An operating system acts as an interface between computer hardware and software, managing computer resources and allowing users to run application programs. There are several types of operating systems including batch, time-sharing, distributed, network, and real-time systems. The five most common operating systems are Ubuntu, Android, Mac OS X, iOS, and Windows, each having their own unique features and compatibility with different hardware.

1. Computer - Operating System
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It is a program with following features:
 An operating system is a program that acts as an interface between the software and the
computer hardware.
 It is an integrated set of specialised programs that are used to manage overall resources
and operations of the computer.
 It is specialised software that controls and monitors the execution of all other programs
that reside in the computer, including application programs and other system software.
Objectives of Operating System
 To make a computer system convenient to use in an efficient manner
 To hide the details of the hardware resources from the users
 To provide users a convenient interface to use the computer system
 To act as an intermediary between the hardware and its users and making it easier for the
users to access and use other resources
 To manage the resources of a computer system
 To keep track of who is using which resource, granting resource requests, according for
resource using and mediating conflicting requests from different programs and users
 To provide efficient and fair sharing of resources among users and programs

2. Characteristics of Operating System
 Memory Management -- keeps tracks of primary memory i.e. what part of it is in use by
whom, what part is not in use etc. and allocates the memory when a process or program
requests it.
 Processor Management -- allocates the processor(CPU) to a process and deallocates
processor when it is no longer required.
 Device Management -- keeps track of all devices. This is also called I/O controller that
decides which process gets the device, when, and for how much time.
 File Management -- allocates and de-allocates the resources and decides who gets the
resources.
 Security -- prevents unauthorized access to programs and data by means of passwords
and similar other techniques.
 Job accounting -- keeps track of time and resources used by various jobs and/or users.
 Control over systemperformance -- records delays between request for a service and
from the system.
 Interaction with the operators -- The interaction may take place via the console of the
computer in the form of instructions. Operating System acknowledges the same, does the
corresponding action and informs the operation by a display screen.
 Error-detecting aids -- Production of dumps, traces, error messages and other
debugging and error-detecting methods.
 Coordination between other software and users -- Coordination and assignment of
compilers, interpreters, assemblers and other software to the various users of the
computer systems.

3. Types of Operating System
Operating systems are there from the very first computer generation. Operating systems keep
evolving over the period of time. Following are few of the important types of operating system
which are most commonly used.
Batch operating system
The users of batch operating system do not interact with the computer directly. Each user
prepares his job on an off-line device like punch cards and submits it to the computer operator.
To speed up processing, jobs with similar needs are batched together and run as a group. Thus,
the programmers left their programs with the operator. The operator then sorts programs into
batches with similar requirements.
The problems with Batch Systems are following.
 Lack of interaction between the user and job.
 CPU is often idle, because the speeds of the mechanical I/O devices is slower than CPU.
 Difficult to provide the desired priority.
Time-sharing operating systems
Time sharing is a technique which enables many people, located at various terminals, to use a
particular computer system at the same time. Time-sharing or multitasking is a logical extension
of multiprogramming. Processor's time which is shared among multiple users simultaneously is
termed as time-sharing. The main difference between Multiprogrammed Batch Systems and
Time-Sharing Systems is that in case of Multiprogrammed batch systems, objective is to
maximize processor use, whereas in Time-Sharing Systems objective is to minimize response
time.
Multiple jobs are executed by the CPU by switching between them, but the switches occur so
frequently. Thus, the user can receives an immediate response. For example, in a transaction
processing, processor execute each user program in a short burst or quantum of computation.
That is if n users are present, each user can get time quantum. When the user submits the
command, the response time is in few seconds at most.
Operating system uses CPU scheduling and multiprogramming to provide each user with a small
portion of a time. Computer systems that were designed primarily as batch systems have been
modified to time-sharing systems.
Advantages of Timesharing operating systems are following
 Provide advantage of quick response.
 Avoids duplication of software.

4.  Reduces CPU idle time.
Disadvantages of Timesharing operating systems are following.
 Problem of reliability.
 Question of security and integrity of user programs and data.
 Problem of data communication.
Distributed operating
System(Multiprogramming/Multitasking)
Distributed systems use multiple central processors to serve multiple real time application and
multiple users. Data processing jobs are distributed among the processors accordingly to which
one can perform each job most efficiently.
The processors communicate with one another through various communication lines (such as
high-speed buses or telephone lines). These are referred as loosely coupled systems or distributed
systems. Processors in a distributed system may vary in size and function. These processors are
referred as sites, nodes, computers and so on.
The advantages of distributed systems are following.
 With resource sharing facility user at one site may be able to use the resources available
at another.
 Speedup the exchange of data with one another via electronic mail.
 If one site fails in a distributed system, the remaining sites can potentially continue
operating.
 Better service to the customers.
 Reduction of the load on the host computer.
 Reduction of delays in data processing.
Network operating System
Network Operating System runs on a server and and provides server the capability to manage
data, users, groups, security, applications, and other networking functions. The primary purpose
of the network operating system is to allow shared file and printer access among multiple
computers in a network, typically a local area network (LAN), a private network or to other
networks. Examples of network operating systems are Microsoft Windows Server 2003,
Microsoft Windows Server 2008, UNIX, Linux, Mac OS X, Novell NetWare, and BSD.
The advantages of network operating systems are following.
 Centralized servers are highly stable.
 Security is server managed.
 Upgrades to new technologies and hardwares can be easily integrated into the system.

