[Slothie]

I remember building a joystick interface for my PET back in the day that used a capacitor charging up through the potentiometer in the joystick and a comparator, so you would pulse a line to short out the capacitor and time how long it took for the capacitor to charge up. Thus you could get a very el-cheapo A-D conversion. I think I got it working well enough, but I don't recall actually writing any games to use it! But it might be an interesting and era-appropriate way to do it. Alternatively you could use a 555 to make a variable width pulse and time that, which would also be era-appropriate.