It is really simple: the game accepts three character long inputs as commands. 15 instructions are documented int the TIS-100 datasheet (the game's manual).

So there are 26*26*26 = 17576 possible combinations which may or may not be a legal command.

The plan is to use an Arduino Leonardo in HID mode to enter all these combinations, a blank right after a combination followed by F12 (the hotkey for screenshots). I allow the game a pause of 200 ms after entering the blank to bring up the error popup that is shown whenever the input is not a known command. And another 500 ms delay to allow the system to take the screenshot and save it.

It should take roughly four hours to complete the cycle.

To avoid having to look though all 17576 screenshots by myself I will use GNU-octave to look through the images and search for the red error message. Every image without this message contains a command. So there should be at least 16 (it is known that there is at lease one undocumented command).