RISC OS

RISC OS

A screenshot of RISC OS 4
Developer	RISCOS Ltd (6.xx), Castle Technology Ltd / RISC OS Open Ltd (5.xx)
Working state	Current
Source model	Proprietary (6.xx), 'Shared Source' (5.xx)
Latest release	6.20 or 5.16 / December, 2009 / January 23, 2010
Platforms	ARM
License	Proprietary with Castle Shared Source License
Official website	www.riscos.com

This article is about the operating system created by Acorn Computers. For the MIPS Magnum operating system, see RISC/os.

RISC OS (Template:Pron-en) is a computer operating system which was originally developed by Acorn Computers Ltd in Cambridge, England for their range of desktop computers, based on the ARM chipset. It was first released in 1987 under the name Arthur, in 1988 with the addition of cooperative multitasking it was renamed as RISC OS 2.00. The operating system takes its name from the RISC (Reduced Instruction Set Computing) architecture used on supported systems.

Since 1988, RISC OS has been bundled with nearly every Acorn computer model, including the Archimedes range of computers, and RiscPC and A7000 computers. After the breakup of Acorn in 1998, development of RISC OS was continued by several companies, including RISCOS Ltd and Castle Technology Ltd. Also since 1998, RISC OS has been bundled with a number of ARM based Acorn Clone personal computers such as the Iyonix and A9home.

Features

OS core

• Stored in ROM – This gives a fast bootup time and safety from operating system corruption. RISC OS 4 and 5 are stored in 4 MB of Flash ROM, allowing the operating system to be updated without having to replace the ROM chip.
• Module-based – The operating system is made up of a number of modules. These can be added 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 its version of RISC 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.
• Single user, co-operative multitasking – While most current desktop operating systems use pre-emptive multitasking (PMT) and multithreading, RISC OS remains with a co-operative multitasking system. Many users have called for the OS to migrate to PMT. The OS also has only rudimentary memory protection, and all users have full superuser privileges.
• Volume-oriented filesystem – The top level of the file hierarchy is a volume (disc, network share) prefixed by the filesystem type. The OS uses metadata to determine file type; file extensions are not used. Colons are used to separate the filesystem from the rest of the path; the root is represented by a dollar ($) sign and directories by a period (.). Extensions from foreign filesystems are shown using a forward slash ('example.txt' becomes 'example/txt'). For example, ADFS::HardDisc4.$. is the root of the disc named HardDisc4 using the ADFS filesystem.
• Image filing systems – A filesystem can present a file of a particular type as a volume in its own right – similar to an automatic loopback device. This allows transparent handling of archives and similar files – they just appear like a directory with some special properties, and files inside the archive (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 filesystems simply put a reference inside the image file and go elsewhere for the data.
• Filters and vectors – Almost everything in RISC OS has a defined ABI. The OS provides many ways the programmer can intercept and modify operation of the OS, which makes it simple to write modules that modify OS behaviour – either in the GUI or deeper. Through this mechanism there is a multitude of third-party programs which allow customising of the OS look and feel.

The Desktop

• Intuitive window manager – Launched during the time of Windows 2.0 and Mac OS System 6, the RISC OS WIMP incorporates three-buttoned mouse operation (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).
• Icon bar – Similar to the NextStep/Mac OS X dock and the Windows taskbar. The bar holds icons which represent mounted disc drives and RAM discs, running applications and system utilities. These icons have their own context-sensitive menus and support drag and drop behaviour. These represent the running application as a whole, irrespective of whether it has open windows.
• Filer-based – The GUI is centred around the concept of files. The Filer displays the contents of a disc. Applications are run from the Filer view, files can be double clicked to open them, and dragged to and from the Filer view from applications to save and load them.
• Full drag-and-drop support – The user is able to copy and move data between application windows and disc locations via the Filer by direct manipulation.
• Self-contained application view – Applications are represented by a directory whose name begins with an exclamation mark (pronounced pling). 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 installation and removal.
• Sub-pixel positioning anti-aliasing – The outline font manager provides anti-aliasing of fonts. RISC OS has included this feature since before January 1989 and was one of the first operating systems to include such a feature.
• Consistent look and feel across encouraged across applications – Introduced by Acorn with RISC OS 3, the RISC OS Style Guide is a 130-page document specifying application appearance and behaviour.

