RISC OS

{{Main|History of RISC OS}}

File:Riscos logo generic cogwheel richard hallas lg cogwheel x1.svg

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.{{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}}

File:Acorn Archimedes A3020 ( Bletchley Park, translucent ).png

Acorn officially halted work on the OS in January 1999, renaming themselves 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 boxes. In 2000, as part of the acquisition of Acorn Group plc by MSDW Investment, RISC OS was sold to Pace Micro Technology,{{ 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 }} who later sold it to Castle Technology Ltd.

In May 2001, RISCOS Ltd launched RISC OS Select, a subscription scheme allowing users access to the latest RISC OS 4 updates. These upgrades are released as soft-loadable ROM images, separate to the ROM where the boot OS is stored, and are loaded at boot time. Select 1 was shipped in May 2002, with Select 2 following in November 2002 and the final release of Select 3 in June 2004. In the same month, RISC OS 4.39, dubbed RISC OS Adjust, was released. RISC OS Adjust was a culmination of all the Select Scheme updates to date, released as a physical set of replaceable ROMs for the RiscPC and A7000 series of machines.

Meanwhile, in October 2002, Castle Technology released the Acorn clone Iyonix PC. This ran a 32-bit (in contrast to 26-bit) variant of RISC OS, named RISC OS 5. RISC OS 5 is a separate evolution of RISC OS based upon the NCOS work done by Pace. The following year, Castle Technology bought RISC OS from Pace for an undisclosed sum. In October 2006, Castle announced a shared source license plan, managed by RISC OS Open Limited, for elements of RISC OS 5.

In October 2018, RISC OS 5 was re-licensed under the Apache 2.0 license.{{Cite news|url=https://www.theregister.com/2018/10/23/risc_os_open_source/|title=Roughly 30 years after its birth at UK's Acorn Computers, RISC OS 5 is going open source|first=Richard|last=Speed|work=The Register|access-date=26 October 2021|archive-date=23 September 2021|archive-url=https://web.archive.org/web/20210923185001/https://www.theregister.com/2018/10/23/risc_os_open_source/|url-status=live}}

In 2018 RISC OS Developments acquired Castle Technology Ltd including its intellectual property.{{cite web |url=https://www.riscosdev.com/ |title=Home |website=riscosdev.com}}

In December 2020, the source code of RISC OS 3.71 was leaked to The Pirate Bay.{{cn|date=May 2022}}

{{clear}}

The OS is single-user and employs cooperative multitasking (CMT).{{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 |date=December 1996 |volume=21 |issue=12 |page=49 |oclc=208951251 |issn=0360-5280}} While most current desktop OSes use preemptive multitasking (PMT) and multithreading, {{nowrap|RISC OS}} remains with a CMT system. By 2003, many users had called for the OS to migrate to PMT.{{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}} The OS memory protection is not comprehensive.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 }}{{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}}

The core of the OS is stored in 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 modules. 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 calls in other operating systems.

Most of the OS has defined application binary interfaces (ABIs) to handle filters and vectors. The OS provides many ways in which a program can intercept and modify its operation. This simplifies the task of modifying its behaviour, either in the GUI, or deeper. As a result, there are several third-party programs which allow customising the OS look and feel.

