Thanks guys. So the actual CPU used didn't have an MMU, and when MMUs were available the usual problems of compatibility with an existing codebase got in the way. It makes sense now.
Changing bus size is a big deal, isn't it? I want to go all 64-bit with my system, but still have some 32-bit hardware I'd like to run.
I forgot to reply to this yesterday:
Solar wrote:
The greatest feature -- which kept Amiga in the market long after Commodore croaked -- was the Expansion Port, which routed all the necessary signals to have newer CPUs on expansion boards.
Yeah, replacing the CPU is awesome! Atari 800 had a good expansion bus, but not that good.
You could override memory access but not the CPU. Actually I might be wrong about that; it depends if DMA lines were brought out to the port... but no, there weren't the 2 DMA lines needed to stop the 6502 and still receive DMA signal from the ANTIC. It's probably possible on the 130XE by using RAM not currently bank-switched for video. OTOH I don't think it's possible to RAM-expand the 130XE without soldering internally. It is possible to plug RAM expansions into the earlier models. Hmm... with external memory, an external CPU could be made to work, but still requires a lot of thought on how to synchronize it with the DMA & everything else. I suspect I'd find it easier to design my own computer with video output and all.
Thinking about all this made me realise what a lot goes into an expansion port like the Amiga's!