Editing RISC OS (section)

== Backward compatibility ==
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.<ref>{{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}}</ref> 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]]). [[Patch (computing)|Patch]]ing of applications for the StrongARM was facilitated and Acorn's ''UnsqueezeAIF'' software unsqueezed images according to their [[AIF header]].<ref name="riscos an 295 1996">{{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}}</ref> The incompatibilities prompted release by ''The ARM Club'' of its ''Game On!''<ref name="acornarcade foggy 2001">{{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}}</ref><ref name="armclub 2001">{{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}}</ref> and ''StrongGuard'' software.<ref name="acornarcade foggy 2001" /><ref name="armclub 2009">{{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}}</ref><ref name="acornarcade mckillop 2000">{{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}}</ref> 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)<ref name="riscos 2006">{{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}}</ref> to stop "trashing the desktop".<ref name="drobe williams 2006">{{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}}</ref>

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 computing|26-bit]]}} addressing modes. Most applications under active development have since been rewritten.<ref name="riscworld thompson v7i2">{{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}}</ref><ref name="riscworld thompson v8i2">{{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}}</ref><ref name="wrocc versions">{{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}}</ref> [[Static code analysis]] to detect {{nowrap|26-bit}}-only sequences can be undertaken using ''ARMalyser''.<ref name="riscworld brett v7i1">{{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}}</ref> Its output can be helpful in making 32-bit versions of older applications for which the source code is unavailable.<ref name="riscworld brett v3i1">{{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}}</ref><ref name="riscworld brett v7i1" /><ref name=armclub>{{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}}</ref> Some older 26-bit software can be run without modification using the [[Aemulor]] emulator.<ref name="wrocc versions" /><ref name="drobe williams 2002" /><ref name="drobe williams 2003" />

Additional incompatibilities were introduced with newer [[ARM core]]s, 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.<ref>{{Cite web |url=https://www.riscosopen.org/wiki/documentation/show/ARMv7%20compatibility%20primer#TOC3.11 |title=ARMv7 compatibility primer in Documentation<!-- Bot generated title --> |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 }}</ref>