Berkeley Software Distribution

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BSD Unix
Beastie, the BSD daemon by Poul-Henning Kamp
DeveloperCSRG, UC Berkeley
OS familyUnix
Working stateSuperseded by derivatives (see below)
Source modelOpen source
Latest release4.4-Lite2 / 1995
Kernel typeMonolithic
LicenseBSD license
Official websiteN/A

Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) 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.

BSD is one of several branches of Unix operating systems. Another one is evolved from UNIX System V developed by AT&T's Unix System Development Labs. A third consists of the GNU/Linux operating systems which draw from Unix System V and BSD, as well as Plan9, and non-UNIX operating systems.

BSD was widely identified with the versions of Unix available for workstation-class systems. This can be attributed to the ease with which it could be licensed and the familiarity it found among the founders of many technology companies during the 1980s. This familiarity often came from using similar systems—notably DEC's Ultrix and Sun's SunOS—during their education. While BSD itself was largely superseded by the System V Release 4 and OSF/1 systems in the 1990s (both of which incorporated BSD code), in recent years modified open source versions of the codebase (mostly derived from 4.4BSD-Lite) have seen increasing use and development.


Unix, filiation on Unix systems.

PDP-11 beginnings

The earliest distributions of Unix from Bell Labs in the 1970s included the source code to the operating system, allowing researchers at universities to modify and extend Unix. The first Unix system at Berkeley was a PDP-11 installed in 1974, and the computer science department used it for extensive research thereafter.

Other universities became interested in the software at Berkeley, and so in 1977 Bill Joy, then a graduate student at Berkeley, assembled and sent out tapes of the first Berkeley Software Distribution (1BSD). 1BSD was an add-on to Sixth Edition Unix rather than a complete operating system in its own right; its main components were a Pascal compiler and Joy's ex line editor.

The Second Berkeley Software Distribution (2BSD), released in 1978, included updated versions of the 1BSD software as well as two new programs by Joy that persist on Unix systems to this day: the vi text editor (a visual version of ex) and the C shell.

Later releases of 2BSD contained ports of changes to the VAX-based releases of BSD back to the PDP-11 architecture. 2.9BSD from 1983 included code from 4.1cBSD, and was the first release that was a full OS (a modified Version 7 Unix) rather than a set of applications and patches. The most recent release, 2.11BSD, was first released in 1992, with maintenance updates from volunteers continuing until 2003.

VAX versions

A VAX computer was installed at Berkeley in 1978, but the port of Unix to the VAX architecture, UNIX/32V, did not take advantage of the VAX's virtual memory capabilities. The kernel of 32V was largely rewritten by Berkeley students to include a virtual memory implementation, and a complete operating system including the new kernel, ports of the 2BSD utilities to the VAX, and the utilities from 32V was released as 3BSD at the end of 1979. 3BSD was also alternatively called Virtual VAX/UNIX or VMUNIX (for Virtual Memory Unix), and BSD kernel images were normally called /vmunix until 4.4BSD.

The success of 3BSD was a major factor in the Defense Advanced Research Projects Agency's (DARPA) decision to fund Berkeley's Computer Systems Research Group (CSRG), which would develop a standard Unix platform for future DARPA research in the VLSI Project. CSRG released 4BSD, containing numerous improvements to the 3BSD system, in October 1980.

4BSD (November 1980) offered a number of enhancements over 3BSD, notably job control in the previously-released csh, delivermail (the antecedent of sendmail), "reliable" signals, and the Curses programming library.

4.1BSD (June 1981) was a response to criticisms of BSD's performance relative to the dominant VAX operating system, VMS. The 4.1BSD kernel was systematically tuned up by Bill Joy until it could perform as well as VMS on several benchmarks. (The release would have been called 5BSD, but the name was changed to avoid confusion with AT&T's UNIX System V release. One early, never-released test version was in fact called 4.5BSD.)

4.2BSD would take over two years to implement and contained several major overhauls. Before its official release came three intermediate versions: 4.1a incorporated a modified version of BBN's preliminary TCP/IP implementation; 4.1b included the new Berkeley Fast File System, implemented by Marshall Kirk McKusick; and 4.1c was an interim release during the last few months of 4.2BSD's development.

The official 4.2BSD release came in August 1983. It was notable as the first version released after the 1982 departure of Bill Joy to co-found Sun Microsystems; Mike Karels and Marshall Kirk McKusick took on leadership roles within the project from that point forward. On a lighter note, it also marked the debut of BSD's daemon mascot in a drawing by McKusick that appeared on the cover of the printed manuals distributed by USENIX.


