RISC OS

RISC OS, which stands for Reduced Instruction Set Computing Operating System, is a British graphical user interface-based operating system for ARM-processor based computers or similar devices.

Features
Stored in ROM - This allows for faster bootup (sub-30 seconds), safety from corruption and security against viruses. Versions 4 and 5 are stored in 4MB of Flash ROM, enabling Flash updates to the ROM.
Module-based - Modules can be freely added and replaced, including soft-loading of modules not present in ROM at boot time. This modular design has lead to developer RISC OS Ltd 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 API calls in Microsoft Windows.
Single User, Co-Operative Multitasking, Single Threaded - Despite almost all other current desktop operating systems moving towards pre-emtive multitasking (PMT) and multithreading, RISC OS remains a co-operative multitasking system. Although this is preferential for RISC OS' many embedded applications, many desktop developers and users have called for the OS to migrate to PMT. The OS also has only rudimentary memory protection.
Proprietary ADFS filesystem - The OS uses meta-data 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 $ sign and directories by a period (.). Extensions from foreign filesystems are shown using a /. For example: ADFS::HardDisc4.$. is the root of HardDisc4 in ADFS format.
Self contained application view - Applications are contained in a directory, which, if its first character is '!' (pronounced pling) is normally treated by the filer as an application: clicking on such a directory would launch the application, rather than open the directory, and although the application's resources and executable files are contained within the directory, they remain hidden from the user. All application files are stored within this single directory, allowing for drag and drop install and removal.
Intuitive window manager - The RISC OS WIMP incorporates three-buttoned mouse operation, context-sensitive menus, window order control (i.e. send to back) and dynamic window focus (i.e. allows windows to be in focus at any position on the stack, including when not 'on top' and not visible). Launched during the time of Windows 3.0 and Mac OS System 7, the RISC OS WIMP GUI was well ahead of its time.
Drag and Drop - The user is able to copy and move data between application windows and disc locations via the filer by direct manipulation. This includes moving ('cut and paste'), copying and file saving and opening.
Iconbar - Similar to the Windows taskbar and Mac OS dock but released prior to both, RISC OS was a pioneer of this feature. 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.
Sub-pixel anti-aliasing - The Outline font manager provides broadcast quality anti-aliasing of fonts, drawn in real time onto the screen. Introduced in 1990, RISC OS was one of the first operating systems to include such a feature
Consistent look and feel across all applications - Introduced by RISC OS developer Acorn with Version 2, the RISC OS Style Guide is a detailed, 130 page document setting out the rules on application appearance and behaviour. This has ensured that applications appear and behave in the same way from the user's perspective, aiding ease of use.

Bundled applications
Applications bundled with RISC OS vary slightly between versions, but usually include the following core apps:

!Paint - a basic bitmap-based drawing program.
!Draw - a surprisingly sophisticated vector-based drawing program (Object Based).
!Calc - a basic calculator application.
!Edit - a text editor.

Early years (Arthur)
Main article: Arthur (operating system)

A screenshot of Arthur's GUI desktop and its bundled accessory applicationsThe OS was designed in Cambridge, England by Acorn for the 32-bit ARM based Acorn Archimedes, and released in its first version in 1987, as the Arthur operating system.

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. It had co-operative multitasking with some limitations, but was not multithreaded. It used the ADFS filesystem for both floppy and hard discs. It initially ran from a 512 KB ROM module. The WIMP interface offered all the standard features and fixed many of the bugs that had hindered Arthur. It lacked 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). The main advantage of the OS was its ROM; it booted very quickly and while it was easy to crash it was impossible to break. Its high performance was because much of the system was written in ARM assembly language. The OS was organised as a relatively small kernel which defined a standard software interface to which extension modules were required to conform. Much of the system's functionality was implemented in modules coded in the ROM, though these could be supplanted by more evolved versions loaded into RAM. Among the kernel facilities were a general mechanism, named the callback handler, which allowed a supervisor module to perform process multiplexing. This facility was 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.

A slightly updated version RISC OS 2.01 was released later to support the ARM3 processor that was shipped with the [[Acorn Archimedes|Archimedes A540, Acorn R225/R260] and Acorn BBC A3000].


RISC OS 3

A typical RISC OS 3.7 sessionRISC OS 3.00 was released with the A5000 in 1991; it was almost four times the size of RISC OS 2 and ran from a 2MB ROM. It improved multitasking and also placed some of the more popular base applications in the ROM.


RISC OS 3.1
RISC OS 3.1 was released later which was 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 version suitable for those machines. Three variants were released RISC OS 3.10 the base version, RISC OS 3.11 a slight update that fixed some serial port issues, and RISC OS 3.19 which was a German translation.


RISC OS 3.5
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 was the most advanced OS for its time with a drag-and-drop Graphical User Interface (GUI) whereas IBM Compatible PCs were still operating in DOS command line format. RISC OS and indeed Arthur OS were both ahead of their time with the 'It just works' approach to creating an operating system.


