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UNICOS is a range ofUnix and laterLinuxoperating system (OS) variants developed byCray for itssupercomputers. UNICOS is the successor of theCray Operating System (COS). It provides networkclustering and source codecompatibility layers for some other Unixes. UNICOS was originally introduced in 1985 with theCray-2 system and later ported to other Cray models. The original UNICOS was based onUNIX System V Release 2, and had manyBerkeley Software Distribution (BSD) features (e.g.,computer networking andfile system enhancements) added to it.

CX-OS was the original name given to what is now UNICOS. This was a prototype system which ran on aCray X-MP in 1984 before the Cray-2 port. It was used to demonstrate the feasibility of usingUnix on a supercomputer system, before Cray-2 hardware was available.

The operating system revamp was part of a larger movement inside Cray Research to modernize their corporate software: including rewriting their most importantFortran compiler (cft to cft77) in a higher-level language (Pascal) with more modern optimizations and vectorizations.

As a migration path for existing COS customers wishing to transition to UNICOS, a Guest Operating System (GOS) capability was introduced into COS. The only guest OS that was ever supported was UNICOS. A COS batch job would be submitted to start up UNICOS, which would then run as a subsystem under COS, using a subset of the systems CPUs, memory, and peripheral devices. The UNICOS that ran under GOS was exactly the same as when it ran stand-alone: the difference was that thekernel would make certain low-level hardware requests through the COS GOS hook, rather than directly to the hardware.

One of the sites that ran very early versions of UNICOS wasBell Labs, where Unix pioneers includingDennis Ritchie ported parts of theirEighth Edition Unix (includingSTREAMSinput/output (I/O)) to UNICOS. They also experimented with a guest facility within UNICOS, allowing the stand-alone version of the OS to host itself.

Cray released several different OSs under the name UNICOS, including:

1. UNICOS: the original Cray Unix, based on System V. Used on theCray-1,Cray-2,X-MP,Y-MP,C90, etc.
2. UNICOS MAX: aMach-basedmicrokernel used on theT3D's processing elements, together with UNICOS on the host Y-MP or C90 system.
3. UNICOS/mk: aserverized version of UNICOS using theChorusmicrokernel to make adistributed operating system. Used on theT3E. This was the last Cray OS really based on UNICOS sources, as the following products were based on different sources and simply used the "UNICOS" name.
4. UNICOS/mp: not derived from UNICOS, but based onIRIX 6.5. Used on theX1.
5. UNICOS/lc: not derived from UNICOS, but based onSUSE Linux. Used on theXT3,XT4 andXT5. UNICOS/lc 1.x comprises a combination of
   1. the compute elements run theCatamount microkernel (which itself is based onCougaar)
   2. the service elements run SUSE Linux
6. Cray Linux Environment (CLE): from release 2.1 onward, UNICOS/lc is now called Cray Linux Environment
   1. the compute elements runCompute Node Linux (CNL) (which is a customizedLinux kernel^[1])
   2. the service elements runSUSE Linux Enterprise Server

• Scientific Linux, aLinux distribution byFermilab andCERN
• Rocks Cluster Distribution, a Linux distribution for supercomputers

• "UNICOS 10.0.1.1 Release Overview".Cray Inc. Archived fromthe original on 2020-01-07. Retrieved2009-02-12.
• "UNICOS/mk 2.0.6 Release Overview". Cray Inc. Archived fromthe original on 2020-01-07. Retrieved2009-02-12.
• "UNICOS/mp 3.1 Release Overview". Cray Inc. Archived fromthe original on 2020-01-07. Retrieved2009-02-12.
• "Cray XT System Software Release Overview". Cray Inc. Archived fromthe original on 2020-01-07. Retrieved2009-02-12.
• Gannett, Fred (2007-10-25)."Cray Supercomputer FAQ and other documents".
• Ritchie, Dennis (September 2000)."Experiences with the Cray X/MP". Archived fromthe original on 2013-03-17.

• 1984 software
• Cray software
• Linux distributions
• Microkernel-based operating systems
• Microkernels
• Supercomputer operating systems
• Unix distributions
• UNIX System V

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