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1. 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.

2. Early computers lacked 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. 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.

3. In general, the operating system is the first layer of software loaded into computer memory when it starts up. As the first software layer, all other software that gets loaded after it depends on this software to provide them with various common core services. These common core services include, but are not limited to: disk access, memory management, task scheduling and user interfacing. Since these basic common services are assumed to be provided by the OS, there is no need to re-implement those same functions over and over again in every other piece of software that you may use. The portion of code that performs these core services is called the "kernel" of the operating system.

4. Operating system kernels had been evolved from libraries that provided the core services into unending programs that control system resources because of the early needs of accounting for computer usage and then protecting those records. 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.

Modern Operating Systems

5. As of 2005, the major operating systems in widespread use on general-purpose computers have consolidated into two main families

(a) Unix like family and

(b) Microsoft like family.

6. Mainframe computers and embedded systems use a variety of different operating systems, many with no direct connection to Windows or Unix.

7. The Unix-like family is a more diverse group of operating systems, with several major sub-categories including SystemV, 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 confirm to the definitions that they have cooperatively developed. The name is commonly used to refer to the large set of operating systems which resemble the original Unix.

8. 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. Some proprietary Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that vendor's proprietary hardware while others can run on the vendor's proprietary hardware and also on industry-standard PCs. Sun's formerly proprietary Solaris (it is becoming open-source under the CDDL license) is one such versatile but true Unix (it can run on Sun's servers but also on smaller x86 systems).

9. 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.

10. The Microsoft-like 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.

Unix SystemV

11. System V, previously known as AT&T System V, was one of the versions of the Unix computer OS. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4. System V Release 4, or SVR4, was the most successful version, and the source of several common Unix features, such as "SysV init scripts" (/etc/init.d), used to control system startup and shutdown, and the System V Interface Definition (SVID), a standard defining how System V systems should work.

12. While AT&T sold their own hardware which ran System V, many customers ran a version from a reseller, based on AT&T's reference implementation. Popular SysV derivatives include Dell SVR4 and Bull SVR4. The most widely used versions of System V today are SCO Open Server,Based on System V Release 3, and Sun Microsystems Solaris Operating Environment and SCO Unix Ware, both based on System V Release 4.

13. System V was an enhancement over AT&T's first commercial UNIX called System III. Traditionally, System V has been considered one of the two major "flavors" of UNIX, the other being BSD. However, with the advent of UNIX implementations developed from neither code base, such as Linux and QNX, this generalization is not as accurate as it once was, and in any case standardization efforts such as POSIX are tending to reduce the differences between implementations.

14. There are five releases of SVR, namely:

(a) SVR 1: The first version of System V was released in 1983. It introduced features such as the vi editor and cursors from the Berkley Software Distribution of UNIX developed at the University of California, Berkley (UCB). It also added support for inter-process comn using messages, semaphores and shared memory.

(b) SVR 2: System V Release 2 was released in 1984. It added Unix shell functions and the SVID.

(c) SVR 3: System V Release 3 was released in 1987. It included STREAMS, remote file sharing (RFS), shared libraries and the Transport Layer Interface(TLI).

(d) SVR 4: System V Release 4.0 was announced on 1 Nov1989 and was released in 1990. A joint project of Unix Systems Labs and Sun Microsystems, it combined technology from Release 3 as well as 4.3 BSD, Xenix and Sun OS. TCP/IP and csh support from BSD. Network file system(NFS), memory mapped files, a new shared library system support from Sun OS. Other improvements were ksh, ANSI C, internationalization support, ABI and support for standards such as POSIX , X/Open and SVID 3. SVR 4.1 added asynchronous I/O. SVR 4.2 added support for the Veritas file system, access control lists(ACLs), and dynamically loadable kernel modules.

        (e) SVR 5: Produced by the SCO group.

Berkley Software Distribution (BSD)

15. Berkeley Software Distribution (BSD) is the UNIX derivative distributed by the University of California, Berkeley starting in the 1970s. The name is also used collectively for the modern descendants of these distributions.

16. BSD pioneered many of the advances of modern computing. Berkeley's Unix was the first to include library support for the IP stacks, Berkeley sockets. By integrating sockets with the UNIX operating system file descriptors, users of their library found it almost as easy to read and write data across the network, as it was to put data on a disk.

17. The AT&T laboratory eventually released their own STREAMS library, which incorporated much of the same functionality in a software stack with better architectural layers, but the already widely-distributed sockets library, together with the unfortunate omission of a function call for polling a set of open sockets (an equivalent of the select call in the Berkeley library), made it difficult to justify porting applications to the new API.

18. Today, it continues to be used as technology testbed by academic organizations, as well as high-technology examples in a lot of commercial and free products. It is increasingly being used on embedded devices as well. The general quality of its source code design and clean writing, as well as its documentation (especially reference manual pages, commonly referred to as "man pages"), make such systems a heaven for programmers.

19. It is an interesting fact that BSD operating systems can run native software of several other operating systems on the same architecture, using binary compatibility. This, much faster than emulation, allows for instance to run applications intended for Linux on a BSD operating system at full speed. This makes BSD not only suitable for server environments, but also for workstation ones, considering the increasing availability of commercial or closed-source software for Linux. It also allows to migrate old commercial software which only used to run on commercial UNIX platforms to a modern BSD operating system, while retaining functionality of the previous system until it can fully be replaced by a better alternative.

