Operating system

In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations.

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

The operating system takes care such that other applications are able to use memory, input and output devices and have access to the file system. If multiple applications are running, the operating system schedules these such that all processes have sufficient processor time where possible and do not interfere with each other.

Examples of operating systems

• Unix
• GNU/Linux
• Microsoft Windows
• Mac OS
• MVS

Classifications and Terminology

An operating system is conceptually broken into three sets of components: a shell, a kernel and low-level system utilities. As the name implies the shell is an outer wrapper to the kernel which in turn talks directly to the hardware.

            Hardware <-> Kernel <-> Shell <-> Applications 

In some operating systems the shell and the kernel are completely separate entities, allowing you to run varying combinations of shell and kernel (eg Unix), in others their separation is only conceptual (eg Windows).

Kernel design ideologies include those of the monolithic kernel, the microkernel and the exokernel. Traditional commercial systems such as Unix and Windows use a monolithic approach, the trend in more modern systems is to use a microkernel (such as in Qnx[?], BeOS, Windows NT etc) there are a few exceptions such as Linux which still use a monolithic kernel . The microkernel approach is also very popular among research OSes. Many embedded systems use ad-hoc exokernels.

See Also

• History of operating systems
• List of operating systems
• Operating system advocacy
• monolithic kernel -- microkernel -- exokernel
• real-time operating system -- time-share[?] -- multitasking -- embedded system -- single-user[?] -- multi-user
• asymmetric and symmetric multiprocessing (SMP) -- clustering -- distributed computing
• orthogonally persistent[?] -- capabilities versus access control lists
• object-oriented operating system

External links

• Operating Systems Projects (http://www.cs.arizona.edu/people/bridges/os/full)
• TUNES Review of Operating Systems (http://tunes.org/Review/OSes)
• How Operating Systems Work, shown by a tiny self-developed Operating System (http://www.visual-opal.de)