Chase H.Q.: Routine at F269

F269: Play the siren sound effect (128K)

If the siren is enabled this produces an alternating rising and falling siren sound effect. AY channels A and B are used. Lives at $805D when relocated.
play_siren_sfx_128k	F269	LD A,($A239)	Return if siren disabled
	F26C	AND A
	F26D	RET Z
	F26E	LD A,($A213)	Load channel A fine pitch
	F271	LD B,$AA	Load 50-50 alternating pattern (self modified @ F260 above)
	F273	RLC B	Jump to increasing case if top bit was set
	F275	JR C,pss_increasing	Jump to increasing case if top bit was set
Decreasing case.
pss_decreasing	F277	SUB $03	Decrease fine pitch by 3
	F279	CP $5A	Jump to set registers if new fine pitch >= 90
	F27B	JR NC,pss_set_regs	Jump to set registers if new fine pitch >= 90
	F27D	JR pss_change	Otherwise went too far: jump to pss_change
Increasing case.
pss_increasing	F27F	ADD A,$03	Increase fine pitch by 3
	F281	CP $8C	Jump to set registers if new fine pitch < 140
	F283	JR C,pss_set_regs	Jump to set registers if new fine pitch < 140
Arrive here if new fine pitch is outside of 90..139.
pss_change	F285	EX AF,AF'	Preserve new fine pitch
	F286	LD A,B	Store rotating pattern (self modifying F271 above)
	F287	LD ($8066),A	Store rotating pattern (self modifying F271 above)
	F28A	EX AF,AF'	Restore new fine pitch
Arrive here if new fine pitch is 90..139.
pss_set_regs	F28B	LD ($A213),A	Update channel A fine pitch
	F28E	SUB $04	and store 4 less to channel B fine pitch (~23Hz higher than A)
	F290	LD ($A215),A
	F293	LD A,($A21A)	Set mixer to enable tone A & B
	F296	AND $3C
	F298	LD ($A21A),A
	F29B	JR write_audio_registers_128k	Exit via write_audio_registers_128k