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UNIX System V

UNIX System V

Unix System V, commonly abbreviated SysV (and usually pronounced, though rarely written as System 5), was one of the versions of the Unix operating system. 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. The system also forms the basis of the System V Interface Definition (SVID), a standard defining how System V systems should work.

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. Popular SysV derivatives include Dell SVR4 and Bull SVR4. The most widely used versions of System V today are IBM's AIX, based on System V Release 3, and Sun Microsystems' Solaris Operating System and HP's HP-UX, both based on System V Release 4.

System V was an enhancement over AT&T's first commercial Unix called System III (there was never an outside release of System IV). Traditionally, System V has been considered one of the two major "flavors" of UNIX, the other being BSD. However, with the advent of Unix-like systems developed from neither code base, such as Linux and QNX, this generalization is not as accurate as it once was, and in any case standardisation efforts such as POSIX are tending to reduce the differences between implementations.

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.


The first version of System V (also called System V.0 or System V Release 1, SVR1) was released in 1983. Developed by AT&T's Unix System Development Labs (USDL), a merger of the Unix Support Group and the PWB group, it was based on System III and the Bell Labs internal UNIX/TS 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. 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 is the definitive description of the System V Release 2 kernel.

Apple Computer's A/UX operating system was based on this release (later versions had many extensions from SVR3, SVR4, and BSD), although it was heavily integrated with the Macintosh Toolbox. The first release of HP-UX was also an SVR2 derivative.


System V Release 3 was released in 1987. 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 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:

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) sold enhanced and supported x86 versions. SVR4 was even ported to the Amiga as Amiga Unix and Atari as ASV SVR4 Unix 1991.


Built by a consortium of Intel based resellers (including Unisys, ICL and NCR Corporation). It provided a limited multi-processor capability. This allowed operating system calls to be processed from any processor, but interrupt servicing only from a "master" processor.


Release 4.1 added asynchronous I/O.


Release 4.2, developed in 1992 added support for the Veritas filesystem, access control lists (ACLs), and dynamically loadable kernel modules.

Again, several versions of SVR4.2 appeared, including Univel (later SCO) UnixWare 1, UHC UnixWare, and Consensys.


Release 4.2MP, completed late 1993, added support for multiprocessing. It was released as UnixWare 2 in 1995.


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. 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 manufacturer.


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