RISC OS 3.6
RISC OS 3.60 followed in 1995, the OS was updated and featured 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. Also the hardware support was improved; Risc PCs could now use ARM 7 processors and Acorn's A7000 machine with its ARM 7500 processor was also supported.


RISC OS 3.7
RISC OS 3.70 followed in 1996, the primary changes in the OS for this version was supporting 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).

RISC OS 3.71 is a small update released to support the hardware in the Acorn A7000+ with its ARM 7500FE (FE offering Floating Point Maths hardware support. This was normally emulated in one of the RISC OS Software modules) processor.

RISC OS provides a very intuitive graphical user interface, with features such as context sensitive menus and pervasive drag and drop, and excellent consistency across applications thanks to Acorn's detailed Style Guide.


Demise of Acorn Computers Ltd
Acorn halted work in all areas except set-top boxes in late 1998, and the company was renamed to Element 14 (the 14th element of the periodic table being silicon). RISC OS development was halted during the development of OS 4.0 for the RISC PC II ("Phoebe"), whose completion was also cancelled. A beta version, OS 3.8 for the original RISC PC, was previously released to developers.

This led to a number of rescue efforts, including 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 (RISCOS Ltd)
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) 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 patches, separate to the Flash ROM where the main OS is stored, and are loaded at boot time. The scheme was devised to accelerate RISCOS Limited'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.

Released in April 2004, RISCOS Ltd's current version is RISC OS Select Edition 3 (RISC OS 4.39), with the ROM based version being dubbed RISC OS Adjust. RISCOS Ltd sold its 500th Adjust ROM in early 2006.

In 2004, RISCOS Ltd began work on a 32-bit version of RISC OS Adjust (Adjust 32), which is compatible with the latest ARM processors and available in both embedded and desktop form. The first machine to make use of the updated OS, the Advantage6 A9 (Photo of Portable Desktop Version), was released in May 2006 after a 12 month Beta testing process. Matt Edgar of Advantage6 became (in)famous for saying the machine would be released "When it's ready!" Both 26-bit and 32-bit builds of new RISC OS 4 releases will now be compiled from the same source, but will have to be modified to run on each individual machine supported, as the OS has no hardware abstraction layer (HAL) at present. Instead it has a hardware-abstracted kernel, which allows specific code to be substituted for each platform supported.


RISC OS 5 (Castle Technology)
RISC OS 5 is a separate evolution by Castle Technology Ltd based upon work done by Pace Micro Technology 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, since a minor 26-bit ARM CPU function was removed by Intel from the XScale. 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 [2]. 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. Whether a unified version will be released is yet to be seen, however, as RISCOS Ltd have continued development of their stream of the OS in preparation to launch Version 6.


Shared Source Initiative (Castle Technology Ltd / RISC OS Open Ltd)
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 full license has not yet been released.

The SSI will initially make phased releases of the OS, starting with the following components:

RISC OS build environment
Shared C Library
Main bundled applications (!Paint, !Edit, !Draw, etc)
Other disc-based applications (!Boot, !System, !Scrap, !Unicode, !Configure)
Acorn's !Browse (a.k.a. Phoenix) web browser, WebServe and related Fetcher modules
USB Printer Manager, printer drivers and printer dumper modules
Configuration plug-ins, screen savers, and some other development-related modules
In a Drobe forum, ROOL director Andrew Hodgkinson said the SSI would release as many components as possible, but it was too early to say how much of the OS that might be.

He said: "The ultimate goal would be to have a complete OS there - perhaps, for example, you could build yourself an Iyonix ROM. But that's putting the cart before the horse. We cannot promise being able to reach such a position at this stage, so we're not doing so."

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.

At this stage it is unclear whether RISC OS Ltd, developer of RISC OS 4, will co-operate with the SSI.

The company 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 6, that have a very high priority for inclusion in future releases of RISC OS 6."

ROL 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."


RISC OS 6 (RISCOS Ltd)
Shortly after Castle announced the SSI, ROL announced RISC OS 6, 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 6 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, will be available for free download by subscribers to the Select scheme, both present and those whose subscription was renewed after 30th May 2004 - the last time the company delivered a Select release - but has since lapsed.

Select Edition 4 will be the first product to be based on RISC OS 6. Originally slated for release around mid-2005, it has been subject to lengthy delays due to the company's commitment to porting Adjust 32 to the A9home, which is taking much longer than expected. Many subcribers have signalled their anger with ROL, alleging that customers' Select fees were used to fund the A9home port while they have received nothing since May 2004.

Select 4 will include new user functionality; the details of which are yet to be confirmed. As yet RISCOS Ltd have not announced any release dates for RISC OS 6 products, and have refused to speculate on any development progress.

Preview 1 and Select 4 will initially be compatible with A9home, 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 don't have the rights.

An Iyonix-compatible version of RISC OS 6 is described as a possibility - From the RISC OS 6 FAQ [3] : "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."