CREDIT CHECK
CREDIT_CHECK ($C00450)
BIOS routine called to check if there's enough credits for a player.
Input: Credits necessary in BIOS_CREDIT_DEC1 ( $10FDB0) for player 1, and BIOS_CREDIT_DEC2 ( $10FDB1) for player 2. Those locations are set to 0 if there aren't enough credits.
Call CREDIT_DOWN after.
Credits can be read through $D00034 and $D00035.