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Operating system

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In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. Additionally, it provides a foundation upon which to run application software such as word processing programs and web browsers.



Early computers lacked operating systems (see History of operating systems). A human operator would manually load and run programs. When programs were developed to load and run other programs, it was natural to draw their name from the human job they replaced.

Today, the term is most often used colloquially to mean all the software which "comes with" a computer system before any applications are installed.



It is also noteworthy that some people use "kernel" to mean the core piece of the OS that deals most directly with the hardware, and have a slightly broader definition of "operating system". They would define "operating system" to refer to the kernel plus some of the basic computer programs and libraries that are necessary to use the kernel. An interesting essay about the difference between the kernel and the operating system, from the perspective of a broader definition of OS, can be found here: Linux and GNU. It should be stressed that neither definition is completely accepted among the computer science community.

Common core services


Programs communicate with each other through Application Programming Interfaces, or APIs, similar to how humans interact with programs through User interfaces. This is especially true between application programs and the OS. The OS's common core services are accessed by application programs through the OS's APIs. Thus an OS enables the communication between hardware and software. CPU scheduling is also a main function of the operating system.

Today's operating systems

As of 2005, the major operating systems in widespread use on general-purpose computers (including personal computers) have consolidated into two families: the Unix-like family and the Microsoft Windows family. Mainframe computers and embedded systems use a variety of different operating systems, many with no direct connection to Windows or Unix.

The Unix-like family is a more diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "Unix" is a trademark of The Open Group which licenses it for use to any operating system that has been shown to conform to the definitions that they have cooperatively developed. Unix-type operating systems can also be called Un*x so the registered trademark symbol (R) doesn't have to be used after it is written. The name is commonly used to refer to the large set of operating systems which resemble the original Unix. Unix systems run on a wide variety of machine architectures. Unix systems are used heavily as server systems in business, as well as workstations in academic and engineering environments. Free software Unix variants, such as Linux and BSD are increasingly popular, and have made inroads on the desktop market as well. Apple's Mac OS X, a BSD variant, has replaced Apple's earlier (non-Unix) Mac OS in a small but dedicated market, becoming one of the most popular Unix systems in the process.

The Microsoft Windows family of operating systems originated as a graphical layer on top of the older MS-DOS environment for the IBM PC. Modern versions are based on the newer Windows NT core that first took shape in OS/2. Windows runs on 32- and 64-bit Intel and AMD computers, although earlier versions also ran on the DEC Alpha, MIPS and PowerPC architectures (and there was work in progress to make it work also on the SPARC architecture). Today, Windows is a popular desktop operating system, enjoying a near-monopoly of around 90% of the worldwide desktop market share. It is also widely used on low-end and mid-range servers, supporting applications such as Web servers and database servers.

Mainframe operating systems, such as IBM's z/OS, and embedded operating systems such as QNX, eCos, and PalmOS, are usually unrelated to Unix and Windows.

Older operating systems which are still used in niche markets include the Windows-like OS/2 from IBM; VMS from Hewlett-Packard (formerly DEC); Mac OS, the non-Unix precursor to Apple's Mac OS X; and AmigaOS, the first GUI based operating system with advanced multimedia capabilities available to the general public.


Examples of operating systems

Classifications and terminology




Kernel design ideologies include those of the monolithic kernel, microkernel, and exokernel. Traditional commercial systems such as UNIX and Windows (including Windows NT), and the newer Linux, use a monolithic approach, while the trend in more modern systems is to use a microkernel (such as in AmigaOS, QNX, BeOS, Mac OS X etc). The microkernel approach is also very popular among research OSs. Many embedded systems use ad hoc exokernels.

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