Operating System

The operating system is the most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.

For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop -- it makes sure that different programs and users running at the same time do not interfere with each other. The operating system is also responsible for security, ensuring that unauthorized users do not access the system.

Types of Operating System :

• Windows

  Examples : Window XP, Window 7, Window Vista, Window 8, Window 98

• Linux

  Examples : Fedora, Unbantu

• Unix

  Example : Red Hate

• Mac

  Example : Apple 

Classification of Operating systems

1. Multi-user: Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.
2. Multiprocessing : Supports running a program on more than one CPU.
3. Multitasking : Allows more than one program to run concurrently.
4. Multithreading : Allows different parts of a single program to run concurrently.
5. Real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time. 

An Operating System Performs these services :

1. In a multitasking operating system where multiple programs can be running at the same time, the operating system determines which applications should run in what order and how much time should be allowed for each application before giving another application a turn.
2. It manages the sharing of internal memory among multiple applications.
3. It handles input and output to and from attached hardware devices, such as hard disks, printers, and dial-up ports.
4. It sends messages to each application or interactive user (or to a system operator) about the status of operation and any errors that may have occurred.
5. It can offload the management of what are called batch jobs (for example, printing) so that the initiating application is freed from this work.
6. On computers that can provide parallel processing, an operating system can manage how to divide the program so that it runs on more than one processor at a time.