RISC OS  is a computer operating system
originally designed by Acorn Computers
Ltd in Cambridge, England. First
released in 1987, it was specifically
designed to run on the ARM chipset,
which Acorn had designed concurrently
for use in its new line of Archimedes
personal computers. RISC OS takes its
name from the RISC architecture
supported.
Between 1987 and 1998, RISC OS was
bundled with every ARM-based Acorn
computer model. These included the Acorn
Archimedes range, Acorn's R line of
computers, RiscPC, A7000 and also
prototype models such as the Acorn
NewsPad and Phoebe computer. A version
of the OS was also used in Oracle's
Network Computer and compatible systems.
After the break-up of Acorn in 1998,
development of the OS was forked and
separately continued by several
companies, including RISCOS Ltd, Pace
Micro Technology and Castle Technology.
Since then, it has been bundled with a
number of ARM-based desktop computers
such as the Iyonix and A9home. As of
2012, the OS remains forked and is
independently developed by RISCOS Ltd
and the RISC OS Open community.
Most recent stable versions run on the
ARMv3/ARMv4 RiscPC, the ARMv5 Iyonix,
ARMv7 Cortex-A8 processors and Cortex-A9
processors. There is a development
version for the Raspberry Pi. SD card
images have been made available free of
charge to Raspberry Pi users with a full
graphical user interface version and a
command-line interface only version.
History
RISC OS was originally released in 1987
as Arthur 1.20. The next version, Arthur
2, became RISC OS 2 and was made
available in April 1989. RISC OS 3.00
was released with 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
develop a desktop version of RISC OS
from Element 14, and continued the
development of RISC OS 3.8, releasing it
as RISC OS 4 in July 1999. Meanwhile,
Element 14 had also kept a copy of RISC
OS 3.8 in house, which they developed
into NCOS for use in set-top boxes. In
2000, Element 14 sold RISC OS to a
company called Pace Micro Technology,
who later sold it to Castle Technology
Ltd.
In May 2001 RISCOS Ltd launched RISC OS
Select, a subscription scheme allowing
users access to the latest RISC OS 4
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. In the
same month, RISC OS 4.39, dubbed RISC OS
Adjust, was released. RISC OS Adjust was
a culmination of all the Select Scheme
updates to date, released as a physical
set of replaceable ROMs for the RiscPC
and A7000 series of machines.
Meanwhile, in October 2002, Castle
Technology released the Acorn clone
Iyonix PC. This ran a 32-bit variant of
RISC OS, known as RISC OS 5. RISC OS 5
is a separate evolution of RISC OS based
upon the NCOS work done by Pace. The
following year, Castle Technology bought
RISC OS from Pace for an undisclosed
sum. In October 2006, Castle announced a
source sharing license plan for elements
of RISC OS 5. This Shared Source
Initiative is managed by RISC OS Open
Limited.
Supported hardware
Versions of RISC OS run or have run on
the following hardware.
Note that RISC OS Open Limited adopted
the 'even numbers are stable' version
numbering scheme post version 5.14,
hence some table entries above include
two latest releases – the last stable
one and the more recent development one.
RISC OS has also been used by both Acorn
and Pace Micro Technology in various TV
connected Set Top Boxes, sometimes
referred to instead as NCOS.
RISC OS can also run on a range of
computer system emulators that emulate
the earlier Acorn machines listed above.
Features
= OS core=
The OS is single-user and employs
cooperative multitasking. While most
current desktop OSes use preemptive
multitasking and multithreading, RISC OS
remains with a CMT system. By 2003, many
users had called for the OS to migrate
to PMT. The OS memory protection is not
comprehensive.
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, similar to system
calls in other operating systems.
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.
= File system=
The file system is volume-oriented: the
top level of the file hierarchy is a
volume 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 are separated by a full
stop. Extensions from foreign
filesystems are shown using a slash. For
example, ADFS::HardDisc4.$ is the root
of the disc named HardDisc4 using the
ADFS filesystem. RISC OS filetypes can
be preserved on other systems by
appending the hexadecimal type as '.xxx'
to filenames. When using cross-platform
software, filetypes can be invoked on
other systems by naming appending
'/[extension]' to the filename under
RISC OS.
