Knight Lore: Routine at B7A9

The spike_ball_fall block (spike_ball_fall). Deadly to touch. It hangs where the room put it until it is let go, then falls under gravity to whatever is below and stays there.

Only one spiked ball in a room falls at a time: $5BBF is set while one is on its way down, and a waiting ball lets go with a chance of 1 in 16 each frame when it is clear. None falls at all while $5BC0 is non-zero. That byte is set on entering a room to bit 0 of the room number and is cleared when the player picks something up, so in odd-numbered rooms the balls wait until then.

The spike_ball_fall block (spike_ball_fall). Deadly to touch. It hangs where the room put it until it is let go, then falls under gravity to whatever is below and stays there. Only one spiked ball in a room falls at a time: $5BBF is set while one is on its way down, and a waiting ball lets go with a chance of 1 in 16 each frame when it is clear. None falls at all while $5BC0 is non-zero. That byte is set on entering a room to bit 0 of the room number and is cleared when the player picks something up, so in odd-numbered rooms the balls wait until then.
upd_63	B7A9	CALL set_both_deadly_flags	deadly both ways; pixel offsets
	B7AC	CALL upd_6_7	deadly both ways; pixel offsets
	B7AF	LD A,($5BC0)	held in this room
	B7B2	AND A
	B7B3	RET NZ
	B7B4	BIT 2,(IX+$0D)	already falling
	B7B8	JR NZ,spiked_ball_drop
	B7BA	LD HL,$5BBF	another ball is falling
	B7BD	LD A,(HL)
	B7BE	AND A
	B7BF	RET NZ
	B7C0	LD A,($5BA5)	1 in 16: let go
	B7C3	CP $10
	B7C5	RET NC
	B7C6	SET 2,(IX+$0D)	falling, and claim the one-at-a-time flag
	B7CA	LD (HL),$01
	B7CC	RET