In 1988 Acorn released their next OS for the Archimedes. Unlike previous OSes the name had changed from Arthur to RISC OS. Sometimes people get confused as to whether they're separate developments or just a renaming.
In truth it is just a rename. The commonly accepted reason for this is the release of the Dudley Moore comedy film 'Arthur 2 - On the rocks' also in 1988, which would have had led to marketing disaster.
If you want to be sure of the rename, let us check the source code.
In the current RISC OS (5) kernel files called Arthur2, Arthur3 and ArthurSWIs are still present, 26 years later.
Even more compelling is this snippet from the RISC OS 2 kernel source code.
GBLS SystemName SystemName SETS "RISC OS" ; ", p.k.a. Arthur a.k.a. Richard III"
RISC OS, previously known as Arthur also known as Richard III.
So RISC OS is a simple rename for version 2. That all makes sense, but 'also known as Richard III'? Well that is a bit of cockney rhyming slang.
RISC OS a.k.a. Turd
I am slightly disappointed that, in the release of the RISC OS source code by ROOL, all the bad language was removed. There's a lot of fun to be had in programmer's honest views on what they work on :-).