I haven't tested too much yet (or read the posts in this thread so far, so I apologize if someone's already said this before), but I really like it so far

Couple things:

• It crashes without warning on a TI-83 Plus. (But then this is only a beta, so I can forgive you for not checking )
• Is there a particular reason you use GRAPH to end calibration? I think any of 2nd, CLEAR, ENTER, or ON would be a bit more intuitive. (On the other hand, it forces people like me to read the manual )
• The hand with the gun looks great, but it's a bit too detailed compared to the scenery IMHO. Maybe make it bit more blurry? (I know that's an odd request lol)
• The brick-wall background looks great when zoomed in close, but it acts a bit weird in the distance (it tricked me into thinking I was turning the wrong way at one point). I can get used to it though.

Or gray with black borders, since the game's in grayscale anyway? I have no idea how that would complicate the code, though.

Hmm I forgot to bring this up before but the recent return of Coelurus reminded me that Gemini was created in the days of Mirage OS 1.1, Ion and OS 1.12.

You should maybe check the code to make sure it's compatible with OS 1.19, because I know a lot of old games from 1999-2002 tend to crash regularly on OS 1.14 or above. While your game might crash sometimes, Gemini itself crashed more if I used something else than MirageOS 1.1 and/or OS 1.12.

Oh right I forgot OS 1.12 doesn't run on the 15 MHz models. Still, problems started to occur in OS 1.14, though (not just with Gemini. I know ARPGCS for instance crashed every exit or close), and OS 1.13 runs on 15 MHz models (I really don't recommend it though lol)

Yeah the 83+SE ran OS 1.13 through 1.19 while the 83+ ran 1.12 through 1.19. The 83+SE was discontinued since July 2004, though. Not sure for how long the 83+ will be available, since certain countries no longer have them for sale individually for students.