Acorn's 32bit Operating System for ARM CPUs has gone through many changes in the nearly 20 years since its launch. It's name has changed from Arthur to RISC OS to NCOS, and its owners have changed from Acorn to Element 14(?) to Pace to Castle. This is a brief record of the different versions of which I have copies. There are other versions, mainly softloaded development versions (including EPROMed versions for the A500), for details of these and more details in general see Matthias Seifert's Software Evolutions.
| Author | OS Name | OS Version | Date of Utility Module | OS Identifier | ROM Part Number | Comments |
| Acorn | Arthur | 0.30 | 17 Jun 1987 | &A0 | 0276,322-01 0276,323-01 0276,324-01 0276,325-01 | A305, A310 |
| Acorn | Arthur | 1.10 | A300 service manual ROM fitting instructions: The ROM set numbered 1, 2, 3 and 4 although labelled Arthur 1.1 is in fact Arthur 1.2 | |||
| Acorn | Arthur | 1.20 | 25 Sep 1987 | &A0 | 0277,022-02 0277,023-03 0277,024-02 0277,025-02 | A440 |
| Acorn | RISC OS | 2.00 | 05 Oct 1988 | &A1 | 0283,022-01 0283,023-01 0283,024-01 0283,025-01 | A3000 & A4x0/1 |
| Acorn | RISC OS | 2.01 | 05 Jul 1990 | &A2 | 0270,601-01 0270,602-01 0270,603-01 0270,604-01 | A540 and R260 |
| Acorn | RISC OS | 2.06 | ? | ? | n/a | Development version for ARM3 A500 |
| Acorn | RISC OS | 3.00 | 25 Sep 1991 | &A3 | 0270,251-01 0270,252-01 0270,253-01 0270,254-01 | early A5000 |
| Acorn | RISC OS | 3.10 | 30 Apr 1992 | &A4 | 4 ROM Set 0296,041-01 0296,042-01 0296,043-01 0296,044-01 | |
| Acorn | RISC OS | 3.10 | 30 Apr 1992 | &A4 | 2 ROM Set 0296,061-01 0296,062-01 | A4 Extension ROM (0296-063-01) with Power management module |
| Acorn | RISC OS | 3.11 | 29 Sep 1992 | &A4 | 4 ROM set: 0296,041-02 0296,042-02 0296,043-02 0296,044-02 | |
| Acorn | RISC OS | 3.11 | 29 Sep 1992 | &A4 | 2 ROM set: 0296,061-02 0296,062-02 | A3010, A3020 & A4000 |
| Acorn | RISC OS | 3.19 | 09 Jun 1993 | &A4 | 0296,241-01 0296,242-01 0296,243-01 0296,244-01 | German RISC OS 3.11 |
| Acorn | RISC OS | 3.50 | 18 Feb 1994 | &A5 | 0297,521-01 0297,522-01 | RiscPC 600 |
| Acorn | RISC OS | 3.60 | 13 Apr 1995 | &A6 | 1203,101-01 1203,102-01 | RiscPC 700 |
| Acorn | RISC OS | 3.61 | 1 Aug 1995 | &A6 | 1206,520 1206,521 | AOM STB2 |
| Acorn | RISC OS | 3.70 | 30 Jul 1996 | &A7 | 1203,191-01 1203,192-01 | StrongARM support |
| Acorn | RISC OS | 3.71 | 19 Feb 1997 | &A7 | 1203,261-01 1203,262-01 | |
| Acorn | RISC OS | 3.80 | 28 Apr 1998 | &A7 | 1203,364-01 | softloaded Development version for Pheobe Only released to developers |
| RISCOS Ltd | RISC OS | 4.00 | ? | &A8 | ? | Initial release sold at RISCOS SE Show July 99 |
| RISCOS Ltd | RISC OS | 4.01 | ? | &A8 | ? | Sold july-Aug 99 |
| RISCOS Ltd | RISC OS | 4.02 | 10 Aug 1999 | &A8 | 19990818-001 | Final release |
| RISCOS Ltd | RISC OS | 4.03 | 09 Nov 1999 | &A8 | 19991214-001 | RiscStation |
| RISCOS Ltd | RISC OS | 4.03 | 09 Nov 1999 | &A8 | 20000330-002 | Microdigital Mico |
| RISCOS Ltd | RISC OS | 4.03 | ? | &A8 | ? | Castle Kinetic Risc PC |
