26-12-2012, 02:04 PM
Operating system
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INTRODUCTION
An operating system (OS) is a collection of software that manages computer hardware resources and
provides common services for computer programs. The operating system is a vital component of the system
software in a computer system. Application programs require an operating system to function.
Time-sharing operating systems schedule tasks for efficient use of the system and may also include
accounting for cost allocation of processor time, mass storage, printing, and other resources.
For hardware functions such as input and output and memory allocation, the operating system acts as an
intermediary between programs and the computer hardware,[1][2] although the application code is usually
executed directly by the hardware and will frequently make a system call to an OS function or be interrupted
by it. Operating systems can be found on almost any device that contains a computer—from cellular phones
and video game consoles to supercomputers and web servers.
Examples of popular modern operating systems include Android, BSD, iOS, Linux, Mac OS X, Microsoft
Types of operating system
Real-time
A real-time operating system is a multitasking operating system that aims at executing real-time
applications. Real-time operating systems often use specialized scheduling algorithms so that they can
achieve a deterministic nature of behavior. The main objective of real-time operating systems is their
quick and predictable response to events. They have an event-driven or time-sharing design and often
aspects of both. An event-driven system switches between tasks based on their priorities or external
events while time-sharing operating systems switch tasks based on clock interrupts.
Multi-user
A multi-user operating system allows multiple users to access a computer system concurrently.
Time-sharing systems and Internet servers can be classified as multi-user systems as they enable
multiple-user access to a computer through the sharing of time. Single-user operating systems, as
opposed to multi-user operating systems, are usable by a single user at a time. Being able to use
multiple accounts on a Windows operating system does not make it a multi-user system. Rather, only
the network administrator is the real user. But for a UNIX-like operating system, it is possible for two
users to log in at a time and this capability of the OS makes it a multi-user operating system.
Multi-tasking vs. single-tasking
A multi-tasking operating system allows more than one program to be running at a time, from the point
of view of human time scales. A single-tasking system has only one running program. Multi-tasking
can be of two types: pre-emptive or co-operative. In pre-emptive multitasking, the operating system
slices the CPU time and dedicates one slot to each of the programs. Unix-like operating systems such
as Solaris and Linux support pre-emptive multitasking, as does AmigaOS. Cooperative multitasking is
achieved by relying on each process to give time to the other processes in a defined manner. 16-bit
versions of Microsoft Windows used cooperative multi-tasking. 32-bit versions, both Windows NT and
Win9x, used pre-emptive multi-tasking. Mac OS prior to OS X used to support cooperative
multitasking.
Distributed
A distributed operating system manages a group of independent computers and makes them appear to
be a single computer. The development of networked computers that could be linked and
communicate with each other gave rise to distributed computing. Distributed computations are carried
out on more than one machine. When computers in a group work in cooperation, they make a
distributed system.
Embedded
Embedded operating systems are designed to be used in embedded computer systems. They are
designed to operate on small machines like PDAs with less autonomy. They are able to operate with a
limited number of resources. They are very compact and extremely efficient by design. Windows CE
and Minix 3 are some examples of embedded operating systems.
Microcomputers
The first microcomputers did not have the capacity or need for
the elaborate operating systems that had been developed for
mainframes and minis; minimalistic operating systems were
developed, often loaded from ROM and known as monitors.
One notable early disk operating system was CP/M, which was
supported on many early microcomputers and was closely
imitated by Microsoft's MS-DOS, which became wildly popular
as the operating system chosen for the IBM PC (IBM's version
of it was called IBM DOS or PC DOS). In the '80s, Apple
Computer Inc.