By now, you can probably see that the term operating system vcovers many roles and functions. That is the case, at least in part, because of the myriad designs and uses of computers. Computers are present within toasters, cars, ships, spacecraft, homes, and businesses. They are the basis for game machines, cable TV tuners, and industrial control systems.
To explain this diversity, we can turn to the history of computers. Although computers have a relatively short history, they have evolved rapidly. Computing started as an experiment to determine what could be done and quickly moved to fixed-purpose systems for military uses, such as code breaking and trajectiory plotting, and governmental users, such as census calculation. Those early computers evolved into general-purpose, multifuctino mainframes, and that's when operating systems were born. In the 1960s, Moore's Law predicted that the number of transistors on an integrated circuit would double every 18 months, and that prediction has held true, Computers gained in functionality and shrank in size, leading to a vast number of users and a vast number and variety of operating systems. (See Appendix A for more details on the history of operating systems.)
How, then, can we define what an operating system is? In general, we have no completely adequate definition of an operating system. Operating systems exist becase they offer a resonable way to solve the problem of creating a usable computing system. The fundamental goal of computer systems is to execute programs and to make solving user problems easier. Computer hardware is constructed toward this goal. Since bare hardware alone is not particularly easy to use, application programs are developed. These programs require certain common operations, such as those controlling the I/O devices. The common functions of controlling and allocating resources are then brought together into one piece of software:the operating system.
In additions, we have no universally accepted definition of what is part of the operationg system. A simple viewpoint is that it includes everything a vendor ships when yuu order "the operating system. "The features included, however, vary greatly across systems. Some systems take up less than a megabyte of space and lack even a full-screen editor, whereas others require gigabytes of spce and lack even a full-screen deitor, whereas others require gigabytes of space and lack even a full-screen ditor , whereas others require gigabytes of space and are based entirely on graphical windowing systems. A more common definition, and the one that we usually follow, is that the operating system is the one program running at all times on the computer - usually called the kernel. Along with the kernel, there are two other types of programs: system programs not associated with the operating of the system.
The matter of what constitutes an operating system become increasingly important as personal computers became more widespread and operating system grew increasingly sophisticated. In 1998, the United States Department of Justice filed suit against Microsoft, in essence claiming that Microsoft included too much functionality in its operating systems and thus prevented application vendors from competing.(For example, a web browser was an integral part of Microsoft's operating systems.) As a result, Microsoft's operating systems.)As a result, Microsoft was found guilty of using its operating-system monopoly to limit competition.
Today, however, if we look at operating systems for mobile devices, we see that once again the number of features constituting the operating system in increasing. Mobile operating systems-Apple's iOS and Google's Android-Features a core kernel along with middleware that supports databases, multimedia, and graphics(to name only a few).
In summary, for our purposes, the operating system includes the always-running kernel, middleware frameworks that ease application development and provide features, and system programs that aid in managing the system while it is running. Most of this text is concerned with the kernel of general purpose operating systems, but other components are discussed as needed to fully explain operating system design and operation.
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