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 and volunteers. To complicate things, since 2002 there have been two separate ‘strands’ of development, by two different companies. The situation can quickly start to get confusing, especially for newcomers.

Here we aim 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.

Two versions of the OS

There are two versions of RISC OS out in the wild. The history is complicated, but very little matters for those wishing to use the system today – except, perhaps, for answering questions about why things are as they are!

When Acorn was wound up in 1998, it was developing RISC OS 3.8. Ownership of the OS was passed to Pace, whilst the rights to develop it for desktop users went to a company called RISCOS Ltd who turned it into RISC OS 4. This could run on the same systems that RISC OS 3.8 could (the RiscPC and A7000 series), and over the years was also made available for machines from RiscStation, MicroDigital and Advantage 6. In addition, it could run on any emulator which could make itself look like one of these, which in practice meant both the Virtual Acorn and RPCEmu systems.

In 2002, when Castle Technology came to release their new Iyonix PC range of computers, the new XScale ARM processor did not contain the by now ‘legacy’ instructions needed by RISC OS 4 and its predecessors – instructions which dated back to the original Acorn systems from 1987. Since Pace, who owned RISC OS, had been updating their version of the OS independently of RISCOS Ltd to support these new processors, Castle licensed this version and called it RISC OS 5. Since the demands of the new processor placed new restrictions on the software which could run on the OS, there were now two different versions of RISC OS available.

In 2006, RISCOS Ltd responded by producing an updated version of RISC OS 4 under the title RISC OS Six. Development continued until 2009, although it never broke free from the rapidly-aging hardware on which it depended. In contrast, RISC OS 5 was opened up, first under a shared source licence and finally, in 2018, under the truly Open Source Apache Licence. Since 2018 is has been ported to numerous ARM development boards, dedicated motherboards, laptops and – perhaps most significantly – the range of Raspberry Pi computers. It will also run on a RiscPC, so it can be used on the emulators as well.

Which version do I want to use?

If you’re new to the platform, the answer is almost certainly RISC OS 5. In most cases, the hardware determines ths version of the OS to use.

Any hardware developed since 2010 will need to use RISC OS 5, since the other versions will not recognise the processor or peripherals. Different builds are made available for the different boards, either via the RISC OS Open website or from the supplier who sold you the hardware.

If you have a RiscPC or an emulator, you have a choice, since RISC Os 4, 5 and Six will all work – it will come down to personal preference and what software you wish to run. RISC OS 5 is actively being developed, but may not work as well with old software that hasn’t seen updates since 2002 or so; RISC OS 4 or Six haven’t seen updates since 2009, but will still work with software last developed in the 1990s.

For other old hardware, you will most likely need to use RISC OS 4 or Six.

What hardware does RISC OS run on?

RISC OS can only be used on “native” ARM-based computers, which can be viewed as distant descendants of the original RISC machines made by Acorn. These can either be physical desktop machines, development boards, or software emulators which allow RISC OS to run on “emulated” hardware within a window on a Windows, Mac OS or Linux desktop.

These days, people putting the system to serious use can have a wide range of systems. Many will be using RISC OS 5 on a modern desktop or laptop computer released in the past few years, while many more will have a system that they have assembled themselves from a Raspberry Pi or other ARM development board. Emulation is a popular way to run either version of the system, especially amongst those who extensively use other operating systems and therefore have another computer in regular use, and there is also a sizable chunk of people who have never moved on from their trusty 1990s RiscPC.

Details of the hardware (and emulators) that RISC OS will run on 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 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. Back in the 1990s, these were Read Only Memory (ROM) – which 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. In fact, on systems like the Raspberry Pi and some development boards, the OS is held on an SD card – meaning that an upgrade is simply a matter of copying some files into the right place and then rebooting.

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 a suitable version of the OS is in ROM (or flash, or SD card), later versions can be ‘soft-loaded’ into memory from the hard disc as the system boots and used to replace the one in ROM. Upgrades to RISC OS 4 and RISC OS Six can be loaded in this was as long as RISC OS 4.02 or later is fitted in the ROMs, and on RISC OS 5 it’s a common way to load test builds of the OS over a stable version.

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.

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?