Bundled applications

Applications bundled with RISC OS vary slightly between versions, but typically include:

• !Alarm – an iconbar clock program, with ability to set alarms
• !Calc – a basic calculator application
• !Chars – a special character insertion utility
• !Clock - a desktop Clock gadget.
• !Draw – a vector-based (or object-based) drawing program
• !Edit – a text editor, with limited support for developing BASIC programs.
• !FontPrint - Acorn to Postscript font conversion
  • T1ToFont - Postscript Type 1 to Acorn Outline font format conversion.
• !Help – an interactive, context sensitive help feature giving help for items under the mouse pointer
• !Lander - A cut down version of Zarch
• !Madness - a desktop toy
• !Magnifier - Accessibility tool, magnifying a portion of the Desktop
• !Mailman - a basic Email client
• !Maestro – a simple scorewriter, with playback
• !Paint – a basic pixel-based drawing program
• !Patience - one player card game
• !Puzzle - another game
• !SiCalc – another calculator application
• !Squash - File Compressor
• !Usage - a desktop CPU load gadget
• !65Host - a BBC micro Emulator
• !65Tube - a BBC micro with 65c02 2nd processor Emulator
• BBC Basic V & VI - Basic programming languages, with inbuilt ARM assembler

Learning Curve Editions included:

• First Word Plus - Word processor
• Genesis Plus - Spreadsheet
• Acorn DTP - Desk top publisher
• PC Emulator
• A Selection of Games

Arthur

Arthur is an early operating system (OS) that was used on Acorn ARM-cpu-based computers from about 1987 until RISC OS 2 was completed and made available in April 1989. It was the operating system of the earliest Archimedes ARM machines.

It was bundled with a desktop graphical user interface (GUI). It features a colour-scheme typically described as "technicolour". Its earlier revisions were very buggy.

The graphical desktop runs on top of a command-line driven operating system which owe much to Acorn's earlier MOS operating system for its BBC Micro range of 8-bit microcomputers.

Arthur, as originally conceived, was intended to deliver similar functionality to the operating system for the BBC Master series of computers, MOS, as a reaction to the fact that a more advanced operating system research project (ARX) would not be ready in time for the Archimedes.

Name

Arthur is said to stand for "A Risc-based operating system by THURsday". (Paul Fellows who lead the project says "It stood for ARM-on-Thursday, reflecting the time-scale that we were given to develop it in".)

The Arthur project team, led by Paul Fellows, was given just five months to develop it entirely from the ground up -- with the directive "just make it like the BBC micro". It was intended as a stop-gap until the revolutionary operating system which Acorn had under development (ARX) could be completed. However, the latter was delayed time and again, and was eventually dropped when it became apparent that the Arthur development could be extended to have a window manager and full desktop environment. Also, it was small enough to run on the first 512K machines with only a floppy disc, whereas ARX required 4 megabytes and a hard drive.

The first reliable release was version 1.2.

Internal Development

No other versions were then released externally, but internally the development of the desktop and window management continued, with the addition of a cooperative multi-tasking system, invented by Neil Raine, which cleverly used the memory management hardware to swap-out one task, and bring in another between call-and-return from the WIMP-Poll call that applications were obliged to make to get messages under the desktop. This transformed it from a single-application-at-a-time system, to one that could operate a full multi-tasking desktop. This transformation took place at version 1.6 though it was not made public until released, with the name change from Arthur to RISC OS, as version 2.0.

Compatibility

Most software made for Arthur 1.2 can be run under RISC OS 2 and later because, underneath the desktop, the original Arthur OS core, API interfaces and modular structures remain as the heart of all versions. (A few titles will not work, however, because they used undocumented features, side effects or in a few cases APIs that became deprecated).

RISC OS 2

RISC OS was a rapid development of Arthur 1.2 after the failure of the ARX project. The first release was to be called Arthur 2, but was renamed to RISC OS 2, and was first sold as RISC OS 2.00 in April 1989. The operating system implements co-operative multitasking with some limitations but is not multithreaded. It uses the ADFS filesystem for both floppy and hard disc access. It initially ran from a 512 KB ROM module. The WIMP interface offers all the standard features and fixes many of the bugs that had hindered Arthur. It lacks virtual memory and extensive memory protection (applications are protected from each other, but many functions have to be implemented as 'modules' which have full access to the memory). At the time of release, the main advantage of the OS was its ROM; it booted very quickly and while it was easy to crash it was impossible to permanently break the OS from software. Its high performance was due to much of the system being written in ARM assembly language. The OS is organised as a relatively small kernel which defines a standard software interface to which extension modules are required to conform. Much of the system's functionality is implemented in modules coded in the ROM, though these can be supplanted by more evolved versions loaded into RAM. Among the kernel facilities are a general mechanism, named the callback handler, which allows a supervisor module to perform process multiplexing. This facility is used by a module forming part of the standard editor program to provide a terminal emulator window for console applications. The same approach made it possible for advanced users to implement modules giving RISC OS the ability to do pre-emptive multitasking.

