Here's a full list of the Global Variables used by Shmupkit, I've tried to leave as many as possible free for you own use in your games.
VARIABLES USED BY SHMUPKIT ========================== A - available for use in your code B - available for use in your code C - ENEMY COLOUR D - KILLS TO GET BONUS E - # ENEMIES IN WAVE F - ENEMIES LEFT G - ENEMY RELEASE DELAY H - ENEMY WEAPON FIRE PROBABILITY I - X DISTANCE FOR ENEMY WEAPON TRIGGERING J - ENEMY SPRITE TYPE K - ENEMY IMAGE L - LEVEL/WAVE M - PLAYER X FOR USE IN OTHER EVENTS N - PLAYER Y FOR USE IN OTHER EVENTS O - available for use in your code P - POINTS FOR A KILL Q - HEIGHT FOR ALIEN WEAPON RELEASE R - available for use in your code S - SPRITE COUNTER T - TRIGGER U - available for use in your code V - available for use in your code W - ENEMY WEAPON Z - MUSIC (reserved)