When I started to look at the instruction set of the 816 more closely (and a coworker of mine was writing an emulator at the same time) it became clearer to me

When I started to look at the instruction set of the 816 more closely (and a coworker of mine was writing an emulator at the same time) it became clearer to me why Mensch went the route he did; a) there just wasn't room in the 8-bit instruction encoding in the '02 to cleanly add 16-bit (or 24-bit etc) versions of each instruction. So instead of moving out into multibyte instructions, he introduced the mode switch and b) he seems to have done this by doing very minimal surgery to the 6502 core itself, basically adding some 16-bit shunts (depending on mode) in front of the decoder to just change the register sizes and behaviors really. (apart from the improvements like adding relocatable direct page and movable stack, like the 6809 got) c) apple wanted something that fit in a 40-pin DIP, to fit into existing motherboard design constraints presumably... and for it to support full 6502 compatibility. so we got this awful address bus multiplexing thing, and even after moving up into other packaging, he never fixed that (oddly he later made a microcontroller, the 65C265, that has all 24 pins without multiplexing). If he'd just started with PDIP-64 like the MC68000, so much agony would have been saved. I think it's a shame really. We could have had a 16/24 version of the 6502 with fewer compromises if the 816 had been a more "serious" project from Apple. But at the time the IIgs was in progress and WDC commissioned to work on the chip, I suspect there was already the politics within Apple about killing the Woz-line in favour of the 68k Mac. We had IIgs systems in my primary school, and they only ever got used as Apple II clones. I had an Atari ST, and while the 68000 was a great chip, it was clearly targeted towards "VAX" class or workstation type applications originally. Its overall complexity is quite high, and interrupt responsiveness very slow, when compared to the 6502. It would have been interesting to see a Mensch-designed 16/[24/32]-bit processor that kept the same minimalist philosophy as the 6502. Paired with a chipset like what was in the Amiga, I think it would been really cool. Though maybe the ARM line is that, in some ways.