
backing storage defragmenting a disk
drive recovering data from damaged
file checking a disk for faults and repairing
them formatting a floppy disk checking the
files on a disk for computer viruses.

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A processing method is the way a particular
operating system deals with input. There are
three main types of processing method real-time,
transaction and batch processing. Real-time
processing systems process input data so quickly
that the resulting output can affect further
input. It is used for applications where it is
essential that the computer responds straight
away to input. Examples of applications where
real-time processing is used are missile defence
systems, automatic pilot systems on aircraft and
monitoring intensive care patients in a hospital.

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Transaction, or on-line, processing, is used for
applications where input needs to be dealt with
straight away but it is not critical if there is
a slight delay in the time that it takes for the
computer to respond to requests. Examples of
applications where transaction processing is used
include the on-line seat booking systems used by
airlines and the stock control systems used by
catalogue companies like Argos. A system where
transaction processing is used will always give
an up-to-the-minute picture of the current
situation.

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A batch processing system does not respond to
input straight away. Instead, input is collected
together into a batch while the system is
off-line. When a batch is ready to be processed
the system goes on-line to carry out the
processing of the data. Batch processing is
non-interactive. This means that the user cannot
get an immediate response to input as they would
with an interactive system. Examples of
applications that use batch processing include
producing gas, electricity or water bills and
marking OMR sheets from multiple choice
examinations.