I have in mind then running a strange hybrid OS which is basically RISC OS, but emulates 6502 code when required, using the Acorn MOS for some bits of IO, running languages and filesystems from sideways ROM (or emulated sideways ROMs), etc. It feels like it would be possible to borrow a lot of techniques from from Springboard, !65Host and !65Tube.
It would also be possible to assist with a smidgen of custom hardware. The first thing which occurs to me is a specialised memory controller which, as well as all the usual stuff, can:
- Present a region of bytes in physical memory as a series of 64-bit words in virtual memory, each encoding something like ADR PC,next_instruction : ADD PC,r4,#___<<n, providing efficient dispatch for emulated 6502 instructions
- Present BBC Micro screen memory re-ordered as VIDC screen memory