At first operating systems were designed to help applications interact with the computer hardware. While this is still the case, the importance of the operating system has grown to the point where (at least in the minds of many users) the operating system defines the machine. Most users engaged in the Mac - PC - UNIX battle are arguing about the operating systems on these machines, not the hardware platform itself.
The operating system provides a layer of abstraction between the user and the bare machine. Users and applications do not see the hardware directly, but view it through the operating system. This abstraction can be used to hide certain hardware details from users and applications. Thus, changes in the hardware are not seen by the user (even though the OS must accommodate them).
An OS takes care for all input and output in a computer system. It manages users, processes, memory management, printing, telecommunication, networking etc.
OS sends data to a disk, the printer, the screen and other peripherals connected to the computer, and because every machine is build different, commands for in or output will have to be treated differently too. In almost all cases an Operating System is not one large big behemoth but consists of many small system programs governed by the core or kernel of the OS. Because of the compactness of these small supporting programs it is easier to rewrite parts or packages of the OS than to redesign an entire program.
What is an Operating System?
An operating system is a layer of software which takes care of technical aspects of a computer's operation. An operating system is the program that, after being initially loaded into the computer by a boot program, manages all the other programs in a computer. The other programs are called applications or application programs. The 1960’s definition of an operating system is “the software that controls the hardware”. We see an operating system as the programs that make the hardware useable. In brief, an operating system is the set of programs that controls a computer. Some examples of operating systems are UNIX, Mach, MS-DOS, MS-Windows, Windows/NT, Chicago, OS/2, MacOS, VMS, MVS, and VM. The application programs make use of the operating system by making requests for services through a defined application program interface (API). In addition, users can interact directly with the operating system through a user interface such as a command language or a graphical user interface (GUI). An operating system performs these services for applications: • It manages the sharing of internal memory among multiple applications. • 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. • It handles input and output to and from attached hardware...