Chase H.Q.: Routine at 8204

8204: Generates engine noise (48K)

Used by the routine at start_siren_hook.
setup_engine_sfx_48k	8204	LD HL,($A24A)	Load speed into HL
	8207	RR H	Bottom bit of H moves to carry (H now unused)
This entry point is used by the routine at page_in_stage_128k.
setup_engine_sfx_48k_0	8209	LD A,L	Speed low byte
	820A	RRA	Halve speed, shifting carry in as MSB
	820B	CPL	A = ~A
	820C	SRL A	Divide by four
	820E	SRL A	Divide by four
	8210	OR $01	A \|= 1
	8212	LD L,A	HL = 0x0300 \| A
	8213	LD H,$03	HL = 0x0300 \| A
	8215	LD A,($A253)	Jump if in high gear
	8218	AND A
	8219	JR NZ,setup_engine_sfx_48k_1
	821B	SRL L	L >>= 1
setup_engine_sfx_48k_1	821D	LD A,($A23B)	Jump if tunnel_sfx == 0
	8220	AND A
	8221	JR Z,setup_engine_sfx_48k_2
	8223	LD H,$01	H = 1
setup_engine_sfx_48k_2	8225	LD A,L	Self modify iterations -- L
	8226	LD ($8244),A	Self modify iterations -- L
	8229	LD A,H	Self modify delay loop -- H
	822A	LD ($8249),A	Self modify delay loop -- H
	822D	NEG	Self modify delay loop -- (5 - A)
	822F	ADD A,$05
	8231	LD ($8251),A
This entry point is used by the routine at start_siren_hook.
play_engine_sfx	8234	LD A,($A230)	Return if perp_caught_phase is >= 3 (when car stops)
	8237	CP $03
	8239	RET NC
	823A	LD A,$00	Self modifying counter produces 0,1,2
	823C	INC A
	823D	AND $03
	823F	LD ($823B),A
	8242	RET NZ	Return if nonzero
This produces the engine sound effect.
	8243	LD C,$03	<Self modified> iterations
setup_engine_sfx_48k_3	8245	XOR A	Output 0
	8246	OUT ($FE),A	Output 0
	8248	LD B,$00	Delay for B cycles (self modified above)
setup_engine_sfx_48k_4	824A	DJNZ setup_engine_sfx_48k_4	Delay for B cycles (self modified above)
	824C	LD A,$18	Output EAR + MIC
	824E	OUT ($FE),A	Output EAR + MIC
	8250	LD B,$00	Delay for B cycles (self modified above)
setup_engine_sfx_48k_5	8252	DJNZ setup_engine_sfx_48k_5	Delay for B cycles (self modified above)
	8254	DEC C	Loop until iterations is zero
	8255	JR NZ,setup_engine_sfx_48k_3	Loop until iterations is zero
	8257	RET	Return