> [the Z80's] instruction set reminded me too much of the idiotic intel 80x86 family

That is because the z80 was a (mostly) binary compatible superset of the 8080 instruction set, and the 8086 (while not binary compatible) was intentionally modeled after the 8080 such that there was a program that could take an 8080 program and do the 1:1 instruction mapping from 8080 to 8086 and get a working 8086 program.

> I wonder if it would be possible to retrofit an MMU onto a Z80 design

Zilog made a number of Z80 follow-on CPUs, and some have MMUs. It is amazing how far they took the instruction set; the Z80380 could run Z80 code as-is, but it also extended instructions to support 32b operations and had a mode with a flat 32b linear address space. I don't recall ever hearing any consumer product using it though.

Z80380 user’s manual, for those interested: https://www.manualslib.com/manual/1237352/Zilog-Z80380.html

This guy ran Linux on an ARM emulator on an ATmega. A similar feat should be possible on a Z80.

http://dmitry.gr/?r=05.Projects&proj=07.%20Linux%20on%208bit

I remember that. That was emulation and it was not usable the way a 75 MHz Z80 could be, running code directly on the CPU (even if it is an FPGA).

If you want to see a Unix like OS on an 8-bit CPU, get a Motorola 6809. It was designed with high level languages in mind (four 16-bit index registers, two 8-bit accumulators that can act as a single 16-bit accumulator) and can even do position independent code quite easily (PC-relative addressing modes). Quite an underrated CPU.

You could run something like Inferno without an MMU: "Limbo programs run safely on a machine without memory-protection hardware". It would probably be more efficient than emulating some other architecture. Arguably, it'd be better than Unix :p

FUZIX is a tiny UNIX that runs on a Z80 using memory banking rather than an MMU.

https://github.com/EtchedPixels/FUZIX