CREDIT CHECK

CREDIT_CHECK ($C00450)
System ROM routine called to check if there's enough credits for a player.

Input: Credits necessary in for player 1, and  for player 2. Those locations are set to 0 if there aren't enough credits.

Call CREDIT_DOWN after.

Credits count are located in backup RAM at $D00034 and $D00035. Only the system ROM should use these locations directly.