river-raid: Routine at 6724

6724: Handle fire button input

Used by the routines at scan_cursor, scan_kempston, scan_sinclair and scan_keyboard.
Creates a new missile if none is currently active. Positions missile at plane X + 4, Y = $7E (just above plane). Sets CONTROLS_BIT_FIRE_SOUND to trigger the fire sound effect.
handle_fire	6724	LD A,($5EF3)	Return if missile already active (Y != 0).
	6727	CP $00
	6729	RET NZ
	672A	LD A,($5F72)	Create missile at (plane_X + 4, $7E).
	672D	ADD A,$04
	672F	LD B,$7E
	6731	LD C,A
	6732	LD ($5EF3),BC
	6736	LD HL,$6BB0	Set CONTROLS_BIT_FIRE_SOUND in state_controls.
	6739	SET 0,(HL)	Set CONTROLS_BIT_FIRE_SOUND in state_controls.
	673B	RET