That is awesome. I'm currently down a similar rabbit hole, having found Eightyone zx81 emulator, and z88dk c compiler dev kit for Z80. Trying to make interesting programs that run acceptably fast/slow on those old computers, has a nostalgic haiku feel to it, that i really miss with modern computers. Suddenly you start to care about how you loop and assign in your C program, because doing it the 'wrong' way makes the compiler emit inefficient assembly; it's 'roguelike programming', in the sense that all your choices suddenly have consequences.

So ? is an abbreviation for PRINT. (I don't remember that, but I had a Vic-20.)