Operating System

Operating System

An operating system is a program that acts as an intermediary between a user of a computer and the computer hardware. The purpose of an operating system is to provide an environment in which a user can execute programs. The primary goal of an operating system is thus to make the computer system convenient to use. A secondary goal is to use the computer hardware in an efficient manner.

What Is an Operating System?

An operating system is an important part of almost every computer system. A computer system can be divided roughly into four components: the hardware, the operating system, the applications programs, and the users.

The hardware — the central processing unit (CPU), the memory, and the input/output (I/O) devices — provides the basic computing resources. The applications programs — such as compilers, database systems, games, and business programs — define the ways in which these resources are used to solve the computing problems of the users. There may be many different users (people, machines, other computers) trying to solve different problems. Accordingly, there may be many different applications programs. The operat¬ing system controls and coordinates the use of the hardware among the various applications programs for the various users.

An operating system is similar to a government. The components of a computer system are its hardware, software, and data. The operating system provides the means for the proper use of these resources in the operation of the computer system. Like a government, the operating system performs no useful function by itself. It simply provides an environment within which other programs can do useful work.

We can view an operating system as a resource allocator. A computer system has many resources (hardware and software) that may be required to solve a problem: CPU time, memory space, file storage space, I/O devices, and so on. The operating system acts as the manager of these resources and allocates them to specific programs and users as necessary for tasks. Since there may be many—possibly conflicting—requests for resources, the operating system must decide which requests are allocated resources to operate the computer system efficiently and fairly.

A slightly different view of an operating system focuses on the need to control the various I/O devices and user programs. An operating system is a control program. A control program controls the execution of user programs to prevent errors and improper use of the computer. It is especially concerned with the operation and control of I/O devices.

There is also no universally accepted definition of what is part of the operating system and what is not. A simple viewpoint is that everything a vendor ships when you order “the operating system" should be considered. The memory requirements and features included, however, vary greatly across systems. Some take up less than 1 megabyte of space (1 megabyte is 1 million bytes) and lack even a full-screen editor, whereas others require hundreds of megabytes of space and include spell checkers and entire "window systems." A more common definition is that the operating system is the one program running at all times on the computer (usually called the kernel), with all else being applications programs. This last definition is more common and is the one we generally follow.

It is easier to define operating systems by what they do than by what they are. The primary goal of an operating system is convenience for the user. Operating systems exist because they are supposed to make it easier to compute with them than without them. This view is particularly clear when you look at operating systems for small personal computers.

A secondary goal is efficient operation of the computer system. This goal is particularly important for large, shared multiuser systems. These systems are typically expensive, so it is desirable to make them as efficient as possible. These two goals—convenience} and efficiency—are sometimes contradictory. In the past, efficiency considerations were often more important than conve¬nience. Thus, much of operating-system theory concentrates on optimal use of computing resources.

To see what operating systems are and what operating systems do, let us consider how they have developed over the past 35 years. By tracing that evolution, we can identify the common elements of operating systems, and see how and why these systems have developed as they have.

Operating systems and computer architecture have had a great deal of influence on each other. To facilitate the use of the hardware, operating systems were developed. As operating systems were designed and used, it became obvious that changes in the design of the hardware could simplify them. In this short historical review, notice how operating-system problems led to the introduction of new hardware features.