> architecturally, it had nothing to do with the original BBC Micro.

This is not true. It had lots to do with it.

* The Archimedes used a CPU whose designers (Sophie Wilson and Steve Furber) have specifically said they built it to be conceptually similar to the 6502. Source: I have met both and seen them say this in person.

* The Archimedes ran RISC OS which is a rewrite of the BBC MOS. Source: I have interviewed the project leader, Paul Fellows.

https://www.theregister.com/2022/06/23/how_risc_os_happened/

The Archie came with !6052tube and could run some BBC apps.

Source code: http://bbc.nvg.org/rom/Acorn/os/?C=N;O=A