{{More citations needed section|date=July 2017}}

The file system is volume-oriented: the top level of the file hierarchy is a volume (disc, network share) prefixed by the file system type. To determine file type, the OS uses metadata instead of file extensions. Colons are used to separate the file system from the rest of the path; the root is represented by a dollar ($) sign and directories are separated by a full stop (.). Extensions from foreign file systems are shown using a slash (example.txt becomes example/txt).{{cite web |url=http://www.drobe.co.uk/article.php?id=921 |title=RISC OS filename translation |work=Drobe |date=2 January 2004 |access-date=25 April 2012 |last=Naulls |first=Peter |archive-date=9 July 2011 |archive-url=https://web.archive.org/web/20110709035738/http://www.drobe.co.uk/article.php?id=921 |url-status=live }} For example, ADFS::HardDisc4.$ is the root of the disc named HardDisc4 using the Advanced Disc Filing System (ADFS) file system. {{nowrap|RISC OS}} filetypes can be preserved on other systems by appending the hexadecimal type as ',xxx' to filenames.{{cite web|url=http://www.osnews.com/story/6170/Emulating_RISC_OS_under_Windows/page3/|title=Emulating RISC OS, Page 3/3|website=OSNews|access-date=25 April 2012|archive-date=13 March 2012|archive-url=https://web.archive.org/web/20120313042944/http://www.osnews.com/story/6170/Emulating_RISC_OS_under_Windows/page3/|url-status=live}} When using cross-platform software, filetypes can be invoked on other systems by naming appending '/[extension]' to the filename under {{nowrap|RISC OS}}.{{citation |last=Fitton |first=Gerald |date=August 1994 |title=Fireworkz for Windows |periodical=Archive |publisher=Abbey Press |location=Glastonbury |volume=7 |issue=11 |page=21 |url=http://downloads.abacusline.me.uk/pdf/Arc/2007/GC0701.pdf |oclc=222434223 |access-date=25 April 2012 |archive-date=3 December 2013 |archive-url=https://web.archive.org/web/20131203024536/http://downloads.abacusline.me.uk/pdf/Arc/2007/GC0701.pdf |url-status=live }}

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.

The file system abstraction layer API uses 32-bit file offsets, making the largest single file 4 GiB (minus 1 byte) long. However, prior to RISC OS 5.20 the file system abstraction layer and many RISC OS-native file systems limited support to 31 bits (just under 2 GiB) to avoid dealing with apparently negative file extents when expressed in two's complement notation.{{citation needed|date=July 2017}}

{{Main|List of RISC OS filetypes}}

The OS uses metadata to distinguish file formats. Some common file formats from other systems are mapped to filetypes by the MimeMap module.{{cite web|url=http://www.iyonix.com/32bit/MIMEMap.shtml|title=Iyonix: MIMEMap|work=iyonix.com|url-status=dead|archive-url=https://web.archive.org/web/20160303223854/http://www.iyonix.com/32bit/MIMEMap.shtml|archive-date=3 March 2016}}

The RISC OS kernel is single-tasking and controls handling of interrupts, DMA services, memory allocation and the video display; the cooperative multi-tasking is provided by the WindowManager module.

File:RISCOS 4 scr.png

The WIMP interface is based on a stacking window manager and incorporates three mouse buttons{{cite book|last=Ryan|first=Dan|title=History of Computer Graphics: DLR Associates Series|url=https://books.google.com/books?id=136myWlKpwEC&pg=PA358|access-date=13 June 2013|date=13 April 2011|publisher=AuthorHouse|isbn=978-1-4567-5115-9|page=358|archive-date=6 July 2014|archive-url=https://web.archive.org/web/20140706155034/http://books.google.com/books?id=136myWlKpwEC&pg=PA358|url-status=live}} (named Select, Menu and Adjust), context-sensitive menus, window order control (i.e. send to back) and dynamic window focus (a window can have input focus at any position on the stack). The icon bar (Dock) holds icons which represent mounted disc drives, RAM discs, running applications, system utilities and docked: files, directories or inactive applications. These icons have context-sensitive menus and support drag-and-drop operation. They represent the running application as a whole, irrespective of whether it has open windows.

The GUI functions on the concept of files. The Filer, a spatial file manager, displays the contents of a disc. Applications are run from the Filer view and files can be dragged to the Filer view from applications to perform saves, rather than opening a separate 'Save' dialog box where the user must navigate to a location already visible in the Finder. In addition, files can be directly transferred between applications by dragging a save icon into another application's window.

