Overview

The operating system contains the software that comprises the core functionality of a computer. The operating system or OS contains large libraries of machine code that are responsible for interfacing with every sub-system in the computer. For example the code for reading and writing the hard disk, writing information to the monitor, interfacing with the keyboard, and much more are all contained in the operating system.

Most high-level or application level software uses the operating system to perform any hardware related tasks. The operating system frees the application level software developer from needing to have detailed knowledge of the hardware platform the software is running on. Instead, the software application makes calls to the operating system's API or Application Program Interface. The operating system then executes the necessary machine code nstructions to interface with the desired sub-system or peripheral, such as writing text to the computer's monitor.

A program running under a certain operating system just sends a message to that operating system to draw a button on the screen - the program itself is not concerned with the actual details of drawing on the screen and is instead dependent on the underlying API. One can think of an operating system as the intermediary between the application software and the hardware.

Unix Derivatives

One of the most widely used multi-user networked operating systems in the world, Unix was developed in 1969 by Ken Thompson at the Bell Labs. Dennis Ritchie is also considered to be a co-inventor of Unix. Around 1972-1974, Unix was reimplemented using the C language, and subsequently it went through a number of changes and improvements under different people. By 1991 it turned out to be the most popular multi-user time-sharing general purpose operating system in the world.

Unix has now forked, and is sold by a number of vendors. However, it is also subject to an international level standardization effort. A number of derivatives exist, including:

• FreeBSD - FreeBSD is an operating system running on Intel IA 32 compatible, DEC Alpha, and PC-98 architectures. It is based on BSD UNIX, a flavor of UNIX developed at the University of California at Berkeley. A large group of developers maintain and develop it. At present, it is being ported to other platforms.
  
• GNU-Hurd - GNU Hurd is a free implementation of the Unix kernel, and it is developed as a GNU project under the Free Software Foundation (FSF). It is still under active development, and is not for production use. At present, it runs only on the IA32 architecture.
  
• GNU/Linux - GNU/Linux is one of the most powerful and most secure operating systems around at present. It is distributed freely and is seriously challenging Microsoft's dominance in the server market. It has an Unix like structure, and is fast replacing Unix in all areas. Its works as an operating system for various severs (web, mail, database, DNS, etc) and is used for massively parallel computing tasks. Recently, due to its immense scalability, it has been used in embedded devices.
  Strictly speaking Linux is the kernel or the core of the operating system only. It is devoid of any proprietary code, and is licensed under the GNU GPL (General Public License). It's development is co-ordinated by Linus Torvalds, and hundreds of developers worldwide contribute to the code.
  The kernel runs on a multitude of platforms, including x86, I64, Alpha, PowerPC, Sparc, Amiga, Atari, MIPS, and SGI. The operating system relies on the GNU toolset, and there are other numerous applications to go with it. It is often regarded as the most successful "hacker project", and at present, big commercial organizations like IBM, Sun, HP, Compaq are actively into GNU/Linux deployment and development.
  GNU/Linux is available as distributions (distros in hacker terms) from various vendors, notable ones for the x86 PC being Red Hat, Debian, MandrakeSoft, SUSE, Caldera, etc. Linux is officially pronounced as leeh-nucks.
  
• Minix - Minix is a free Unix clone and, it is distributed along with its source code. The current versions are version 2.0 and 1.5, and the former supports only Intel compatible architecture, and the latter runs on SPARC, Macintosh, Amiga and Atari as well. Minix is also used as a teaching system for students learning operating systems internals.
  
• OpenBSD - Open BSD is a free implementation of the BSD style Unix. It runs on a number of platforms, including DEC Alpha, Amiga m68k, Hewlett-Packard HP300/HP400, x86, MC680x0 (Apple), PowerPC, Motorola MVME147/16x/17x 68K, SPARC, UltraSPARC and VAX.

Microsoft

Microsoft have always made a large contribution to operating systems for "IBM PC & Compatibles" ever since the days of MS-DOS 1.0. MS-DOS was a basic command-line system with text-based user feedback.

Since 1980, the company started to offer Windows as a graphical alternative to the user environment.