5.  Remote access to servers is possible from different locations and types of systems.
The disadvantages of network operating systems are following.
 High cost of buying and running a server.
 Dependency on a central location for most operations.
 Regular maintenance and updates are required.
Real Time operating System
Real time system is defines as a data processing system in which the time interval required to
process and respond to inputs is so small that it controls the environment. Real time processing is
always on line whereas on line system need not be real time. The time taken by the system to
respond to an input and display of required updated information is termed as response time. So in
this method response time is very less as compared to the online processing.
Real-time systems are used when there are rigid time requirements on the operation of a
processor or the flow of data and real-time systems can be used as a control device in a dedicated
application. Real-time operating system has well-defined, fixed time constraints otherwise
system will fail.For example Scientific experiments, medical imaging systems, industrial control
systems, weapon systems, robots, and home-applicance controllers, Air traffic control system
etc.
There are two types of real-time operating systems.
Hardreal-timesystems
Hard real-time systems guarantee that critical tasks complete on time. In hard real-time systems
secondary storage is limited or missing with data stored in ROM. In these systems virtual
memory is almost never found.
Softreal-timesystems
Soft real time systems are less restrictive. Critical real-time task gets priority over other tasks and
retains the priority until it completes. Soft real-time systems have limited utility than hard real-
time systems. For example, Multimedia, virtual reality, Advanced Scientific Projects like
undersea exploration and planetary rovers etc.
Five Common Operating Systems
The operating system serves as the intermediary between the user and the software on all
computing devices.

6. The operating system is the computer's most important piece of software. It manages the
hardware resources, determines which programs are compatible with the system and executes
those programs when the user decides to run them. Its influence on the nuts and bolts of
computer operation makes it a major determining factor when it comes to the user's experience.
A given operating system has not only its own functionality, but a unique look and feel that
creates a specific environment. Each of the world's major operating systems has its own
passionate following, prompting endless debate on which is superior, and for what. Learning
about them in the face of this shifting competitive landscape requires understanding certain
basics principles.
Ubuntu
Ubuntu is based on a version of the Linux operating system known as Debian GNU/Linux. Its
main benefit is that it's free and open source. This means users can alter the code as they see fit.
Ubuntu also stands out because it's updated once every six months, unlike other open source
programs that have no strict upgrade schedule. Most Ubuntu-compatible software is also free and
available in on-line software "repositories." Users can try Ubuntu on their PC using the
program's "Live CD" mode. This allows you to run the system without actually installing it on
your computer. Because Ubuntu programs are less ubiquitous than those for more well-known
systems, users may need to do some research before finding the software solutions they need.
Android
Android is a "mobile" operating system maintained by the Open Handset Alliance, a group of
companies and organizations that contribute to the system's development under the management
of Google Inc. The system is an "open standard," meaning that an entire community of
developers can alter the software, not just Google. Android comes pre-installed on smartphones
and tablet PCs for which it was designed. As of the date of publication, all four major
smartphone carriers -- Verizon, T-Mobile, Sprint and AT&T -- have Android devices available.
A variety of manufacturers also make Android-compatible devices. One of the main benefits of
the system's open source status is that developers can create different versions of its basic
programs, allowing users greater freedom to customize their mobile devices.
RelatedReading: Common Problems in Management Information Systems
Mac OS X
While a version of Mac OS X comes pre-installed on all Apple Macintosh computers, users must
pay if they wish to use newer versions when they become available. The system is not open
source, so only Apple developers have the ability to legitimately make alterations or
improvements to its code. It differs from other computer operating systems in that it only works
on Apple computers. Its key feature is its usability. The most recent version as of the date of
publication includes a "resume" feature that re-opens applications in the same state they were in

7. when you last closed them. It also includes a "Mission Control" feature, which allows you to see
all of your open files at a glance.
IOS
IOS is Apple's mobile operating system. It runs on the company's iPod, iPhone and iPad devices.
Like Mac OS X, it can only be used on Apple's mobile devices. One advantage of iOS is that
more applications are available for it than for any other mobile operating system. As of summer
2011, it has approximately 400,000 to Android's 300,000. This increases the chances that users
will find the type of app they're looking for. The system's key features include multitasking
support, which allows users to run multiple apps simultaneously, and FaceTime, a video chat
program that comes pre-installed on iOS devices.
Windows
Windows is an operating system owned and operated by Microsoft. It differs from open source
systems in that only Microsoft has the ability to make changes to the code. However, it also
differs from Mac OS X in that it can be installed on a variety of different computers from a host
of competing manufacturers, offering the user more freedom of choice when it comes to
hardware. According to Net Market Share, Windows is the most widely used operating system.
The latest version as of the date of publication includes touchscreen support, which combines the
usability of a touchscreen tablet with the power of a desktop computer. It also features "Play To"
and "Remote Media Streaming," which allow you to play media from your computer on other
devices in its vicinity and to access your media when you're away from your computer.