Phil Tipping has built a remarkable “computing machine simulator” which runs one of three sets of microcode, to act as an educational machine or a 32 bit advanced machine with a small bootstrap ROM. There’s a lights-and-switches front panel, including a small screen, and it can drive a printer too.
PlasMa Mini-mainframe simulator project
Phil has ported Tiny Basic, and also a Forth. And with them, there’s a floating point facility, and a Lunar Lander…
It makes no pretence to simulate any specific computer, either in design or performance; its aim is purely to rekindle the hands-on ‘lights and switches magic’ from that era, and explore the challenges of programming a machine from scratch.
There’s also a whole playlist of videos, including a pi spigot written in PleX assembly language, which can be seen here:
Phil also supplies full building instructions - it’s based on an Arduino MEGA.