One unusual and innovative feature of the operating system at the time of its release was its support for high-quality, hinted and anti-aliased outline font rendering, a feature that only became widespread in other operating systems much later. (This was available in the Arthur 1.2 version in 1987)

A slightly updated version RISC OS 2.01 was released later to support the ARM3 processor that was shipped with the Acorn Archimedes A540 and Acorn R225/R260.

RISC OS 3

RISC OS 3.00 was released with the very earliest version of the A5000 in 1991; it is almost four times the size of RISC OS 2 and runs from a 2 MB ROM. It improves multitasking and also places some of the more popular base applications in the ROM. RISC OS 3.00 had several bugs and was replaced by RISC OS 3.1 a few months later; the upgraded ROMs were supplied for the cost of postage only.

RISC OS 3.1 was released later and sold built-in to the A3010, A3020, A4000, A4 and later A5000 models. It was also made available as replacement ROMs for the A5000 and earlier Archimedes machines (this is the last RISC OS version suitable for those machines). Three variants were released – RISC OS 3.10 the base version, RISC OS 3.11 which included a slight update that fixes some serial port issues and RISC OS 3.19 which was a German translation.

RISC OS 3.50 was sold from 1994 with the first Risc PCs. Due to the very different hardware architecture of the Risc PC, including an ARM 6 processor, 16 and 24bit colour and a different IO chip (IOMD), RISC OS 3.50 was not made available for the older Archimedes and A Series ARM 2 and 3 machines. RISC OS 3.5 was somewhat shoehorned into the 2 MB footprint, and moved the ROM applications of RISC OS 3.1 onto the hard drive; this proved so unpopular that they were later moved back into ROM.

RISC OS 3.60 followed in 1995. The OS features much better hard disk access and its networking was enhanced to include TCP/IP as standard in addition to Acorn's existing proprietary Econet system. The hardware support was also improved; Risc PCs could now use ARM 7 processors. Acorn's A7000 machine with its ARM 7500 processor was also supported. RISC OS 3.6 was twice the size of RISC OS 3.5, shipping on 4 MB in two ROM chips.

RISC OS 3.70 was released in 1996. The primary changes in the OS was support for the StrongARM processor that was made available as an upgrade for the Risc PC. This required extensive code changes due to StrongARM's split data and instruction cache (Harvard architecture) and 32-bit interrupt modes.

RISC OS 3.71 is a small update released to support the hardware in the Acorn A7000+ with its ARM 7500FE processor. The FE offered hardware support for floating point mathematics, which until then was usually emulated in one of the RISC OS Software modules).

RISC OS 3.60 also formed the foundation of NC OS, as shipped in the Acorn NetChannel NCs.

Demise of Acorn Computers Ltd

Acorn officially halted work in all areas except set-top boxes in January 1999 and the company was renamed Element 14 (the 14th element of the periodic table being silicon) with a new goal to become purely a Silicon design business (like the previous very successful spin off of ARM from Acorn in 1990). RISC OS development was halted during the development of OS 4.0 for the RiscPC 2 ("Phoebe 2100"), whose completion was also cancelled. A beta version, OS 3.8 ("Ursula") for the original RiscPC, had previously been released to developers. The project code names of Phoebe (for the hardware), Ursula (for the software) and Chandler (for the graphics processor chip) were taken from the names of characters in the TV series Friends (Phoebe and Ursula were twin sisters in the series).

This led to a number of rescue efforts to try and keep the Acorn desktop computer business alive. Acorn held discussions with many interested parties, and eventually agreed to exclusively licence RISC OS to RISCOS Ltd, which was formed from a consortium of dealers, developers and end-users. There were also a number of projects to bring the advantages of the RISC Operating System to other platforms by the creation of the ROX Desktop to provide a RISC OS-like interface on Unix and Linux systems. Two similar projects, Impulse and Eidos's Phoenix, have both stalled.

