Imagesource: https://jimhall.itch.io/

Jim F. Hall aka @jimfhall might be familiar to some. At least to those who like to spend their time with machines that typically render white text on a black background as the communication medium of choice with the user. We are talking about FreeDOS - Jim is initiator and maintainer and fights for the continuance of the product until today.

But we don't want to talk about FreeDOS at all ... much too young and a few bits too much in the requirements. Rather Jim also teaches basic functionality of computers in university courses and was looking for illustrative material ala the Altair 8800 but in hardware form.

Since none of what he could find, was simple enough, Jim simply pulled his own ISA out of the hat and implemented it in several iterations in the form of the Toy CPU Simulator.

This gem runs on every DOS system, that has ever seen the light of day, and has an interface based on the Altair's frontpanel ... (mmmhhh LEDs and Binary ⚫️⚫️🔴⚫️🔴⚫️🔴⚫️).

The source can be found on github but downloading on itch.io will probably lead to faster success. If the combination of simulated CPU / minimal ISA / DOS makes your saliva drip from your teeth, you've come to the right place.