20. Like AT&T Unix, the BSD kernel is monolithic, meaning that device drivers in the kernel run in privileged mode, as part of the core of the operating system. Early versions of BSD were used to form Sun Microsystems' Sun OS, founding the first wave of popular Unix workstations.

GNU General Public License

21. The GNU General Public License (GNU GPL or simply GPL) is a free software license, originally written by Richard Stallman for the GNU project (a project to create a complete free software OS). It has since become the most popular license for free software (or "open source software"). The latest version of the license, version 2, was released in 1991.

22. The GPL grants the recipients of a computer program the following rights, or "freedoms":

(a) The freedom to run the program, for any purpose.

(b) The freedom to study how the program works, and modify it. (Access to the source code is a precondition for this)

(c) The freedom to redistribute copies.

(d) The freedom to improve the program, and release the improvements to the public. (Access to the source code is a precondition for this)

23. In contrast, the end user license that come with proprietary software rarely grant the end user any rights (other than the right to use the software, although it is debatable whether one requires a license for use per se), and may even attempt to restrict activities normally permitted by law, such as reverse engineering.

24. The primary difference between the GPL and more "permissive" free software licenses such as the BSD license is that the GPL seeks to ensure that the above freedoms are preserved in copies and in derivative works. It does this using a legal mechanism known as copyleft, invented by Stallman, which requires derivative works of GPL-licensed programs to also be licensed under the GPL. In contrast, BSD-style licenses allow for derivative works to be redistributed as proprietary software.

25. By some measures, the GPL is the single most popular license for free and Open Source software. As of April 2004, the GPL accounted for nearly 75% of the 23,479 free-software projects listed on Freshmeat, and about 68% of the projects listed on SourceForge. (It should be noted that these two sites are owned by OSTG, a company that advocates Linux and the GPL).

26. The GPL does not give the licensee unlimited redistribution rights. The right to redistribute is granted only if the licensee includes the source code (or a legally-binding offer to provide the source code), including any modifications made. Furthermore, the distributed copies, including the modifications, must also be licensed under the terms of the GPL.

27. This requirement is known as copyleft, and it gets its legal teeth from the fact that the program is copyrighted. Because it is copyrighted, a licensee has no right to modify or redistribute it (barring fair use), except under the terms of the copyleft. One is only required to accept the terms of the GPL if one wishes to exercise rights normally restricted by copyright law, such as redistribution. Conversely, if one distributes copies of the work without abiding by the terms of the GPL (for instance, by keeping the source code secret), they can be sued by the original author under copyright law.

28. Many distributors of GPL'ed programs bundle the source code with the executables. An alternative method of satisfying the copyleft is to provide a written offer to provide the source code on a physical medium (such as a CD) upon request. In practice, many GPL'ed programs are distributed over the Internet, and the source code is made available over FTP. For Internet distribution, this complies with the license.

29. The copyleft only applies when a person seeks to redistribute the program. One is allowed to make private modified versions, without any obligation to divulge the modifications as long as the modified software is not distributed to anyone else. Note that the copyleft only applies to the software and not to its output (unless that output is itself a derivative work of the program); for example, a web portal running a modified GPL content management system is not required to distribute its changes to the underlying software. (It has been suggested that this be changed for version 3 of the GPL).

Free BSD and Linux

30. Almost all code in Free BSD is under the BSD license (one notable exception being the compiler, gcc). The BSD license puts very few restrictions on what can be done with code placed under it. Essentially, the only restrictions are that the user must attribute the previous contributors (i.e. the user can't claim it was all his work), the user cannot claim that the previous contributors endorse the user's product, and the user cannot hold the contributors liable for any mistakes in the code. After meeting those restrictions, essentially anything else can be done with the code, including distributing closed-source modified versions.

31. The Linux kernel and much of the utilities commonly distributed with it are under the GNU General Public License (GPL). The GPL allows free use of the software licensed under it under essentially the same restrictions as the BSDL with the additional requirement that if modified code is distributed, then the changes must be made available in source code for all to use.

32. Generally, Linux is less centralized than Free BSD. Linux by itself is only a kernel. To function as an operating system, other utilities are required. These other utilities are gathered from various sources and collected together with the kernel by various groups in distributions. Kernel and system utilities are developed independently and merged together to form an operating system. This means that the kernel has one version, and all the other utilities in the operating system have others.

33. Free BSD is more centralized. The kernel and basic system utilities are developed, versioned, and distributed together. Other programs, such as X and web browsers,,can be brought in from elsewhere, but the basic system comes from one source and is designed specifically for the Free BSD operating system. Being versioned together in the same CVS tree is an advantage. Changes must consider all affected parts, not just the particular part being changed. This leads to a more cohesive, polished system. In fact, the concept of a kernel version different from the rest of the system does not really exist in Free BSD.


34. The two systems share much of the same functionality. They are often able to run programs coded for the other system. When a complete desktop environment, such as GNOME or KDE is running, the two systems are often difficult to tell apart. Free BSD can also run Linux programs due to a very lightweight Linux subsystem, which is capable of running even commercial Linux software.

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