Application directories are used to store applications. The OS differentiates them from normal directories through the use of an exclamation mark (also called a pling or shriek) prefix. Double-clicking on such a directory launches the application rather than opening the directory. The application's executable files and resources are contained within the directory, but normally they remain hidden from the user. Because applications are self-contained, this allows drag-and-drop installing and removing.

The {{nowrap|RISC OS}} Style Guide encourages a consistent look and feel across applications. This was introduced in {{nowrap|RISC OS 3}} and specifies application appearance and behaviour. Acorn's own main bundled applications were not updated to comply with the guide until {{nowraplinks|RISCOS Ltd}}'s Select release in 2001.{{cite web |url=http://www.iconbar.com/articles/Influential/index1152.html |title=An arbitrary number of possibly influential RISC OS things |work=The Icon Bar |date=23 March 2007 |access-date=27 September 2011 |last=Phil |first=Mellor |quote=Admittedly it wasn't until RISC OS Select was released, almost 10 years later, that the standard Acorn applications (Draw, Edit, and Paint) implemented the style guide's clipboard recommendations, but most products followed it with care. |archive-date=31 March 2012 |archive-url=https://web.archive.org/web/20120331173614/http://www.iconbar.com/articles/Influential/index1152.html |url-status=live }}

RISC OS was the first operating system to provide scalable anti-aliased fonts.{{cite web |url=http://www.osnews.com/story/6170/Emulating_RISC_OS_under_Windows/page1/ |title=Emulating RISC OS under Windows |access-date=12 May 2011 |last=Round |first=Mark |date=26 February 2004 |work=OSnews |quote=Many of the UI concepts that we take for granted were first pioneered in RISC OS, for instance: scalable anti-aliased fonts and an operating system extendable by 'modules', while most of the PC world was still on Windows 3.0. |archive-date=14 November 2010 |archive-url=https://web.archive.org/web/20101114004250/http://www.osnews.com/story/6170/Emulating_RISC_OS_under_Windows/page1/ |url-status=live }}{{cite web |url=http://telcontar.net/Misc/GUI/RISCOS/ |title=The RISC OS GUI |access-date=12 May 2011 |last=Ghiraddje |date=22 December 2009 |publisher=Telcontar.net |quote=Only with Mac OS X did any mainstream graphical interface provide the smoothly rendered, fractionally spaced type that Acorn accomplished in 1992 or earlier. |archive-date=16 July 2011 |archive-url=https://web.archive.org/web/20110716233538/http://telcontar.net/Misc/GUI/RISCOS/ |url-status=live }}{{cite web |url=https://arstechnica.com/old/content/2005/05/gui.ars/6 |title=A History of the GUI |access-date=25 May 2011 |first=Jeremy |last=Reimer |date=May 2005 |publisher=ArsTechnica |quote=[...] in 1987, the UK-based company Acorn Computers introduced their [...] GUI, called "Arthur", also was the first to feature anti-aliased display of on-screen fonts, even in 16-color mode! |archive-date=2 December 2011 |archive-url=https://web.archive.org/web/20111202061840/http://arstechnica.com/old/content/2005/05/gui.ars/6 |url-status=live }}{{cite web |url=http://www.osnews.com/story/21713/Screen_Fonts_Shape_Accuracy_or_On-Screen_Readability_ |title=Screen Fonts: Shape Accuracy or On-Screen Readability? |access-date=13 June 2011 |first=Thom |last=Holwerda |date=23 June 2005 |publisher=OSNews |quote=[...] it was RISC OS that had the first system-wide, intricate [...] font rendering in operating systems. |archive-date=19 November 2011 |archive-url=https://web.archive.org/web/20111119223926/http://www.osnews.com/story/21713/Screen_Fonts_Shape_Accuracy_or_On-Screen_Readability_ |url-status=live }} Anti-aliased fonts were already familiar from Arthur, and their presence in RISC OS was confirmed in an early 1989 preview,{{cite news | first1=Dick | last1=Pountain | title=Screentest: Archie RISC OS | work=Personal Computer World | date=January 1989 | url=http://chrisacorns.computinghistory.org.uk/docs/Mags/PCW/PCW_Jan89_RISCOS.pdf | page=154 | access-date=4 March 2021 | quote=[ArcDraw] can also add text in multiple sizes and fonts to a drawing (including anti-aliased fonts) | archive-date=13 June 2021 | archive-url=https://web.archive.org/web/20210613044349/http://chrisacorns.computinghistory.org.uk/docs/Mags/PCW/PCW_Jan89_RISCOS.pdf | url-status=live }} featuring in the final RISC OS 2 product, launched in April 1989.{{ cite news | url=https://archive.org/details/AcornUser079-Feb89/page/n10/mode/1up | title=RISC OS is ready for April launch | work=Acorn User | date=February 1989 | access-date=31 October 2020 | pages=9 }}

