Alien 8: Routine at AD13

Reached through UPDATES. Knight Lore's spiked ball: deadly both ways, it hangs until a turn when the random number is under 16 (one in sixteen), no other is falling (DROPPING), and the drop latch (DROP_LATCH) is clear -- which in an even-numbered room it is not until something has been picked up. Then it falls, with a sound pitched by its height, lands with a thud (THUD_ON_LANDING), and lets the next one go.

Bit 2 of +D: falling. Once on the floor it can be let go again, but it has nowhere to fall and makes no second thud. Rooms $0C, $1C, $58 and $84 have them (object template 20).

Reached through UPDATES. Knight Lore's spiked ball: deadly both ways, it hangs until a turn when the random number is under 16 (one in sixteen), no other is falling (DROPPING), and the drop latch (DROP_LATCH) is clear -- which in an even-numbered room it is not until something has been picked up. Then it falls, with a sound pitched by its height, lands with a thud (THUD_ON_LANDING), and lets the next one go. Bit 2 of +D: falling. Once on the floor it can be let go again, but it has nowhere to fall and makes no second thud. Rooms $0C, $1C, $58 and $84 have them (object template 20).
CEILING_DROP	AD13	CALL MAKE_DEADLY	deadly both ways
	AD16	CALL DRAW_AT_L12_D8	draw 12 left and 8 down
	AD19	LD A,(DROP_LATCH)	latched: nothing drops
	AD1C	AND A
	AD1D	RET NZ
	AD1E	BIT 2,(IX+$0D)	falling
	AD22	JR NZ,CEILING_DROP_0	falling
	AD24	LD HL,DROPPING	another is falling
	AD27	LD A,(HL)
	AD28	AND A
	AD29	RET NZ
	AD2A	LD A,(RANDOM)	one turn in sixteen
	AD2D	CP $10
	AD2F	RET NC
	AD30	SET 2,(IX+$0D)	let go
	AD34	LD (HL),$01
	AD36	RET
CEILING_DROP_0	AD37	CALL DEC_DZ_AND_UPDATE_UVZ	fall
	AD3A	CALL THUD_ON_LANDING	a thud if it has just landed
	AD3D	BIT 2,(IX+$0C)	landed
	AD41	JR NZ,CEILING_DROP_2	landed
	AD43	CALL BEEP_BY_Z	a sound pitched by its height
CEILING_DROP_1	AD46	JP SET_WIPE_AND_DRAW_FLAGS	redraw
CEILING_DROP_2	AD49	RES 2,(IX+$0D)	landed: another may fall
	AD4D	LD HL,DROPPING
	AD50	LD (HL),$00
	AD52	JR CEILING_DROP_1