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{{Short description|Computer operating system by Acorn Computers Ltd
{{About|the operating system developed by Acorn Computers Ltd|the OS developed by MIPS Computer Systems|MIPS RISC/os}}
{{Use dmy dates|date=January 2025}}
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{{Infobox OS
| name = RISC OS
| logo =
| logo size =
| screenshot = RISC OS 5.30 screenshot.jpg
| screenshot_size = 300px
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== History ==
{{Main|History of RISC OS}}
[[File:Riscos logo generic cogwheel richard hallas lg cogwheel x1.svg|thumb|left|110x110px|Original cogwheel logo]]
The first version of RISC OS was originally released in 1987 as ''Arthur 1.20''. The next version, {{nowrap|Arthur 2}}, became {{nowrap|RISC OS 2}} and was released in April 1989. RISC OS 3.00 was released with the A5000 in 1991, and contained many new features. By 1996, RISC OS had been shipped on over 500,000 systems.<ref>{{cite web|url=http://acorn.chriswhy.co.uk/docs/Acorn/ART/ART_DS013_AcornRISCOS.pdf|title=ART – Acorn RISC OS|work=acorn.chriswhy.co.uk|access-date=8 July 2011|archive-date=27 May 2013|archive-url=https://web.archive.org/web/20130527220851/http://acorn.chriswhy.co.uk/docs/Acorn/ART/ART_DS013_AcornRISCOS.pdf|url-status=live}}</ref>
[[File:Acorn Archimedes A3020 ( Bletchley Park, translucent ).png|thumb|left|An Acorn Archimedes A3020 computer running RISC OS]]
Acorn officially halted work on the OS in January 1999, renaming themselves [[Element 14 (company)|Element 14]]. In March 1999 a new company, [[RISCOS Ltd]], licensed the rights to develop a desktop version of RISC OS from Element 14, and continued the development of RISC OS 3.8, releasing it as ''RISC OS 4'' in July 1999. Meanwhile, Element 14 had also kept a copy of RISC OS 3.8 in house, which they developed into [[NCOS]] for use in [[set-top box]]es. In 2000, as part of the acquisition of Acorn Group plc by MSDW Investment, RISC OS was sold to [[Pace plc|Pace Micro Technology]],<ref name="register19990429">{{ cite news | url=https://www.theregister.com/1999/04/29/arm_denies_role_in_acorn/ | title=ARM denies role in Acorn dismemberment | work=[[The Register]] | date=24 April 1999 | access-date=31 January 2021 | last1=Cullen | first1=Drew }}</ref> who later sold it to [[Castle Technology]] Ltd.
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== Features ==
=== OS core ===
The OS is [[single-user]] and employs [[cooperative multitasking]] (CMT).<ref name="byte tired">{{cite journal |url=https://archive.org/details/eu_BYTE-1996-12_OCR/page/n84/mode/1up |title=A RISC OS for All Seasons |last=Palmer |first=Stewart |journal=[[Byte (magazine)|Byte]] |date=December 1996 |volume=21 |issue=12 |page=49 |oclc=208951251 |issn=0360-5280}}</ref> While most current desktop OSes use [[preemption (computing)|preemptive]] [[computer multitasking|multitasking]] (PMT) and [[thread (computer science)|multithreading]], {{nowrap|RISC OS}} remains with a CMT system. By 2003, many users had called for the OS to migrate to PMT.<ref name="drobe williams pmt 2003">{{cite news|last=Williams|first=Chris|title=Imagining RISC OS and PMT|url=http://www.drobe.co.uk/article.php?id=764|access-date=10 July 2012|newspaper=[[Drobe]]|date=26 July 2003|archive-date=17 June 2018|archive-url=https://web.archive.org/web/20180617115807/http://www.drobe.co.uk/article.php?id=764|url-status=live}}</ref> The OS [[memory protection]] is not comprehensive.<ref>Michael Reed [https://books.google.com/books?id=Lr8ldHhh_wsC&pg=PA82 ''Tech Book 1'' – Published articles Oct 2006 – June 2008] {{Webarchive|url=https://web.archive.org/web/20200726094727/https://books.google.es/books?id=Lr8ldHhh_wsC&pg=PA82 |date=26 July 2020 }}</ref><ref>{{cite web|url=http://www.drobe.co.uk/riscos/artifact1327.html|title=RISC OS Memory Protection|website=Drobe: The Archives|access-date=4 March 2013|archive-date=31 March 2014|archive-url=https://web.archive.org/web/20140331035623/http://www.drobe.co.uk/riscos/artifact1327.html|url-status=live}}</ref> A third party attempt to add preemptive multitasking was started in 1999, as the Wimp2 project,<ref>{{Cite web |title=ned Productions - RISC-OS programs - Wimp2 |url=https://www.nedprod.com/programs/RISC-OS/Wimp2/ |access-date=2025-06-07 |website=www.nedprod.com}}</ref> and similar goal added to the RISC OS Open roadmap in 2015.<ref>{{Cite web |title=RISC OS Open: RISC OS Roadmap in Library |url=https://www.riscosopen.org/wiki/documentation/show/RISC%20OS%20Roadmap |archive-url=https://web.archive.org/web/20250401115500/https://www.riscosopen.org/wiki/documentation/show/RISC%20OS%20Roadmap |archive-date=1 April 2025 |access-date=2025-06-07 |website=www.riscosopen.org |language=en |url-status=live }}</ref> In addition support for Posix / SysV / BSD pthread (preemptive-threading) was made available through the provision of UnixLib 3.6 in 1992, a C library introduced to permit the porting of the standard GNU / BSD tool chains to RISC OS, by Alun Jones, then of the Higher Education National Software Archive (HENSA). As of 2025 UnixLib is available through the PackMan package manager.
