Looks like they did quite a few Unix boxes, not that anyone really ever bought them.
http://acorn.chriswhy.co.uk/RISCiXComputers.html#R140The original Archimedes ran an OS called "Arthur", later updated to RISC OS version 2. The RISC OS 3 ROMs that Dave identified as probably being newer than the rest of the unit were indeed an upgrade.
I still have a set of the original RISC OS 2 ROMs, manufactured by Toshiba, sitting in a drawer from when I upgraded my own A3000 back in the early '90s!
It was a nice machine - remarkably fast and capable for its day - but always suffered from a lack of memory, and its early performance advantage over the competition was never really maintained. The ARM2 processor could, on many machines, be removed and upgraded to an ARM3 processor on a carrier board, which added a cache and a speed boost to 25 or 33 MHz. RAM upgrades were costly, and the genuine Acorn 2MB upgrade (1MB on the main board + 1MB on the expansion board) on Dave's machine was quite uncommon as there were plenty of cheaper, functionally identical 3rd party alternatives.
After a few years I upgraded from the A3000 to the newer A5000, which had the ARM3 processor, a built in IDE controller and room inside the case for a hard disc - an expensive peripheral which was only just becoming commonplace. It also had a faster video clock, which allowed it to display higher resolutions and was really worthwhile.
The GUI was attractive and usable, and there's never been an easier machine to start programming since. At any time, press F12 and the GUI scrolls up and you have a command line. Type 'BASIC' and you get BBC BASIC version V, which includes a full ARM assembler. C was available as a commercial package from Acorn and I regret not learning it back then - but there was no need, BBC BASIC was so fast and capable.
Despite the fact that RISC OS used co-operative rather than pre-emptive multi-tasking, it was a lovely machine to use, and way ahead of the competition in many ways. Unfortunately it never caught on commercially in the way the PC did. I still miss my A5000.