Last night I wrote a dirty little two-pass assembler and an emulator, which makes it a whole lot easier to write and test software for my machine!
The 'driver' software for my printer, that converts binary numbers to binary coded decimals (+ polling the busy-line of the interface + writing out the data) is working, the (very slow and crude) prime finding algorithm is done too!
It looks like everything is ready to go! Now all I need to do is to load the code into the machine through the binary interface, debug and run it - which will probably take me an afternoon! (code already takes up ~200bytes = 4/5 of the main memory)

My baby will finally proove its's usefulness :D I'll post another video once it works! (timelaps might be necessary hehe)