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.
The file system abstraction layer API
uses 32 bit file offsets, making the
largest single file 4 GiB long. However,
prior to RISC OS 5.20 the file system
abstraction layer and many RISC
OS-native file systems limited support
to 31 bits to avoid dealing with
apparently negative file extents when
expressed in two's complement notation.
= File formats=
The OS uses metadata to distinguish file
formats. Some common file formats from
other systems are mapped to filetypes by
the MimeMap module.
= Kernel=
The RISC OS kernel is single-tasking and
controls handling of interrupts, DMA
services, memory allocation and the
video display.
= Desktop=
The WIMP interface is based around a
stacking window manager and incorporates
three mouse buttons, context-sensitive
menus, window order control and dynamic
window focus. The Icon bar holds icons
which represent mounted disc drives, RAM
discs, running applications, system
utilities and docked: Files, Directories
or inactive Applications. 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, a spatial file
manager, 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
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 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
RISCOS Ltd's Select release in 2001.
= Font manager=
The outline font manager provides
spatial anti-aliasing of fonts, the OS
being the first operating system to
include such a feature, having included
it since before January 1989. Since
1994, in RISC OS 3.5, it has been
possible to use an outline anti-aliased
font in the WindowManager for UI
elements, rather than the bitmap system
font from previous versions.
RISC OS 4 does not support Unicode but
"RISC OS 5 provides a Unicode Font
Manager which is able to display Unicode
characters and accept text in UTF-8,
UTF-16 and UTF-32. Other parts of the
RISC OS kernel and core modules support
text described in UTF-8."
= Bundled applications=
RISC OS is delivered with a number of
desktop applications in the form of
pre-installed software.
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. Successive OS upgrades
have raised more serious issues of
backward compatibility for desktop
applications and games. Applications
still being maintained by their
author(s) or others have sometimes
historically been amended to provide
compatibility.
The introduction of the RiscPC in 1994
and its later StrongARM upgrade raised
issues of incompatible code sequences
and proprietary squeezing. Patching of
applications for the StrongARM was
facilitated and Acorn's UnsqueezeAIF
software unsqueezed images according to
their AIF header. The incompatibilities
prompted release by The ARM Club of its
Game On! and StrongGuard software. They
allowed some previously incompatible
software to be run on new and upgraded
systems. The version of the OS for the
A9home prevented the running of software
without an AIF header to stop "trashing
the desktop".
The Iyonix PC and A9home saw further
software incompatibility because of the
deprecated 26-bit addressing modes. Most
applications under active development
have since been rewritten. Static code
analysis to detect 26-bit only sequences
can be undertaken using ARMalyser. Its
output can be helpful in making 32-bit
versions of older applications for which
the source code is unavailable. Some
older 26-bit software can be run without
modification using the Aemulor emulator.
Additional incompatibilities were
introduced with newer ARM cores, such as
ARMv7 in the BeagleBoard and ARMv6 in
the Raspberry Pi.
See also
Acorn C/C++
Drobe
riscos.info
ROX Desktop, a graphical desktop
environment for the X Window System,
inspired by the user interface of RISC
OS
References
External links
"What is RISC OS?". The RISC OS Products
Directory. 2 November 2006. Archived
from the original on 19 February 2007.
Retrieved 26 August 2014. 
Gilbert, Michael. "Archiology: relics
from Acorn's past". lewisgilbert.co.uk.
Retrieved 26 August 2014. 
Watson, Jamie A.. "Raspberry Pi:
Hands-on with RISC OS". ZDNet. CBS
Interactive. Retrieved 26 August 2014. 
Watts, Robin. "Documentation on Acorn
Computers". Warm Silence Software. Pink
Noise Productions. Retrieved 26 August
2014.
