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 ensures 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
   * OpenVMS

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.

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