crapola / air_buster_md_fix

IPS patch for Air Buster (Sega Genesis)

Air Buster Fix

This is an IPS patch for Air Buster / Aero Blasters on the Sega Genesis that fixes the freeze in stages 4 and 5.

Detailed description

The bug happens in low gravity sections when you press too many directions at the same time (specifically left, down and right).
The faulty function:

loc_0000D26A:
	SUBQ.w	#4, $1A(A1)
	MOVE.w	D1, D2
	ANDI.b	#$0F, D0
	ASL.w	#2, D0
	JMP	loc_0000D27A(PC,D0.l)
; Followed by 11 jump destinations.

At this point, D0's lower nibble contains input information in the form:

direction: RLDU
      bit: 3210

There are 11 functions to jump to, so the valid range for D0 before alignment is (0,10).
A value of 10 corresponds to right (8) + down (2) which is probably the highest you could get using the original hardware. A modern emulator that doesn't filter "wrong" inputs makes it easy to exceed this value and cause a crash.