A new version of the font manager employing "new-style outline fonts" was made available after the release of RISC OS,{{ cite news | url=https://archive.org/details/AcornUser081-Apr89/page/n8/mode/1up | title=New fonts for DTP | work=Acorn User | date=April 1989 | access-date=3 May 2021 | pages=7 }} offering full support for the printing of scalable fonts, and was provided with Acorn Desktop Publisher.{{ cite news | url=https://archive.org/details/AcornUser087-Oct89/page/n135/mode/2up | title=Hold the Font Page | work=Acorn User | date=October 1989 | access-date=14 April 2021 | last1=Bell | first1=Graham | pages=134–135 }} It was also made available separately and bundled with other applications.{{ cite news | url=https://archive.org/details/AcornUser093-Apr90/page/n20/mode/1up | title=Acorn Customer Hotline | work=Acorn User | date=April 1990 | access-date=2 May 2021 | pages=19 }} This outline font manager provides support for the rendering of font outlines to bitmaps for screen and printer use, employing anti-aliasing for on-screen fonts, utilising sub-pixel anti-aliasing and caching for small font sizes.{{ cite conference | url=https://archive.org/details/1989-proceedings-5th-comp-graphics-workshop/page/25/mode/1up | title=The Acorn Outline Font Manager | last1=Raine | first1=Neil | last2=Seal | first2=David | last3=Stoye | first3=William | last4=Wilson | first4=Roger | conference=Fifth Computer Graphics Workshop | location=Monterey, California | date=November 1989 | publisher=USENIX Association | pages=25–36 }} At the time of the introduction of Acorn's outline font manager, the developers of rival desktop systems were either contemplating or promising outline font support for still-unreleased products such as Macintosh System 7 and OS/2 version 2.{{ cite news | url=https://archive.org/details/AcornUser088-Nov89/page/n69/mode/2up | title=My Type | work=Acorn User | date=November 1989 | access-date=7 May 2021 | last1=Bell | first1=Graham | pages=68–69 }}

From 1993, starting with the German RISC OS 3.12, and in 1994 for RISC OS 3.5, it has been possible to use an outline anti-aliased font in the WindowManager for UI elements, rather than the bitmap system font from previous versions.{{cite magazine

| date = July 1993

| title = Acorn signs pre-press deal

| url = https://archive.org/details/AcornUser132-Jul93/page/n11/mode/2up

| magazine = Acorn User

| page = 10

| publisher = Addison-Wesley

| access-date =

}}{{ cite tech report | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/AN/253.pdf | title=Acorn Computers Support Group Application Notice 253 – New features of RISC OS version 3.5 | publisher=Acorn Computers Limited | last1=Murphy | first1=Dean | date=29 March 1994 | access-date=2 March 2022 | issue=0.02 }} RISC OS 4 does not support Unicode but "RISC OS 5 provides a Unicode Font Manager which is able to display Unicode characters and accept text in UTF-8, UTF-16 and UTF-32. Other parts of the RISC OS kernel and core modules support text described in UTF-8."{{cite web|url=http://www.riscos.info/index.php/Unicode_in_RISC_OS|title=Unicode in RISC OS|work=riscos.info|access-date=28 April 2015|archive-date=11 April 2015|archive-url=https://web.archive.org/web/20150411221409/http://www.riscos.info/index.php/Unicode_in_RISC_OS|url-status=live}}

