The most abundant Operating System for the Acorn Archimedes ARM-based computer systems, it was (like the Archimedes itself) technologically far more advanced that its contemporaries when it was released, but fell behind by the mid 90s. It generally runs from ROM, and uses a minimal amount of RAM, and pioneered many advanced features for which its equal still hasn't been found today.

The lineage of RISC OS (pronounced 'Risc OS', but written 'RISC OS') goes all the way back to Acorn MOS and the BBC Micro. The BBC's innovative sideways RAM/sideways ROM system, while primarily designed to extend the RAM available to the BBC's 6502 processor, had the useful side-effect of creating a highly modular operating system (MOS had to be able to deal with whatever ROMs the user decided to plug in, after all!). A standard set of OS interfaces available though BBC Basic abstracted the hardware and the OS from applications: OSCLI, OS_Byte, OS_Word and the abstract character-based VDU driver call.

In designing the Operating System for the new Archimedes machines, one of the most desirable features was backwards compatability: there was an enormous base of educational software available for the BBC Micro, and they wanted to leverage as much of this as possible. RISC OS's stopgap progenitor, Arthur ("A RISC OS Before THURsday"), was based around the same kernel interfaces as MOS, which were included as a subset of the Arthur kernel interface; all well-defined abstract entry points defined by the ARM SWI instruction. Since most BBC Micro applications were written primarily in BASIC and therefore used the abstract interfaces by default, many BBC programs would run on Arthur with little or no conversion at all. For those that needed 6502 code, Acorn supplied an emulator, 65Host, which mimicked the BBC Micro model B hardware almost exactly, though more slowly. It's rare to find a program that will run on a B but not on 65Host.

The sideways ROM concept was generalised to that of a Relocatable Module, much like a dynamically linked library on other platforms, with naming services and interfaces managed by the kernel. Almost all components of the operating system were implemented as modules, including the filesystems, the editor, windowing system, etc. Modules could be part of the RISC OS ROM, reside in ROM on expansion cards (or "podules" as they were known) or be loaded into RAM. In this sense it was a true microkernel OS.

Areas where Arthur was weak were in memory management and process model, and in UI design. The OS was strictly single-tasking, only one 'process' existed at a time, and the whole address space was visible to that process. The windowing system, while clean and well-designed, meant that each application had to build its entire graphical operating environment from scratch, since it was the only process on the machine: file access, clocks, calendars, notepads etc.

The first proper release of RISC OS, which was given the release number 2.00 to reflect its descent from Arthur, knocked most of these problems out of the way in one fell swoop, and was such a reliable and stable platfom that the basic operating system had only one tiny incremental release (purely to add support for the faster ARM 3 processor) in the four years between the initial release and the release of RISC OS 3.

The windowing system (know as WindowManager or simply "The WIMP"), with some extra kernel support, grew the ability to manage multiple protected memory address spaces. The Arthur Wimp SWI interface was extended to allow context switching on event requests, and an inter-process message-passing protocol was provided, giving a safe and robust cooperative multi-tasking environment. Modular applications managed many of the mundane tasks that an application writer would have to have done under Arthur: a unified Filer module handled user-level file management, and communicated with applications via the message passing protocol to tell them where to load or save files from.

Also included was an anti-aliased outline font subsystem, which provided vector fonts, anti-aliased (in a very efficient manner, too) with cached bitmaps. To this day, the only other operating system that seems to come close to a font system as rational and aesthetically pleasing as RISC OS's is BeOS.

The next major revision of RISC OS, version 3 (generally speaking, 3.1; version 3.0 was specific to the A5000) saw some of the standard applications (text, bitmap and vector editors, alarm clock and system configuration GUI) moved into ROM, which was a huge convenience booster for those of us who lived without hard disc drives, as well as providing some interesting graphical features (such as the ability to use customised toosplrites for window components; what the rest of the world now calls 'skins'), and the ability to read/write DOS and Atari format floppy discs.

Also added with RISC OS 3 was Acorn Replay, a full-motion video framework and kernel set of decoders and renderers, which got surprisingly good quality reproduction of MPEG movies even on the likes of an 8MHz A3000. Needless to say, much extreme cleverness was involved, the inner kernels being written by those most familiar with the hardware involved, in typical Acorn fashion.

Despite these improvements, RISC OS 3 still retained the major weaknesses of RISC OS 2. While memory protection between applications was well-supported, crucial kernel areas were left entirely unprotected. This includes the ARM vector table; a sure-fire way to kill any RISC OS machine is to go to BASIC (hit F12 and type BASIC) and issue the command
This amusing command reassigns the Fast Interrupt Request handler to a no-op, and immediately kills the machine next time an interrupt occurs.

