The most important program run on a computer because it manages all the 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 OS has even greater responsibilities and powers. It regulates traffic, making sure that different programs and users running at the same time do not interfere with each other. The operating system also is responsible for security, ensuring that unauthorized users do not access a system. Microsoft Windows NT, LINUX, and UNIX are operating systems.