Support for the characters of RISC OS (and some other historic computers) was added to Unicode 13.0 (in 2020).{{Cite web|url=https://www.unicode.org/versions/Unicode13.0.0/ch22.pdf|title=The Unicode® Standard Version 13.0 – Core Specification|access-date=11 March 2020|archive-date=6 May 2021|archive-url=https://web.archive.org/web/20210506122306/https://www.unicode.org/versions/Unicode13.0.0/ch22.pdf|url-status=live}}

{{Main|List of RISC OS bundled applications}}

{{nowrap|RISC OS}} is available in several distributions, all of which include a small standard set of desktop applications, but some of which also include a much wider set of useful programs. Some of those richer distributions are freely available, some are paid for.

Limited software portability exists with subsequent versions of the OS and hardware. Single-tasking BBC BASIC applications often require only trivial changes, if any.{{citation needed|date=June 2012}} Successive OS upgrades have raised more serious issues of backward compatibility for desktop applications and games.{{cite web|title=ARM architecture versions cheat sheet|url=http://www.starfighter.acornarcade.com/mysite/articles/armv.html|access-date=14 January 2015|archive-date=12 April 2015|archive-url=https://web.archive.org/web/20150412223011/http://www.starfighter.acornarcade.com/mysite/articles/armv.html|url-status=live}} Applications still being maintained by their author(s) or others have sometimes historically been amended to provide compatibility.{{citation needed|date=June 2012}}

The introduction of the RiscPC in 1994 and its later StrongARM upgrade raised issues of incompatible code sequences and proprietary squeezing (data compression). Patching of applications for the StrongARM was facilitated and Acorn's UnsqueezeAIF software unsqueezed images according to their AIF header.{{cite web|title=Application Note 295|url=http://acorn.riscos.com/riscos3/37/37DiscImage/Manuals/AppNote|work=Engineering Support Application Note|publisher=Acorn Computers|access-date=28 June 2012|date=4 October 1996|url-status=dead|archive-url=https://archive.today/20130104150342/http://acorn.riscos.com/riscos3/37/37DiscImage/Manuals/AppNote|archive-date=4 January 2013}} The incompatibilities prompted release by The ARM Club of its Game On!{{cite news|author=Foggy|title=Games FAQ|url=http://acornarcade.com/articles/Games_FAQ/index1004.html|access-date=27 June 2012|newspaper=Acorn Arcade|date=11 July 2001|archive-date=21 March 2012|archive-url=https://web.archive.org/web/20120321065412/http://www.acornarcade.com/articles/Games_FAQ/index1004.html|url-status=live}}{{cite web|title=Game On!|last=Ruck|first=David J.|website=The ARM Club: Software Products|publisher=David J. Ruck|url=http://www.armclub.org.uk/products/gameon/|archive-url=https://web.archive.org/web/20011004235759/http://www.armclub.org.uk/products/gameon/|archive-date=4 October 2001|access-date=27 June 2012}} and StrongGuard software.{{cite web|title=StrongGuard|url=http://www.armclub.org.uk/products/strongguard/|last=Ruck|first=David J.|website=The ARM Club: Software Products|publisher=David J. Ruck|archive-url=https://web.archive.org/web/20090212202624/http://www.armclub.org.uk/products/strongguard/|archive-date=12 February 2009|access-date=27 June 2012}}{{cite news|last=McKillop|first=Keith|title=RISC OS 2000: Show Report|url=http://acornarcade.com/RISC_OS_2000_Show_Report/news971.html|access-date=27 June 2012|newspaper=Acorn Arcade|date=22 November 2000|archive-date=16 October 2015|archive-url=https://web.archive.org/web/20151016230129/http://acornarcade.com/RISC_OS_2000_Show_Report/news971.html|url-status=live}} They allowed some formerly incompatible software to run on new and upgraded systems. The version of the OS for the A9home prevented the running of software without an AIF header (in accord with Application Note 295){{cite web|url=http://www.riscos.com/news/news_items/NewsMay2006.pdf|archive-url=https://www.webcitation.org/6BwhJRm1b?url=http://www.riscos.com/news/news_items/NewsMay2006.pdf|url-status=dead|archive-date=5 November 2012|title=RISCOS Ltd News|publisher=RISCOS Ltd|access-date=28 June 2012|date=May 2006}} to stop "trashing the desktop".{{cite news|last=Williams|first=Chris|title=ROL release C99 SCL to A9home users|url=http://www.drobe.co.uk/article.php?id=1580|access-date=28 June 2012|newspaper=Drobe|date=17 April 2006|archive-date=1 October 2009|archive-url=https://web.archive.org/web/20091001162532/http://www.drobe.co.uk/article.php?id=1580|url-status=live}}

