Activates different IRQ/NMI behavior for various game modes, depending on the following values:
#$00 = Regular level.
#$01 = Mario Start, Time Up, etc. + Title Screen + Castle destruction scene.
#$02 = Overworld.
#$80 = Iggy's and Larry's battle mode.
#$C0 = Reznor's, Morton's, Roy's, and Ludwig's battle mode.
#$C1 = Bowser's battle mode.