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[ Home » About RISC OS » A Guide to RISC OS Versions ]

A Guide to RISC OS Versions

Since the demise of Acorn at the end of the 1990s, development of RISC OS has continued due to the work of a number of different companies. For the first time in its history, two versions of the OS are being developed for desktop users. Add to that the changes required to make the OS run on new hardware, and the situation can start to get confusing.

This quick guide to RISC OS versions aims to aims to explain the options available for anyone wanting to use the system today: if you want information going back to the beginnings in 1989, you should read our brief history of RISC OS. The first section of the guide looks at the background information required to make sense of RISC OS today, before the second goes on to examine the versions of the OS in common use.

More details of the hardware that RISC OS has run on over the years can be found in our guide to RISC OS hardware.

A bluffer’s guide to RISC OS

For anyone unfamiliar with the RISC OS world, a lot of the details can seem confusing. Here are some of the questions that are often asked by potential users and existing users alike.

What hardware does RISC OS run on?

These days, RISC OS can be used on “native” ARM-based desktop machines which can be viewed as distant descendants of the original RISC machines made by Acorn, as well as on “emulated” hardware which runs on Windows, Mac OS X and Linux.

Many people putting the system to serious use will have a recent machine like a RiscPC, Iyonix PC or A9home – or they could be using an emulator such as Virtual Acorn or RPCEmu. Details of the older hardware that RISC OS has run on over the years can be found in our guide to RISC OS hardware.

When will it be available for ‘normal’ hardware?

Due to its dependence on the ARM processor, RISC OS is never likely to end up running natively on mainstream computer hardware. In practice, due to the availability of emulators like Virtual Acorn and RPCEmu, this is no longer a real issue.

What’s that about “26-bit” and “32-bit”?

When discussing RISC OS and its hardware, it’s common to see the terms “26-bit” and “32-bit” mentioned frequently to differentiate old and new systems. This can be confusing, not least because Acorn were describing the Archimedes as “32-bit” back in 1987 – machines that we would now refer to as being “26-bit”.

The first thing to remember is that RISC OS is a 32-bit operating system, and the ARM processor that it runs on is a 32-bit processor, as far as common use of the term goes – this has been true since the systems were first launched back in the 1980s. The “26-bit” and “32-bit” that crop up in discussions about the modern OS are referring to the way in which a section of the processor called the program counter is used – that is, they refer to the way that memory is accessed. This is not something that directly affects users, but it does provide a neat way to identify the two types of system.

When RISC OS was released in 1987, the ARM’s program counter could only operate in 26-bit mode and so RISC OS itself and all of the software that ran on it was 26-bit in this respect. Although the processor worked with 32-bit numbers, it could only access memory with addresses up to 26 bits wide: that’s 64Mbytes.

By the mid 1990s, 64Mbytes was not a lot of memory, and the ARM processors used from the RiscPC onwards could operate in both 26-bit mode or a new 32-bit mode that could access up to 4Gbytes of storage. The new mode was not compatible with most of the software written for RISC OS, so the system continued to run in 26-bit mode (with some clever tricks used to access all the additional memory that the new hardware could accept).

Unfortunately, modern ARM processors, like those used in the A9home and Iyonix PC, no longer have a 26-bit mode; as a result, new versions of RISC OS have to be made 32-bit in order to run on them. The problem is that software must also be updated to run on these systems: most actively developed software is now compatible with 26- and 32-bit, and a lot of that which isn’t can be used with the aid of a software tool called Aemulor. However, versions of RISC OS for use on old hardware are still produced as 26-bit to ensure the widest-possible compatibility with old software.

What does “ROM-based” mean?

RISC OS is a ROM-based system, which means that the OS itself is stored on physical memory chips inside the computer. On all but the latest machines, these are Read Only Memory (ROM) – this used to mean that upgrading the OS required the use of a screwdriver.

Modern hardware has replaced these ROMs with Flash ROMs: these can be upgraded from software, a little like the BIOS in a conventional PC. The Iyonix PC and A9home both come with this facility, which makes installing a new OS a relatively painless affair.

Even users of old machines don’t need to open the case and replace components for the most recent versions of RISC OS. As long as RISC OS 4.02 or later is fitted in the ROMs, later versions of RISC OS 4 and RISC OS 6 can be loaded from the hard disc as the machine boots. A similar option is also available for users of RISC OS 3.5 and later, which allows them to install a copy of RISC OS 4.02 on their machines’ hard discs to avoid having to open the case and replace the ROMs (although for technical reasons, this does not offer an upgrade beyond RISC OS 4.02).

Those emulating a RISC OS system using Virtual Acorn, RPCEmu or similar don’t need to worry about the ROM issue at all: on such systems, the ROMs are simply stored as files on the hard disc of the native Windows, Mac OS or Linux system.

