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talex5 | 3 years ago
v2 at least had 4 modes: user mode, supervisor, IRQ and FIQ. They were encoded in the low 2 bits of r15 (which weren't otherwise needed, since the PC was always word-aligned).
The original Archimedes was intended to run a "preemptively multi tasking, multi-threaded, multi-user, written in Acorn-extended Modula2+" system called ARX. But when that wasn't ready in time, they ended up just porting the 8-bit BBC Micro MOS to the new 32-bit machines!
This is a great read about it all:
http://www.rougol.jellybaby.net/meetings/2012/PaulFellows/in...
klelatti|3 years ago
> Now, the aforementioned, Arthur Norman, who had written the LISP interpreter for the BBC Micro, and was in and out of Acorn the whole time, had come up with a design for a machine with 3 instructions called the SKI machine. Whose instructions were called 'S', 'K' and 'I' and it ran LISP in hardware and he built one of these things, it never worked, it was so big and covered in wire-wrap it was always broken. The idea was to prove that you really didn't need very many instructions for a general purpose machine.
pinewurst|3 years ago