pSOS (Portable Software On Silicon) is areal-time operating system (RTOS), created in about 1982 by Alfred Chao, and developed and marketed for the first part of its life by his company Software Components Group (SCG). In the 1980s, pSOS rapidly became the RTOS of choice for allembedded systems based on theMotorola 68000 series family architecture, because it was written in 68000assembly language and was highly optimised from the start. It was also modularised, with early support for OS-aware debugging, plug-indevice drivers,Internet protocol suite (TCP/IP) stacks, languagelibraries, and disk subsystems. Later camesource code level debugging,multiprocessing support, and furthercomputer networking extensions.

In about 1991, Software Components Group was acquired byIntegrated Systems Inc. (ISI) which further developed pSOS, then renamed as pSOS+, for other microprocessor families, by rewriting most of it in theprogramming languageC. Attention was also paid to supporting successively moreintegrated development environments, culminating in pRISM+.

In July 1994, ISI acquiredDigital Research's modular real-time multi-tasking operating systemFlexOS fromNovell.^[1]

In 1995, ISI offered apSOSystem/NEST package forNovell Embedded Systems Technology (NEST).^[2][1]

In February 2000, ISI was acquired byWind River Systems, the originators of the rival RTOSVxWorks. Despite initial reports that pSOS support would continue, development was halted. Wind River announced plans for a 'convergence' version of VxWorks which will support pSOS system calls, and that no further releases of pSOS will occur.

NXP Semiconductors acquired pSOS forTriMedia from Wind River and continued to support this OS for the TriMediavery long instruction word (VLIW) core.

In March 2000, rival companyExpress Logic released their Evaluation Kit for pSOS+ users, designed to provide a migration path to itsThreadX RTOS.

During August 2000, MapuSoft Technologies Inc. came up with the pSOS OS Changer porting kit which can smoothly move the software to multiple OS such asLinux, VxWorks, and more. It includes anintegrated development environment (IDE) andapplication programming interface (API) optimization along with a profiling tool to measure API timing on target boards (www.mapusoft.com).

In August 2007, RoweBots, a former partner of SCG and ISI,open sourced their pSOS+ compatible version called Reliant. It is available to all that wish to upgrade without application changes.

TheXenomai project supports pSOS+ APIs (and others traditional RTOS APIs) over a Linux-based real-time framework to allow existing industrial applications to migrate easily to a Linux-based environment while keeping stringent real-time guarantees.

Another open sourced alternative isRTEMS, which has support for various APIs, including the "Classic API" (compatible to pSOS) and the POSIX API. Compared to Linux, RTEMS is a closer match to pSOS applications due to its lower memory size and its strict realtime behaviour.

PopularSecure Sockets Layer (SSL), nowTransport Layer Security (TLS), libraries such aswolfSSL still support pSOS.

• FlexOS
• Novell Embedded Systems Technology (NEST)
• UNIX System V STREAMS

• "pSOSystem".Integrated Systems, Inc. (ISI). 1998. Archived fromthe original on 1998-02-19.
• "Express Logic announcement of their Evacuation Kit". Archived fromthe original on 2007-09-28. Retrieved2007-06-13.

• Real-time operating systems
• 1982 software

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