Other weaknesses are the lack of pre-emptive multitasking, virtual memory filesystem caching or asynchronous I/O: these three combined can make disc access painful and tedious. This is, however, an advantage in embedded applications where real time constraints might forbid page faults or swapping, and indeed RISC OS lives on as an embedded OS in Pace set-top boxes, and some low-cost internet TVs.

Following Acorn's demise, further RISC OS development is controlled by RISC OS Ltd, with the release of RISC OS 4 in 2000 introducing some concessions towards vitrual memory amongst other minor improvements.

Information all taken from my pre-existing knowledge of RISC OS. No NDAs or proprietary knowledge were harmed in the production of this writeup.

Current State of RISC OS

Risc OS is a British operating system (OS) owned by Castle Technology that was and is way ahead of its time. Designed to be run from ROM the system is utterly incorruptable by anything short of reprogramming with a pick axe. Choices (eg, backdrops/wallpaper) are saved to disc and changes to the OS (such as Patches) can be stored in Flash Memory so there is no loss of control. Total start up time on my 1999 Risc PC, from pressing the on button to loading up your browser, is just under 16 seconds according to my watch, and that's booting to a 1280x1024 screen mode with a full sized bitmap background, total custom GUI Theme overhaul and mp3 player churning out OMD.

Risc OS is a fully up to date system at the time of writing (Latest new version 2004, Castle are currently consulting on a new version codenamed Merlin). On it's native Risc machines Risc OS takes up virtually no RAM, the access time is measured in nanoseconds, not milliseconds from a hard drive and, in use, everything from the operating system is instant. When a major upgrade comes along you can eaither just pop the old ROMs out and push the new ones in or if you prefer, you can boot all 4mb of OS from your Hard-Drive/CD-ROM. The 4mb of ROM contains the OS, basic apps such as a text editor, bitmap editor, vector editor etc. A Risc OS machine without a hard drive is still a useable computer, useful if your hard drive smegs up but admittedly not a desirable state of affairs.

Risc OS software is pretty impressive. All the industry standard filetypes are supported by the major applications, so there isn't a problem there. For some reason (whether it's something to do with Risc itself or just good coding I don't know) all the apps are a hell of a lot smaller than the Windoze versions. The major document editor takes up around 0.5 meg of your hard drive and only around 600k ram, with the benifit that you can fit the equivilent of MS Works onto a single 1.6Mb floppy (those are the same floppies windows decides to format to 1.44Mb). Windows users usually refuse to belive their eyes, but then I suppose Risc OS apps don't have flight simulators hidden in them :-).

Risc OS dedicated machines such as the Kinetic or the new Iyonix Panther (produced by Castle Tech) can run other operating systems. Linux runs brilliantly, no problems there. So does BSD. Mac OS can't be installed for obvious reasions (more's the pitty). Windows/DOS does run, though it's a tad slow. My (admittedly venerable) Risc PC when running Windows 98 is fine for MS Office 2000 or playing Duke Nukem 3D, but don't try to run any of the latest games via windows. The up side of running Windows/DOS ontop of Risc OS (Risc OS taking the place of a hyper-flexable BIOS and back-up OS) is that it can run Windows inside a window on a Risc OS desktop/GUI with the DOS drives as disc images on your hard drive, which can be treated as standard directories/folders. This way I can back up Drive C (dedicated solely to the Windows OS) by simply copying the disc image as easily as if it were a Word document. In the event of a major disaster rendering Windows unusable, system files can be edited and documents recovered via RISC OS without even being deprived of your MP3 collection.

Risc OS can now be run in a similar way on Windows or Linux thanks to the 'Virtual Acorn' software by Graeme Barnes of 3QD. The latest version of Virtual Acorn, VRPC-Adjust, is a nigh on perfect emulation of a Risc PC running Risc OS Adjust. The emulation package effectively allows Risc OS to access the full resources of your PC letting you take advantage of Risc OS's modern and powerful graphics and DTP software packages. Of course this still requires you to load Windows/Linux as a host OS. As both these Operating systems are significantly larger than Risc OS's 4MB this really boosts the "Switch-On to Work" lag, though once booted Risc OS's ease of use and streamlined apps do give a significant advantage over relying on the huge code-mountains that pass for apps on Windows. Several companies have taken advantage of these factors to produce so-called hybrid machines such as the A6+ from Advantage 6 which are designed specificaly to run RISC OS on x86 based hardware via emulation.

Risc OS dedicated machines such as the Iyonix from Castle Tech do, as stated, run the OS from Rom. The BIG advantage of this is that it never needs to be reinstaled due to file corruption, disc errors etc. A windows user that I know asked me why I prefered Risc OS. I replied by asking him how many times he had reinstalled windows in the past year. He couldn't remember. I asked him how many times I'd had to reinstall Risc OS in the past ten. Think about it.

RISC OS Related Nodes


Computer Models

Hardware Related

Software Related


If you are aware of (or create) any further writeups related to RISC OS please let me know so I can link to them here.

For Screenshots of RISC OS see

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