It's a really great concept for a Zach-like game. I know that the intention is to let the player figure out the instructions themselves, but even having this in mind there should still be some kind of even vague explanation to just get the player started and interested in experimenting with the instructions. Also figuring out the instructions would be a lot easier if the game had an option to manually step through instructions. I really enjoy games like that, so even though I didn't get far, I'll probably save this game for later and try to figure it out and beat it, but for now I'll leave it as I don't have enough time to sit and think through this and there are other games to check out
As a side note, shouldn't the AZ instruction be named ZA? There's the AB instruction that seems to move B to A, so going with that logic AZ would move the value from register Z to A and not A to Z as it's supposed to to get the output, or maybe I'm understanding something wrong, but again it would be easier to get it if I was able to step through the instructions