4.3BSD was released in June 1986. Its main changes were to improve the performance of many of the new contributions of 4.2BSD that had not been as heavily tuned as the 4.1BSD code. Prior to the release, BSD's implementation of TCP/IP had diverged considerably from BBN's official implementation. After several months of testing, DARPA determined that the 4.2BSD version was superior and would remain in 4.3BSD. (See also History of the Internet.)

After 4.3BSD, it was determined that BSD would move away from the aging VAX platform. The Power 6/32 platform (codenamed "Tahoe") developed by Computer Consoles Inc. seemed promising at the time, but was abandoned by its developers shortly thereafter. Nonetheless, the 4.3BSD-Tahoe port (June 1988) proved valuable as it led to a separation of machine-dependent and machine-independent code in BSD which would improve the system's future portability.

Until this point, all versions of BSD had incorporated proprietary AT&T Unix code and therefore required licenses from AT&T for their use. Source code licenses had become very expensive by this point, and several outside parties had expressed interest in a separate release of the networking code, which had been developed entirely outside AT&T and would not be subject to the licensing requirement. This led to Networking Tape 1 (Net/1), which was made available to non-licensees of AT&T code and was freely redistributable under the terms of the permissive BSD license. It was released in June 1989.

4.3BSD-Reno came in early 1990. It was an interim release during the early development of 4.4BSD, and its use was considered a "gamble", hence the naming after the gambling center of Reno, Nevada. This release was clearly moving towards POSIX compliance, and, according to some, away from the BSD philosophy (as POSIX is very much based on System V, and Reno was quite bloated compared to previous releases).

Net/2 and legal troubles

After Net/1, BSD developer Keith Bostic proposed that more non-AT&T sections of the BSD system be released under the same license as Net/1. To this extent, he started a project to reimplement most of the standard Unix utilities without using the AT&T code. For example, vi, which had been based on the original Unix version of ed, was rewritten as nvi (new vi). Within eighteen months, all the AT&T utilities had been replaced, and it was determined that only a few AT&T files remained in the kernel. These files were removed, and the result was the June 1991 release of Net/2, a nearly complete operating system that was freely distributable.

Net/2 was the basis for two separate ports of BSD to the Intel 80386 architecture: the free 386BSD by William Jolitz and the proprietary BSD/386 (later renamed BSD/OS) by Berkeley Software Design (BSDi). 386BSD itself was short-lived, but became the initial code base of the NetBSD and FreeBSD projects that were started shortly thereafter.

BSDi soon found itself in legal trouble with AT&T's Unix System Laboratories subsidiary, then the owners of the System V copyright and the Unix trademark. The USL v. BSDi lawsuit was filed in 1992 and led to an injunction on the distribution of Net/2 until the validity of USL's copyright claims on the source could be determined.

The lawsuit slowed development of the free-software descendants of BSD for nearly two years while their legal status was in question, and as a result systems based on the Linux kernel, which did not have such legal ambiguity, gained greater support. Linux and 386BSD began development at about the same time, and Linus Torvalds has said that if there had been a free Unix-like operating system on the 386 at the time, he likely would not have created Linux. Although it is debatable exactly what effect that would have had on the software landscape since, there is little doubt that it would have been substantial.

4.4BSD and descendants

See also: Comparison of BSD operating systems

The lawsuit was settled in January 1994, largely in Berkeley's favor. Of the 18,000 files in the Berkeley distribution, only 3 had to be removed and 70 modified to show USL copyright notices. A further condition of the settlement was that USL would not file further lawsuits against users and distributors of the Berkeley-owned code in the upcoming 4.4BSD release.

In June 1994, 4.4BSD was released in two forms: the freely distributable 4.4BSD-Lite contained no AT&T source, whereas 4.4BSD-Encumbered was available, as earlier releases had been, only to AT&T licensees.

The final release from Berkeley was 1995's 4.4BSD-Lite Release 2, after which the CSRG was dissolved and development of BSD at Berkeley ceased. Since then, several distributions based on 4.4BSD (such as FreeBSD, OpenBSD, and NetBSD) have been maintained.

In addition, the permissive nature of the BSD license has allowed many other operating systems, both free and proprietary, to incorporate BSD code. For example, Microsoft Windows has used BSD-derived code in its implementation of TCP/IP and bundles recompiled versions of BSD's command line networking tools with its current releases. Also Darwin, the system on which Apple's Mac OS X is built, is partly derived from FreeBSD 5. Various commercial UNIXes, such as Solaris, also contain varying amounts of BSD code.


