IRIX is a computer operating system developed by Silicon Graphics, Inc. (SGI) to run natively on their 32- and 64-bit MIPS architecture workstations and servers. Based on UNIX System V with BSD extensions, it is capable of extremely long uptimes, and its XFS file system is regarded as one of the most advanced journaling file systems in the industry.
The current major version of IRIX is IRIX 6.5 which was released in May 1998. New minor versions of IRIX 6.5 were released every quarter until 2005; since then there have been four further minor releases. Through version 6.5.22, there were two branches of each release: a maintenance release (identified by an m suffix to the version number) that included only fixes to the original IRIX 6.5 code, and a feature release (with an f suffix) that included improvements and enhancements. An overlay upgrade from 6.5.x to the 6.5.22 maintenance release is available for free download, while version 6.5.23 and higher require an active Silicon Graphics support contract, despite only running on Silicon Graphics hardware.
The IRIX name was first used around the time of release 3.0 of the operating system for SGI's IRIS 4D series of workstations and servers, in 1988. Previous releases were identified only by the release number prefixed by "4D1-", eg. "4D1-2.2". The 4D1- prefix continued to be used in official documentation to prefix IRIX release numbers.
IRIX 3.x was based on UNIX System V Release 3 with 4.3BSD enhancements, and incorporated the 4Sight windowing system, based on NeWS and IRIS GL. SGI's own Extent File System (EFS) replaced the System V filesystem.
In 1994, IRIX 6.0 added support for the 64-bit MIPS R8000 processor, but was otherwise similar to IRIX 5.2. Later 6.x releases supported other members of the MIPS processor family in 64-bit mode. IRIX 6.3 was released for the SGI O2 workstation only. IRIX 6.4 improved multiprocessor scalability for the Octane, Origin 2000, and Onyx2 systems. The Origin 2000 and Onyx2 IRIX 6.4 was marketed as "Cellular IRIX", although it only incorporated some features from the original Cellular IRIX distributed operating system project. IRIX 6.4 had severe reliability issues, limiting its adoption. IRIX development stabilized with IRIX 6.5, released in 1998. The current version of IRIX is 6.5.30, released in August 2006.
IRIX has strong support for real-time disk and graphics I/O. IRIX was one of the first Unix versions to feature a graphical user interface for the main desktop environment. IRIX was widely used in the computer animation industry and for scientific visualization due to its once-large application base.
IRIX was a leader in Symmetric Multi-Processing (SMP), scalable from 1 to greater than 1024 processors with a single system image.