Having written a fair share of deeply embedded assembly.. if you're used to writing this close to the metal on a small system, that may have actually have been the most straight-foward way. Since ultimately the score must be rendered to the screen, and that might be its only purpose, there was probably little point in holding a "raw" intermediate value. Adding a constant and a few instructions for rollover don't sound too bad in the grand scheme of things.
figuring out tricks like that looks so fun. my "embedded" experience has been hard real time but usually on things with decent amounts of rom and ram. so tricks like that weren't needed.
