The most important program that runs on any 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 monitor, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers. The operating system is also responsible for security, ensuring that unauthorized users do not access the system. Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run.