RISC OS 4

In 1999 a new company called RISCOS Ltd was founded. They licensed the rights to RISC OS from Element 14 (and eventually from the new owner, Pace Micro Technology) and continued the development of OS 3.8, releasing it as RISC OS 4 in July 1999. According to the company, over 6,400 copies of RISC OS 4.02 were sold up until production was ceased in mid 2005.

In 2002 the company launched RISC OS Select, a subscription scheme allowing users access to the latest OS updates in between major releases. These upgrades are released as soft-loadable ROM images, separate to the Flash ROM where the boot OS is stored, and are loaded at boot time. The scheme was devised to accelerate RISCOS Ltd's development cycle by producing extra income in between major releases. It has also allowed the company to subsidise the retail price of ROM releases, which are generally a culmination of the last few Select upgrades with a few extra minor changes.

In April 2004, RISCOS Ltd released the ROM based version 4.39, being dubbed RISC OS Adjust. (The name was a play on the RISC OS GUI convention of calling the three mouse buttons 'Select', 'Menu' and 'Adjust'.) RISCOS Ltd sold its 500th Adjust ROM in early 2006

In 2004, RISCOS Ltd privately began work on a 32-bit version of RISC OS Adjust (Adjust 32), which is compatible with current ARM processors and designed for both embedded and desktop forms. The first machine to make use of the updated OS is the Advantage6 A9home (Photo of Portable Desktop Version). It was released in May 2006 after a 12 month Beta testing process, although the current build of Adjust 32, namely RISC OS 4.42, is not yet feature complete. Both 26- and 32-bit builds of new RISC OS 4 releases can now be compiled from the same source code, but will have to be modified to run on each individual machine supported, as the OS has no HAL at present. Instead it has a hardware-abstracted kernel, which allows specific code to be substituted for each platform supported.

RISC OS 4 is also available as an emulator for Windows systems. The emulator is called Virtual Acorn and is sold by 3QD Developments [2]. The latest version is Virtual RPC-Adjust: RO 4.39. An Apple Mac version suitable for use with new Intel based Macs and older G5 Macs running Mac OS X is now available and in tests on the latest Apple hardware is shown to be running around 3 times faster than a real Risc PC for many OS based operations and up to 10 times faster on hardware based operations, such as copying files from CD to harddrive.

RISC OS 5

RISC OS 5 is a separate evolution by Castle Technology Ltd based upon work done by Pace for their NCOS based set top boxes. RISC OS 5 was written to support Castle's Iyonix PC Acorn-compatible, which runs on the Intel XScale ARM processor. Although a wealth of software has now been updated, a few older applications can only be run on RISC OS 5 via an emulator called Aemulor, since the ARMv5 XScale processor does not support 26-bit addressing modes. Likewise, RISC OS 5 itself had to be ported to run properly on the new CPU, and abstraction of the graphics and other hardware interfaces created, to allow it, for example, to use standard graphics cards, instead of Acorn's own VIDC chip.

In July 2003, Castle Technology Ltd bought the head licence for RISC OS from Pace Micro. RISCOS Ltd and Castle continued maintaining separate development branches of the RISC OS operating system for some time, but as a result of a lengthy dispute over licensing during 2004 the two companies agreed to merge the two competing streams. After taking over development from Castle, RISC OS Open Ltd later distanced themselves from this announcement, indicating it was technically impossible but indicated they would work together to smooth incompatibilities.

Some pieces of software famous to other platforms such as Microsoft Windows and Linux have been ported to RISC OS, most notably Firefox and Thunderbird.

Shared Source Initiative

In October 2006, Castle Technology Ltd announced a plan to release elements of RISC OS 5 under a unique source sharing license. The Shared Source Initiative (SSI) is a joint venture between Castle and RISC OS Open Limited (ROOL), a newly formed software development company, which aims to accelerate development and encourage uptake of the OS. Under the custom dual license, released source will be freely available and may be modified and redistributed without royalty for non-commercial use, while commercial usage will incur a per-unit license fee to Castle.

The SSI is making phased releases of the OS. Up to October 2008, enough source has been released to build an almost complete Iyonix ROM image.

ROOL will maintain the shared source tree and build an international developer community on a non-profit basis to support and encourage development. Both ROOL and Castle intend to provide RISC OS consultancy to clients requiring embedded ARM solutions, already a major market for the OS.

RISCOS Ltd and ROOL have agreed to cooperate, but expect to continue working on their own forks of the source tree.