BSD pioneered many of the advances of modern computing. Berkeley's Unix was the first Unix to include libraries supporting the Internet Protocol stacks: Berkeley sockets. By integrating sockets with the Unix operating system's file descriptors, it became almost as easy to read and write data across a network as it was to access a disk. The AT&T laboratory eventually released their own STREAMS library, which incorporated much of the same functionality in a software stack with a better architecture, but the wide distribution of the existing sockets library, together with the unfortunate omission of a function call for polling a set of open sockets equivalent to the select call in the Berkeley library, reduced the impact of the new API.

Today, BSD continues to be used as a testbed for technology by academic organizations, as well as finding uses in a lot of commercial and free products and, increasingly, in embedded devices. The general quality of its source code, as well as its documentation (especially reference manual pages, commonly referred to as man pages), make it well-suited for many purposes.

The permissive nature of the BSD license allows companies to distribute derived products as proprietary software without exposing source code and sometimes intellectual property to competitors. Searching for strings containing "University of California, Berkeley" in the documentation of products, in the static data sections of binaries and ROMs, or as part of other information about a software program, will often show BSD code has been used. This permissiveness also makes BSD code suitable for use in open source products, and the license is compatible with many other open source licenses.

It is an interesting fact that BSD operating systems can run much native software of several other operating systems on the same architecture, using a binary compatibility layer. Much simpler and faster than emulation, this allows, for instance, applications intended for Linux to be run at effectively full speed. This makes BSDs not only suitable for server environments, but also for workstation ones, given the increasing availability of commercial or closed-source software for Linux only. This also allows administrators to migrate legacy commercial applications, which may have only supported commercial Unix variants, to a more modern operating system, retaining the functionality of such applications until they can be replaced by a better alternative.

Current BSD operating system variants support many of the common IEEE, ANSI, ISO, and POSIX standards, while retaining most of the traditional BSD behavior. 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' SunOS, founding the first wave of popular Unix workstations.

Significant BSD descendants

See also: Category:BSD and Comparison of BSD operating systems

BSD has been the base of a large number of operating systems. Most notable among these today is perhaps the major open source BSDs, FreeBSD, NetBSD and OpenBSD—sometimes known as the BSDs—which have themselves spawned a number of children, including DragonFly BSD, FreeSBIE, MirOS BSD, DesktopBSD, and PC-BSD. They are targeted at an array of systems for different purposes and are common in government facilities, universities and in commercial use. A number of commercial operating systems are also partly or wholly based on BSD or its descendants, including Sun's SunOS and Apple Computer's Mac OS X. A selection of significant Unix versions and Unix-like operating systems that descend from BSD includes:

While many Unix variants are not derived from BSD, there are very few that do not include some bits of BSD (like 'vi').

BSD Trivia

  • The Information Week magazine rated BSD 4.3 as the Greatest Software Ever Written [1]. Quoting from the article, "BSD 4.3 represents the single biggest theoretical undergirder of the Internet".

See also

Further reading

  • Chris DiBona, Mark Stone, Sam Ockman, Open Source (Organization), Brian Behlendorf and J. Scott Bradner, Open Sources: Voices from the Open Source Revolution. O'Reilly & Associates, 1999. Trade paperback, 272 pages. ISBN 156592582. Online version; Marshall Kirk McKusick, chapter on BSD, "Twenty Years of Berkeley Unix - From AT&T-Owned to Freely Redistributable"
  • Samuel J. Leffler, Marshall K. McKusick, Michael J. Karels, John S. Quarterman, The Design and Implementation of the 4.3BSD UNIX Operating System (Addison Wesley, November, 1989; ISBN 0-201-06196-1)
  • Marshall K. McKusick, Keith Bostic, Michael J. Karels, John S. Quartermain, The Design and Implementation of the 4.4BSD Operating System (Addison Wesley, 1996; ISBN 0-201-54979-4)
  • Marshall K. McKusick, George V. Neville-Neil, The Design and Implementation of the FreeBSD Operating System (Addison Wesley, August 2, 2004; ISBN 0-201-70245-2)
  • Peter H. Salus, The Daemon, the GNU & The Penguin (Forthcoming - currently being serialised on the Groklaw website)
  • Peter H. Salus, A Quarter Century of UNIX (Addison Wesley, June 1, 1994; ISBN 0-201-54777-5)
  • Peter H. Salus, Casting the Net (Addison-Wesley, March 1995; ISBN 0-201-87674-4)

External links