[Tomte]

Mikrosim maybe? The name is because it simulates microcode. That was a huge a-ha moment for me in school.

https://en.wikipedia.org/wiki/MikroSim

http://www.mikrocodesimulator.de/index_eng.php

[paranoidrobot]

Good find, thanks.

That one is looking more familiar the more screenshots I see of it, but I think I might be mixing memories of two programs - this, and "CPU Sim" which I mention in a reply to the sibling comment.