Atic Atac: Routine at 83EA

Used by the routine at UPDATE_KNIGHT.

Monsters are not placed once and left. Called once a frame from PLAYER_TICK: entering a room names it in $5E26 and starts a 32-frame countdown at $5E27; when that expires a creature is dropped into the room, and after that one more on any frame the refresh register's low nibble is zero -- which is why standing still in one place does not make you safe.

It looks for a free slot among only the first three of the eight monster records, and gives up if all three are taken. Those three are also the ones INERT_SPRITE bothers to burn time for, so the spawning slots and the timed slots are the same three.

A new monster is a straight sixteen-byte copy of a template at MONSTER_TEMPLATE -- one LDIR, and the record is complete.

Used by the routine at UPDATE_KNIGHT. Monsters are not placed once and left. Called once a frame from PLAYER_TICK: entering a room names it in $5E26 and starts a 32-frame countdown at $5E27; when that expires a creature is dropped into the room, and after that one more on any frame the refresh register's low nibble is zero -- which is why standing still in one place does not make you safe. It looks for a free slot among only the first three of the eight monster records, and gives up if all three are taken. Those three are also the ones INERT_SPRITE bothers to burn time for, so the spawning slots and the timed slots are the same three. A new monster is a straight sixteen-byte copy of a template at MONSTER_TEMPLATE -- one LDIR, and the record is complete.
SPAWN_MONSTER_INTO_ROOM	83EA	LD A,($5E26)	Only in the room the spawner is watching.
	83ED	LD C,A
	83EE	LD A,($EA91)
	83F1	CP C
	83F2	JR NZ,SPAWN_MONSTER_INTO_ROOM_3	Wrong room: nothing to do.
	83F4	LD HL,$5E27	The countdown to the next one.
	83F7	LD A,(HL)
	83F8	AND A
	83F9	JR Z,SPAWN_MONSTER_INTO_ROOM_4
	83FB	DEC (HL)	Not yet.
	83FC	RET NZ	Not yet.
SPAWN_MONSTER_INTO_ROOM_0	83FD	LD HL,$EE60	Three slots, sixteen bytes apart.
	8400	LD DE,$0010
	8403	LD B,$03
SPAWN_MONSTER_INTO_ROOM_1	8405	LD A,(HL)	A zero sprite means the slot is free.
	8406	AND A
	8407	JR Z,SPAWN_MONSTER_INTO_ROOM_2
	8409	ADD HL,DE
	840A	DJNZ SPAWN_MONSTER_INTO_ROOM_1
	840C	RET
SPAWN_MONSTER_INTO_ROOM_2	840D	EX DE,HL
	840E	LD HL,MONSTER_TEMPLATE	The template.
	8411	LD BC,$0010	The template.
	8414	PUSH DE
	8415	LDIR	Sixteen bytes, and the monster exists.
	8417	POP HL
	8418	PUSH HL
	8419	INC HL
	841A	LD A,($EA91)
	841D	LD (HL),A
	841E	INC HL
	841F	PUSH HL
	8420	LD HL,SPAWN_TYPES
	8423	LD A,($5C78)
	8426	AND $0F
	8428	LD E,A
	8429	LD D,$00
	842B	ADD HL,DE
	842C	LD A,(HL)
	842D	POP HL
	842E	LD (HL),A
	842F	LD DE,($5E1D)
	8433	LD B,E
	8434	CALL RANDOM_POSITION
	8437	LD (HL),A
	8438	LD B,D
	8439	CALL RANDOM_POSITION
	843C	LD (HL),A
	843D	POP DE
	843E	PUSH IX
	8440	LD IX,$0000
	8444	ADD IX,DE
	8446	CALL RANDOM_VELOCITY
	8449	CALL DRAW_THING
	844C	POP IX
	844E	RET
SPAWN_MONSTER_INTO_ROOM_3	844F	LD ($5E26),A
	8452	LD A,$20
	8454	LD ($5E27),A
	8457	RET
SPAWN_MONSTER_INTO_ROOM_4	8458	LD A,R
	845A	AND $0F
	845C	RET NZ
	845D	JR SPAWN_MONSTER_INTO_ROOM_0