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This page attempts to document the different Operating Systems used by Acorn computers from the most common (e.g. BBC MOS or RISC OS) to the most obscure (ARX or Brazil). if I have missed an OS or version please let me know.
Here are more details of RISC OS versions .
| Operating System | Version | Media | Computer | Comments |
| ABC OS | tbd | ROM | for Acorn Business Computers I/O processor. | |
| Acorn DOS | n/a | ROM | System 2, 3, 4 & 5 | ROM was built for specific system configuration |
| Arthur | 0.20 | EPROM | Archimedes, very early A300s | rapidly replaced by 0.30 |
| Arthur | 0.30 | ROM | Archimedes, early A300s | upgraded to 1.20 |
| Arthur | 1.20 | ROM | Archimedes, A300s and A440s | replaced by RISC OS 2.00 Note Arthur ROMs marked 1.10 are actually 1.20 |
| ARX | n/a | n/a | Archimedes | Developed in Acorn's Palo Alto Research Centre for the Archimedes, not completed in time, replaced by Arthur |
| Brazil | tbd | ROM | ARM second processors and Springboard PC card. | OS kernel supporting calls to I/O processor (BBC micro or PC). |
| CP/M | 2.2 | Disc | Z80 2nd processor ABC 1x0 | Licensed from Digital Research |
| DRDOS | tbd | Disc | 80186 Co-Processor | with GEM Licensed from Digital Research |
| Electron OS | 1.0 | ROM | Electron | |
| Flex9 | tbd | Disc | System 3 with 6809 CPU | |
| Galilleo | tbd | ? | Set top box OS, never completed | |
| MOS | 0.10 | ROM | Early BBCs up to Issue 4 | Cassette Filing System only, lots of bits unimplemented. |
| MOS | 1.00 | EPROM | Early BBCs | Supported discs drives |
| MOS | 1.10 | ROM | US BBC | Has support for NSTC tevelsions and some minor graphics changes |
| MOS | 1.20 | ROM | BBC | upgrade for MOS 0.1 & 1.0, Used in most BBC Micros |
| MOS | 2.00 | ROM | BBC B+ | |
| MOS | 3.00 | ROM | Communicator | |
| MOS | 3.20 | ROM | Master | |
| MOS | 3.50 | ROM | Master | Upgrade only, not fitted as standard |
| MOS | 4.00 | ROM | Master ET | |
| MOS | 5.10 | ROM | Master Compact | |
| MS-DOS | 3.00 | Disc | available with PC Emulator . Licensed from Microsoft | |
| NCOS | 1.xx | ROM | Set top boxes and NCs | based on RISC OS 3.6 |
| Pandora | tbd | ROM | 32016 2nd processor ABC2x0 Acorn Cambridge Workstation | Panos kernel more information |
| Panos | 1.x | Disc | 32016 2nd processor Cambridge Co-Processor ABC2x0 Acorn Cambridge Workstation | more information |
| PC DOS | 7 | Disc | Risc PC PCards | Licensed from IBM |
| RISC iX | 1.1x | Disc | R140 | Acorn's Unix (BSD variant) more information |
| RISC iX | 1.2x | Disc | R225, R260 | Acorn's Unix (BSD variant) more information |
| RISC OS | 2.00 | ROM | Archimedes A300s, A400/1, A3000 | Also available as an upgrade from Arthur 1.20 for older Archimedes |
| RISC OS | 2.01 | ROM | R140, R225, R260 | RISC OS variant to boot RISC iX |
| RISC OS | 3.00 | ROM | A5000 | Buggy release rapidly replaced by v3.10 |
| RISC OS | 3.1x | ROM | A4, A3010, A3020, A4000, A5000 | Standard release until the RiscPC launch. Upgrade from previous versions available. |
| RISC OS | 3.50 | ROM | RiscPC600 | Early RiscPC.s |
| RISC OS | 3.60 | ROM | RiscPC600, RiscPC700 & A7000 | pre StrongARM RiscPCs |
| RISC OS | 3.7x | ROM | StrongARM RiscPCs & A7000+ | Available as an upgrade with StorngARM CPU card |
| RISC OS | 3.80 | Disc | Developement version for Phoebe (RiscPC 2) | only released to developers |
| RISC OS | 4.0x | ROM | RiscPCs, A7000+, Kinetic RiscPCs, Microdigital Omega and RiscStation R7500. | Completed version of RISC OS 3.80 from RISCOS Ltd. Also available as an upgrade to older RiscPCs and A7000s |
| RISC OS | 4.05 | ROM | RiscPCs, A7000+, Kinetic RiscPCs etc | Support for interlaced screens in PAL & NTSC on Risc PCs and A7000+s. Includes fixes previously available as ROM patches. Upgrade from RISC OS 3.7 (and 4.02), primarily for users of Millipede Professional Graphics systems. |
| RISC OS | 4.xx Select | disc | RiscPCs, A7000+, Kinetic RiscPCs etc | Softloads on top of RISC OS 4.0x. Only available to subscribers to RISCOS Ltd Select scheme |
| RISC OS | 4.39 Adjust | ROM | RiscPCs, A7000+, Kinetic RiscPCs etc | Update to RISC OS 4.0x Adjust32 is 32bit version for A9home |
| RISC OS | 5.xx | ROM | Iyonix | 32bit version for Castle Iyonix |
| RISC OS | 6.xx | disc | RiscPCs, A7000+ | Select 4 & 5 softload version from RISC OS Ltd. |
| Taos | tbd | n/a | Being investigated by Acorn mid-1990s. never released |