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Unix System V, commonly abbreviated SysV (and usually pronounced — though rarely written — as "System 5"), is one of the first versions of the Unix operating system. It was originally developed by American Telephone & Telegraph (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 commercially the most successful version, being the result of an effort, marketed as Unix System Unification, which solicited the collaboration of the major Unix vendors. It was the source of several commercial common Unix features.

While AT&T sold their own hardware that ran System V (see AT&T Computer Systems), most customers ran a version from a reseller, based on AT&T's reference implementation. A standards document called the System V Interface Definition outlined the default features and behavior of implementations. The most widely used versions of System V today are IBM's AIX, based on System V Release 3, and Sun's OpenSolaris and HP's HP-UX, both based on System V Release 4.

In the 1980s and early 90s, System V was considered one of the two major "flavors" of UNIX, the other being Berkeley Unix (BSD). During the period of the Unix wars System V was known for being the primary choice of manufacturers of large multiuser systems, in opposition to BSD's dominance of desktop workstations. However, with standardization efforts such as POSIX and the commercial success of Linux, this generalization is not as accurate as it once was.

Unix history tree





System V succeeded AT&T's previous commercial Unix called System III in 1983. There was never an external release of System IV.[1] The first version (also called System V.0 or System V Release 1, SVR1) was released in 1983. Developed by AT&T's UNIX Support Group (USG), it was based on the Bell Labs internal USG UNIX 5.0. System V also included features such as the vi editor and curses from the Berkeley Software Distribution of UNIX developed at the University of California, Berkeley (UCB); it also improved performance by adding buffer and inode caches. System V ran on the DEC VAX and PDP-11 machines. It also added support for inter-process communication using messages, semaphores, and shared memory.


System V Release 2 was released in 1984. It added shell functions and the SVID. New kernel features included record and file locking, demand paging, and copy on write.[2] The concept of the "porting base" was formalized, and the DEC VAX 11/780 was named for this Release. The "porting base" is the so-called original version of a Release, from which all porting efforts for other machines emanate. Maurice J. Bach's The Design of the UNIX Operating System[3] is the definitive description of the System V Release 2 kernel.

Apple Computer's A/UX operating system was initially based on this release. The first release of HP-UX was also an SVR2 derivative.[4]


System V Release 3 was released in 1986.[5] It included STREAMS, the Remote File System (RFS), the File System Switch (FSS) virtual file system mechanism, a restricted form of shared libraries, and the Transport Layer Interface (TLI) network API. The final version was Release 3.2 in 1988, which added binary compatibility to Xenix on Intel platforms; SCO Xenix System V/386 was based upon 3.2. The AT&T 3B2 became the official "porting base". IBM's AIX operating system is an SVR3 derivative.


System V Release 4.0 was announced on October 18, 1988[6] and was released in 1990. A joint project of Unix System Laboratories and Sun Microsystems, it combined technology from Release 3 as well as 4.3BSD, Xenix, and SunOS. New features included:

The primary platforms for SVR4 were Intel x86 and SPARC; the SPARC version, called Solaris 2 (or, internally, SunOS 5.x), was developed by Sun. The relationship between Sun and AT&T was terminated after the release of SVR4, meaning that later versions of Solaris did not inherit features of later SVR4.x releases. Sun would in 2005 release most of the source code for Solaris 10 (SunOS 5.10) as the open source OpenSolaris project, creating the only open-source (heavily modified) System V implementation available.

Many versions of SVR4 appeared, because of hardware vendors (HP, SGI) adapting it to their platform, and because porting houses (SCO, Microport, ESIX, UHC)[7] sold enhanced and supported x86 versions. SVR4 was even ported to the Amiga as Amiga Unix and Atari as ASV SVR4 Unix 1991. Other popular SysV derivatives include Dell SVR4[8] and Bull SVR4. A consortium of Intel based resellers including Unisys, ICL, NCR Corporation, and Olivetti developed SVR4.0MP with multiprocessing capability (allowing system calls to be processed from any processor, but interrupt servicing only from a "master" processor).[9] Release 4.1 ES (Enhanced Security) added security features required for Orange Book B2 compliance and Access Control Lists and support for dynamic loading of kernel modules.[10][11]

In 1992, AT&T USL engaged in a joint venture with Novell, called Univel. That year saw the release System V.4.2 as Univel UnixWare, featuring the VERITAS File System. Other vendors included UHC and Consensys. Release 4.2MP, completed late 1993, added support for multiprocessing and It was released as UnixWare 2 in 1995.[12]

System V.4 as modified by Sun is now an open source product in the form of OpenSolaris.


The Santa Cruz Operation (SCO), developers of XENIX, eventually acquired the System V Release 4.2 codebase and the UnixWare trademark from Novell, while other vendors (Sun, IBM, HP) continued to use and extend System V Release 4. The Unix trade mark is now owned by The Open Group, which grants it to any operating system that meets its Single Unix Specification (SUS) - in effect, a successor to the System V Interface Definition. The SUS, however, is also met by Apple's Mac OS X, a derivative of BSD, and several OSes that are not derived from either BSD or System V.

System V Release 5 was developed in 1997 by the Santa Cruz Operation (SCO) as a merger of SCO OpenServer (an SVR3-derivative) and UnixWare, with a focus on large-scale servers.[13] It was released as SCO UnixWare 7. SCO's successor, The SCO Group, also based SCO OpenServer 6 on SVR5, but the codebase is not used by any other major developer or reseller. System V Release 6 was announced by SCO to be released by the end of 2004, but was apparently cancelled.[14] It was supposed to support 64-bit systems.[15]


  1. ^ Tanenbaum, Andrew S. (2001). Modern Operating Systems. Prentice Hall. pp. 675. "Whatever happened to System IV is one of the great unsolved mysteries of computer science."  
  2. ^ Goodheart, Berny; James Cox (1994). The Magic Garden Explained. Prentice Hall. pp. 11. ISBN 0-13-098138-9.  
  3. ^ Bach, Maurice (1986). The Design of the UNIX Operating System. Prentice Hall. ISBN 0-13-201799-7.  
  4. ^ Rosen, p. 33.
  5. ^ Rargo, Stephan A. (April 10, 1993), UNIX System V Network Programming, Addison-Wesley Professional, ISBN 978-0201563184  
  6. ^ Amdahl, Control Data Corporation, et al. (October 18, 1988). "SEVERAL MAJOR COMPUTER AND SOFTWARE COMPANIES ANNOUNCE STRATEGIC COMMITMENT TO AT&T'S UNIX SYSTEM V, RELEASE 4.0". Press release. Retrieved 2007-01-01.  
  7. ^ Eric S. Raymond, A buyer's guide to UNIX versions for PC-clone hardware, posted to Usenet November 16, 1994.
  8. ^ Technologists notes - A brief history of Dell UNIX, 10 January 2008,, retrieved 2009-02-18  
  9. ^ Unix Internatl. and USL release early version of SVR4 multiprocessing software, 17 June 1991,, retrieved 2009-04-22  
  10. ^ UNIX INTERNATIONAL REVIEWS THE UNIX SYSTEM V.4 STORY SO FAR, 13 August 1992,, retrieved 2008-10-31  
  11. ^ Bishop, Matt (December 2, 2002), Computer Security, Addison Wesley, p. 505, ISBN 0201440997  
  12. ^ UnixWare 2 Product Announcement Questions& Answers, 1995,  
  13. ^ Kenneth H. Rosen (1999). UNIX: The Complete Reference. McGraw-Hill Professional, pp. 23, 32.
  14. ^
  15. ^ SCO UNIX Roadmap

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