The core of the OS is stored in [[read-only memory|ROM]], giving a fast [[bootup]] time and safety from operating system corruption. RISC OS 4 and 5 are stored in {{nowrap|4 MB}} of [[flash memory]], or as a [[ROM image]] on [[SD card]] on single board computers such as the Beagleboard or Raspberry Pi, allowing the operating system to be updated without having to replace the ROM chip. The OS is made up of several [[Loadable kernel module|module]]s. These can be added to and replaced, including soft-loading of modules not present in ROM at run time and on-the-fly replacement. This design has led to OS developers releasing rolling updates to their versions of the OS, while third parties are able to write OS replacement modules to add new features. OS modules are accessed via software interrupts (SWIs), similar to [[system call]]s in other operating systems.
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=== File system ===
{{More citations needed section|date=July 2017}}
The [[file system]] is volume-
A file system can present a file of a given type as a volume of its own, similar to a [[loop device]]. The OS refers to this function as an image filing system. This allows transparent handling of archives and similar files, which appear as directories with some special properties. Files inside the image file appear in the hierarchy underneath the parent archive. It is not necessary for the archive to contain the data it refers to: some symbolic link and network share file systems put a reference inside the image file and go elsewhere for the data.
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| ArchiEmu<ref>{{cite web |title=Tellima |url=http://www.tellima.nl/riscos/ |url-status=live |archive-url=https://web.archive.org/web/20151222081521/http://www.tellima.nl/riscos/ |archive-date=22 December 2015 |access-date=13 December 2015}}</ref> || RISC OS || 0.53.3 – 7 December 2014
|-
| ArcEm<ref>{{cite web |title=ArcEm – The Acorn Archimedes Emulator |url=
|-
| Arculator<ref>{{cite web |title=Arculator – The Acorn Archimedes Emulator |url=http://b-em.bbcmicro.com/arculator/ |url-status=live |archive-url=https://web.archive.org/web/20130621075124/http://b-em.bbcmicro.com/arculator/ |archive-date=21 June 2013 |access-date=11 September 2019 |publisher=B-em.bbcmicro.com}}</ref> || Windows, Linux || 2.2 – 24 June 2023
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| Red Squirrel<ref>{{cite web |title=Red Squirrel Acorn Archimedes Emulator |url=http://www.redsquirrel.fsnet.co.uk/redsquirrel.html |url-status=live |archive-url=https://web.archive.org/web/20051227213827/http://www.redsquirrel.fsnet.co.uk/redsquirrel.html |archive-date=27 December 2005 |access-date=2 September 2013 |publisher=Redsquirrel.fsnet.co.uk}}</ref> || Archimedes, Risc PC, A7000 || 0.6 – 28 October 2002
|-
| RPCEmu<ref>{{cite web |title=RPCEmu |url=https://www.marutan.net/rpcemu/ |url-status=live |archive-url=https://web.archive.org/web/20130316043123/http://www.marutan.net/rpcemu/ |archive-date=16 March 2013 |access-date=7 November 2017 |publisher=Marutan.net}}</ref> || Risc PC, A7000, Phoebe || Windows, Linux, macOS, [[OpenBSD]] || 0.9.
|-
|[[VirtualAcorn|VirtualRPC]]|| Risc PC || Windows, macOS || 5 September 2014<ref>{{cite web |title=VirtualAcorn |url=http://www.virtualacorn.co.uk/news.htm |work=virtualacorn.co.uk}}</ref>
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