Modern RISC OS systems

As far as present day users of RISC OS are concerned, there are three versions of the OS in common use: RISC OS 4, RISC OS 5 and RISC OS 6. The exact versions which can be used will vary depending upon the available hardware. Some systems may still be running RISC OS 3, but an upgrade is recommended if the machine in question is used regularly.

The systems can be divided into two groups: those which are “26-bit” and those which are “32-bit”, as described above. These numbers refer to the “addressing mode” of the ARM processor: older models offered both, while more recent designs only work in 32-bit mode.

Older versions of RISC OS rely on the presence of 26-bit mode in the processor, and so can only be run on hardware which supports this (including emulated systems). The Iyonix PC and A9home have 32-bit-only processors, and so require 32-bit versions of RISC OS.

RISC OS 4.02/4.03 and RISC OS 5

These two systems are broadly similar in features, and between them they are likely to be the minimum sensible specification for a machine in regular use. RISC OS 4.02 and 4.03 is a 26-bit OS from RISCOS Ltd, and can be used on machines such as the RiscPC and A7000 from Acorn and Castle Technology, machines from RiscStation and Microdigital, and emulated systems such as Virtual Acorn and RPCEmu.

RISC OS 5, on the other hand, is 32-bit. While it was once only available to those with an Iyonix (and the only OS that would run on that hardware), it is now seeing active development and versions are available for other ARM-based systems such at the BeagleBoard (including ARMini and BIK), along with the TouchBook. Experimental versions are also running on the Raspberry Pi, the RiscPC, and the RPCemu emulator.

Compared to RISC OS 3, the two versions offer an improved hard disc format with support for long filenames and more than 77 files per directory. They also include a facility known as the “Nested Wimp”, which is required by a number of modern pieces of software, as well as many other facilities and improvements to provide better hardware and software support.

RISC OS 4.02 is usually supplied as a physical ROM chip, to be installed in the computer, although a “soft-loaded” version is available for installation on disc as an upgrade for RISC OS 3.5 and above, and ROM images are supplied for use with emulated systems. RISC OS 5 is always supplied as a ROM image, either for use in flash ROM or as a disc image.

Development of RISC OS 4 continued as part of the Select scheme, while RISC OS 5 was developed by Castle up to version 5.13. This strand of the OS was then taken up by RISC OS Open as part of the shared source release. Iyonix, ARMini, BIK and Beagleboard owners now have access to a 5.18 ROM which includes many improvements, while an experimental 5.19 is running on a wider range of systems.

RISC OS Select and RISC OS 4.39/Adjust

Developments made by RISCOS Ltd after the releases of RISC OS 4.02 have been made under the banner of the Select Scheme, and are therefore often referred to as “RISC OS Select”. The scheme is based around an annual subscription, with members receiving updates as and when they become available.

Three releases of Select were made with the RISC OS 4 label, which culminated in RISC OS 4.39. All three releases were initially supplied as “softloads” which are installed on disc and replace the version of RISC OS in ROM after the machine starts. Although this slows down the machine’s start-up, it makes installing updates a lot simpler.

In addition, the third release (RISC OS 4.39) is also available as a set of physical ROMs for insertion into the RiscPC. This update provides users with a recent version of RISC OS as the base on their system, from which further softloads can be made in the future. In ROM form, RISC OS 4.39 is often referred to by the marketing name of “RISC OS Adjust”, to differentiate it from the softloaded versions supplied as part of the Select Scheme.

The various Select updates provide improvements to the RISC OS user interface, with an updated appearance to the Desktop and improved functionality. The bundled applications, including Draw and Paint, have also seen significant updates. The OS has a number of improvements behind the scenes which can be used by software developers, but at the time of writing uptake by developers has been low.

RISC OS 6 and 32-bit

Following the release of RISC OS 4.39, the RISCOS Ltd version of the OS was also converted to 32-bit operation so that it could be used on the A9home; at the same time, 26-bit builds can also be produced for use on the RiscPC and Virtual Acorn systems.

The first release was RISC OS 6.06 in 2007, followed a year later by RISC OS 6.10. Several more releases followed, and in 2010 users can access RISC OS 6.20. Aside from the ability to run on modern display hardware, the main developments have been in the Desktop user interface: adding features which make RISC OS even easier to use.

Development of RISC OS 6 continues under the banner of the Select Scheme, although (in a similar way to RISC OS Adjust) users who do not subscribe to the scheme can buy one-off softload copies of the OS for use on their machine.

Further reading

For space reasons, this guide to the versions of RISC OS is only an introduction – at least as far as the history of the OS is concerned. Much more detailed information can be found on the following sites.

If you have any questions about the RISC OS operating system, why not drop us a line?

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