| RISCOS Ltd | RISC OS | 4.04 | ? | &A8 | ? | Castle Kinetic Risc PC 300MHz |
| RISCOS Ltd | embedded RISC OS | 4.06 | 8 Jan 2003 | &A8 | 20030120-002 | Orpheus development version |
| RISCOS Ltd | RISC OS | 4.29 | 23 Apr 2002 | &A9 | 20020428-001 | soft-loaded RISC OS Select Issue 1 Release 9 |
| RISCOS Ltd | RISC OS | 4.33 | 30 Oct 2002 | &A9 | 20021118-001 | soft-loaded RISC OS Select 2 Issue 3 |
| RISCOS Ltd | RISC OS | 4.37 | 11 Aug 2003 | &A9 | 20030915-001 | soft-loaded RISC OS Select 3 Issue 3 |
| RISCOS Ltd | RISC OS | 4.39 | 10 Mar 2004 | &A9 | 20040317-001 | RISC OS Adjust 1 Issue 1 |
| RISCOS Ltd | RISC OS | 4.39 | 28 Apr 2004 | &A9 | 20040510-003 | soft-loaded RISC OS Select 3 Issue 4 |
| RISCOS Ltd | RISC OS | 4.40 | n/a | &A9 | n/a | Adjust32 for A9home, 1st release |
| RISCOS Ltd | RISC OS | 4.41 | n/a | &A9 | n/a | Adjust32 for A9home, 2nd release |
| RISCOS Ltd | RISC OS | 4.42 | 15 Mar 2006 | &A9 | 20060316-002 | Adjust 32 for A9home retail version |
| Acorn | NCOS | 1.06 | 26 Oct 1996 | 2203,021-01 2203,022-01 | OS Version Module Version 4.50 Date 08 Nov 1996 | |
| Acorn | NCOS | 1.11 | 26 Oct 1996 | 2203,222-01 2203,223-01 | OS Version Module Version 4.50 Date 24 Feb 1998 | |
| Pace | RISC OS-NC | 5.13 | 26 Jan 2000 | &A7 | 2208,670 2208,671 | OS Version Module Version 5.13 Date 19 Oct 1999 Bush Internet Surf Set IBX-100 |
| Pace | RO-STB | 4.0.0 | 25 Sep 2000 | OS Version Module 5.30 Date 28 June 2000 Build STB4001910 Pace DSL4000 | ||
| Castle | RISC OS | 5.03 | &AA | Flash ROM | Castle Iyonix | |
| Castle | RISC OS | 5.05 | 17 Feb 2004 | &AA | Flash ROM | Castle Iyonix |
| Castle | RISC OS | 5.06 | &AA | Flash ROM | Castle Iyonix | |
| Castle | RISC OS | 5.07 | &AA | Flash ROM | Castle Iyonix | |
| Castle | RISC OS | 5.08 | 04 Nov 2004 | &AA | Flash ROM | Castle Iyonix |
| Castle | RISC OS | 5.09 | 18 May 2005 | &AA | Flash ROM | Castle Iyonix |
| Castle | RISC OS | 5.10 | 7 Oct 2005 | &AA | Flash ROM | Castle Iyonix |
| Castle | RISC OS | 5.11 | 22 Feb 2006 | &AA | Flash ROM | Castle Iyonix |
| Castle | RISC OS | 5.12 | 26-Jun-06 | &AA | Flash ROM | Castle Iyonix |
| Castle | RISC OS | 5.13 | 23-Feb-07 | &AA | Flash ROM | Castle Iyonix |
| RISC OS OpenLtd | RISC OS | 5.14 | Flash ROM | soft-loaded or Flash ROM. Castle Iyonix | ||
| RISC OS Open Ltd | RISC OS | 5.16 | 18 Jan 2010 | &AA | Flash ROM | soft-loaded or Flash ROM. Castle Iyonix |
| RISCOS Ltd | RISC OS | 6.02 | 08 Oct 2006 | &A9 | 20061024-iii-virginia | Preview. Select 4i1 |
| RISCOS Ltd | RISC OS | 6.06 | 19 Mar 2007 | &A9 | 20070424-001 | soft-loaded. Select 4i2 |
| RISCOS Ltd | RISC OS | 6.10 | 20 Apr 2008 | &A9 | 20080422-001 | soft-loaded. Select 4.4 |
| RISCOS Ltd | RISC OS | 6.14 | 1 Dec 2008 | &A9 | 20081201-001 | soft-loaded. Select 5i1 |
| RISC OS Ltd | RISC OS | 6.16 | 19 Apr 2009 | &A9 | 20090419-001 | soft-loaded. Select 5i2 |
Images of some of these ROMs are available from Tribbeck.com, but please note the copyright.