The Iyonix PC ({{nowrap|RISC OS 5}}) and A9home (custom {{nowrap|RISC OS 4}}) saw further software incompatibility because of the deprecated {{nowrap|26-bit}} addressing modes. Most applications under active development have since been rewritten.{{cite news|last=Thompson|first=Matt|title=Iyonix Issues|url=http://www.apdl.co.uk/riscworld/volume7/issue2/iyonix/|access-date=27 June 2012|volume=7|issue=2|newspaper=RISC World}}{{cite news|last=Thompson|first=Matt|title=Iyonix Issues|url=http://www.apdl.co.uk/riscworld/volume8/issue2/iyonix/|access-date=27 June 2012|volume=8|issue=2|newspaper=RISC World}}{{cite web|title=A Guide to RISC OS Versions|url=http://www.wrocc.org.uk/riscos/versions.shtml|work=WROCC website|publisher=Wakefield RISC OS Computer Club|access-date=27 June 2012|archive-date=7 January 2013|archive-url=https://web.archive.org/web/20130107011934/http://www.wrocc.org.uk/riscos/versions.shtml|url-status=live}} Static code analysis to detect {{nowrap|26-bit}}-only sequences can be undertaken using ARMalyser.{{cite news|last=Brett|first=Paul|title=PD World|url=http://www.apdl.co.uk/riscworld/volume7/issue1/pd/|access-date=27 June 2012|newspaper=RISC World|volume=7|issue=1}} Its output can be helpful in making 32-bit versions of older applications for which the source code is unavailable.{{cite news|last=Brett|first=Paul|title=PD World|url=http://www.apdl.co.uk/riscworld/volume3/issue1/pd/|access-date=27 June 2012|newspaper=RISC World|volume=3|issue=1}}{{cite web|title=Free Software|url=http://www.armclub.org.uk/free/|last=Ruck|first=David J.|website=The ARM Club|publisher=David J. Ruck|access-date=27 June 2012|archive-date=26 June 2012|archive-url=https://web.archive.org/web/20120626180450/http://www.armclub.org.uk/free/|url-status=live}} Some older 26-bit software can be run without modification using the Aemulor emulator.