RISCOS Ltd said on their website: "We await the full details of the licensing terms and conditions that will be applicable to RISC OS 5 source code. When these are known we shall be able to review the situation. However the current expectation is that there are very few features that are present in RISC OS 5 that are missing in RISC OS Six, that have a very high priority for inclusion in future releases of RISC OS 6."

Managing Director Paul Middleton told Drobe News that the company would not be open sourcing its OS code in the same way.

He said: "It is probably worth pointing out that the 'open sourcing' of RISC OS is going to solely cover RISC OS 5 versions. We do not intend to 'open source' RISC OS 4 versions as some people seem to have assumed."

"I would point out though that we have always been happy to work with developers who require source level access to RISC OS, in the same way that Acorn made sources available for particular projects. The difference between us and ROOL is that we do require any changes made to be fed back to us, as we only want one version of RISC OS 4 to be available."

More recently however, concern has been expressed concerning the exact licensing terms and ownership of RISC OS, with some commentators claiming that recent efforts to port RISC OS to platforms other than Iyonix might not be permitted. Ports of RISC OS 5 to the BeagleBoard and Risc PC/A7000 are, however, under way.

RISC OS Six

Shortly after Castle announced the SSI, RISCOS Ltd announced RISC OS Six, the next generation of their stream of the operating system. Significant portability, stability and internal structure improvements, including full 26/32 bit neutrality, have laid the foundations for the company's future releases, all of which will be based on Version 6.

RISC OS Six is now highly modularised, with legacy and hardware specific features abstracted, and other code separated for easier future maintenance and development. Teletext support, device interrupt handler, software-based graphics operations, the real-time clock, the mouse pointer, CMOS RAM support, and hardware timer support have been abstracted out of the kernel and into their own separate modules. Legacy components, like the VIDC driver, and obsolete functionality for the BBC Micro have been abstracted too. AIF and transient utility executable checking has been introduced also to protect against rogue software, while graphics acceleration modules are provided for the SM501 graphics chip in the A9home and for ViewFinder AGP podule cards.

A beta-version of RISC OS 6, Preview 1, was available for free download by subscribers to the Select scheme, both present subscribers and those whose subscription was renewed after 30 May 2004 but has since lapsed.

Select 4 is the first product to be based on RISC OS Six. Originally slated for release around mid-2005, it had been subject to delays due to the company's commitment to support the porting of RISC OS to a 32bit neutral environment which became Adjust 32 on the A9home.

Select 4 includes new user functionality. RISCOS Ltd released Select 4 issue 2 to subscribers on 30 April 2007, this version of RISC OS is numbered 6.06. Select 4 issue 3 was released in September of the same year and provided updates to 6.06 including initial support for filer short cuts.

On 26 April 2008 RISCOS Ltd released Select 4 issue 4 with many new features.

Select 4 releases are initially compatible with only Acorn Risc PC and A7000 machines. RiscStation R7500, MicroDigital Omega and Mico computers will not officially be supported, as the company does not have test machines available and requires proprietary software code to which they do not have the rights. Lack of detailed technical information about the MicroDigital Omega has also been cited as being another reason why support of that hardware is difficult. Subsequent versions of Select 4 will also be compatible with the A9home.

RISCOS Ltd. released their latest offering, RISC OS 6.20 in December 2009 Select 6 issue 1.

An Iyonix-compatible version of RISC OS 6 is described as a possibility. From the RISC OS 6 FAQ : "Some people have assumed that because we have not made any definite announcements with respect to Select 4 on the Iyonix, that we are not interested in doing the work. The facts are however that our resources are limited, and priority has been given to working with partners who actively want RISC OS Select features on their products."

Emulators

There are several free emulators available for Windows, Mac, Linux and other platforms. These include:

• Archie
• Arculator
  • RPCEmu
• ArcEm
• Red Squirrel
• Acorn-Pattenden
• Riscose
• ARM2

These do not come with the operating system and so legal means need to be found to find an appropriate version for the emulator. RISC OS 4.02 licensed for use with emulators may be purchased for £5.

Commercial emulators Virtual A5000 and Virtual RPC can be regarded as a fully working RISC OS systems. The latter is bundled with RISC OS 4.02 or 4.39, and runs on Microsoft Windows and Apple Mac OS X. It is generally regarded as a much upgraded version of Red Squirrel as the two are derivatives.

See also

• Application directory

External links

• Arthur Lives! - a guide by Ben Jefferys
• Arthur OS Emulator (original site has been taken down - discovered 2009/01/26)
• What is RISC OS?
• RISCOS Ltd
• RISC OS Open Ltd