User:Trevj/RISC OS

RISC OS is a series of graphical user interface-based computer operating systems (OSes) designed for ARM architecture systems. It takes its name from the RISC (Reduced Instruction Set Computing) architecture supported. The OS was originally developed by Acorn Computers for use with their 1987 range of Archimedes personal computers using the Acorn RISC Machine processors. It comprises a command-line interface and desktop environment with a windowing system.

From 1988 to 1998, the OS was bundled with nearly every ARM-based Acorn computer model, including the Archimedes range, RiscPC and A7000. A version of the OS (called NCOS) was used in Oracle's Network Computer and compatible systems. After the breakup of Acorn in 1998, development of the OS was forked and independently continued by several companies, including RISCOS Ltd, Pace Micro Technology and Castle Technology. Since 1998 it has been bundled with a number of ARM-based Acorn clone computers such as the Iyonix and A9home. , the OS remains forked and is developed by RISCOS Ltd and the RISC OS Open community.

Most recent stable versions run on the ARMv3/ARMv4 RiscPC (or under emulation via VirtualAcorn or RPCEmu), the ARMv5 Iyonix and ARMv7 Cortex-A8 processors (such as that used in the BeagleBoard and Touch Book). In 2011, a port for the Cortex-A9 PandaBoard was announced.

History
RISC OS was originally released in 1987 as Arthur 1.20. The next version, Arthur 2, became RISC OS 2 and was completed and made available in April 1989. RISC OS 3.00 was released with the very earliest version of the A5000 in 1991 and contained a series of new features. By 1996 RISC OS had been shipped on over 500,000 systems.

Acorn officially halted work on the OS in January 1999, renaming themselves Element 14. In March 1999 a new company called RISCOS Ltd 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 on ROM were sold up until production was ceased in mid 2005.

In May 2001 the company launched RISC OS Select, a subscription scheme allowing users access to the latest OS 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. RISCOS Ltd released the ROM based version 4.39 the same month, dubbed RISC OS Adjust as 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 October 2002, the Acorn clone Iyonix PC was released by Castle Technology. This runs RISC OS 5, a separate evolution based upon the NCOS work done by Pace for set-top boxes. In October 2006, Castle announced a source sharing license plan for elements of RISC OS 5. This Shared Source Initiative (SSI) is managed by RISC OS Open Limited.

Also in October 2006, RISCOS Ltd announced RISC OS Six, the next generation of their stream of the operating system. The first product to be launched under the name was the continuation of the Select scheme, Select 4. A beta-version of RISC OS 6, Preview 1 (Select 4i1), was available in 2007 as a free download to all subscribers to the Select scheme, while in April 2009 the final release of Select 5 was shipped.

The latest release of RISC OS from RISCOS Ltd is Select 6i1, shipped in December 2009.

Supported hardware
Versions of RISC OS run or have run on the following hardware.

In addition a commercial emulator available from VirtualAcorn is available as a standalone product or pre-bundled with Microsoft Windows PCs from Microdigital, Advantage Six and R-Comp.

RISC OS has also been used by Acorn and Pace Micro Technology, in various TV connected Set Top Boxes.

OS core
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 4 MB of flash memory, allowing the operating system to be updated without having to replace the ROM chip. The OS is made up of a number of 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.

The OS is single-user and employs co-operative multitasking (CMT). While most current desktop OSes use pre-emptive multitasking (PMT) and multithreading, RISC OS remains with a CMT system. Many users have called for the OS to migrate to PMT. The OS also has rudimentary memory protection, and all users have full superuser privileges.

The file system is volume-oriented: the top level of the file hierarchy is a volume (disc, network share) prefixed by the filesystem type. To determine file type, the OS uses metadata instead of file extensions. 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 ( becomes  ). For example,  is the root of the disc named HardDisc4 using the ADFS filesystem.

A file system can present a file of a particular type as a volume in its own right, similar to a loop device. The OS refers to this functionality 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 filesystems put a reference inside the image file and go elsewhere for the data.

Most of the OS has defined ABIs to handle filters and vectors. The OS provides many ways in which the programmer can intercept and modify its operation. This simplifies the task of modifying its behaviour, either in the GUI or deeper. As a result, there is a number of third-party programs which allow the OS look and feel to be customised.

Kernel
The kernel controls handling of interrupts, DMA services, memory allocation and the video display.

Desktop
The WIMP interface (see link for explanation; Acorn used wimp to refer to their gui) incorporates three mouse buttons (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 holds icons which represent mounted disc drives, RAM discs, running applications and system utilities. These icons have context-sensitive menus and support drag-and-drop behaviour. They represent the running application as a whole, irrespective of whether it has open windows.

The GUI is centred around the concept of files. The Filer 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. Application directories are used to store applications. The OS differentiates them from normal directories through the use of a pling (exclamation mark, also called shreik) 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 installation and removal.

The outline font manager provides anti-aliasing of fonts, the OS being the first operating system to include such a feature,   having included it since before January 1989 The RISC OS Style Guide encourages a consistent look and feel across applications. This was introduced in RISC OS 3 and specifies application appearance and behaviour. Acorn's own main bundled applications were not updated to comply with the guide until 's Select release in 2001.

Bundled applications
RISC OS is delivered with a number of desktop applications in the form of pre-installed software.