Additional incompatibilities were introduced with newer ARM cores, such as ARMv7 in the BeagleBoard and ARMv8 in the {{nowrap|Raspberry Pi 3}}. This includes changes to unaligned memory access in ARMv6/v7 and removal of the SWP instructions in ARMv8.{{Cite web |url=https://www.riscosopen.org/wiki/documentation/show/ARMv7%20compatibility%20primer#TOC3.11 |title=ARMv7 compatibility primer in Documentation |access-date=16 October 2021 |archive-date=16 October 2021 |archive-url=https://web.archive.org/web/20211016142701/https://www.riscosopen.org/wiki/documentation/show/ARMv7%20compatibility%20primer#TOC3.11 |url-status=live }}

RISC OS has also been used by both Acorn and Pace Micro Technology in various TV connected set-top boxes, sometimes referred to instead as NCOS.{{Citation needed|date=December 2024}} A special cut down RISC OS Pico (for 16MiB cards and larger) styled to start up like a BBC Micro was released for BASIC's 50th anniversary.{{cite web |date=5 January 2014 |title=Happy birthday, BASIC |url=https://www.riscosopen.org/news/articles/2014/05/01/happy-birthday-basic |url-status=live |archive-url=https://web.archive.org/web/20181109020010/https://www.riscosopen.org/news/articles/2014/05/01/happy-birthday-basic |archive-date=9 November 2018 |access-date=8 May 2018}} SD card images have been released for downloading free of charge to Raspberry Pi 1, 2, 3, & 4 users with a full graphical user interface (GUI) version{{cite web |title=Downloads |url=http://www.raspberrypi.org/downloads |url-status=live |archive-url=https://web.archive.org/web/20130114192342/http://www.raspberrypi.org/downloads |archive-date=14 January 2013 |access-date=2 September 2013 |publisher=Raspberry Pi}} and a command-line interface only version (RISC OS Pico, at 3.8 MB).{{cite web |title=RISC OS Open: Raspberry Pi |url=https://www.riscosopen.org/content/downloads/raspberry-pi |url-status=live |archive-url=https://web.archive.org/web/20210320161349/https://www.riscosopen.org/content/downloads/raspberry-pi |archive-date=20 March 2021 |access-date=25 April 2015 |work=riscosopen.org}}

Versions of RISC OS run or have run on the following hardware: (RISC OS Open Limited adopted{{cite web |date=12 January 2017 |title=Stable release criteria |url=https://www.riscosopen.org/content/documents/stable-releases |url-status=live |archive-url=https://web.archive.org/web/20181109020018/https://www.riscosopen.org/content/documents/stable-releases |archive-date=9 November 2018 |access-date=12 January 2017}} the 'even numbers are stable' version numbering scheme post version 5.14, hence some table entries above include two latest releases – the last stable one and the more recent development one.)

RISC OS can also run on a range of computer system emulators that emulate the earlier Acorn machines listed above:

• Acorn C/C++
• ArtWorks
• Drobe
• ROX Desktop, a graphical desktop environment for the X Window System, inspired by the user interface of RISC OS
• Sibelius (scorewriter), originally an application for RISC OS, rewritten for Windows in 1998
• RISC OS character set

{{Reflist|30em|refs=

{{cite news|last=Williams|first=Chris|title=Aemulor: Number of apps working on Iyonix 'growing daily'|url=http://www.drobe.co.uk/article.php?id=488|access-date=21 June 2012|newspaper=Drobe|date=19 November 2002|archive-date=2 February 2014|archive-url=https://web.archive.org/web/20140202124533/http://www.drobe.co.uk/article.php?id=488|url-status=live}}

{{cite news|last=Williams|first=Chris|title=Aemulor sees the light of day|url=http://www.drobe.co.uk/riscos/artifact602.html|access-date=27 June 2012|newspaper=Drobe|date=25 March 2003|archive-date=2 February 2014|archive-url=https://web.archive.org/web/20140202131856/http://www.drobe.co.uk/riscos/artifact602.html|url-status=live}}

}}

• {{Official website|www.riscosopen.org/content}} RISC OS Open

{{RISC OS|state=expanded}}

{{Acorn computers, clones and compatibles}}

{{Operating system}}

Category:Acorn Computers operating systems

Category:ARM operating systems

Category:Desktop environments

Category:Free software operating systems

Category:Software using the Apache license

Category:Window-based operating systems

Category:1987 software