Chase H.Q.: Routine at F251

F251: Start the siren sound effect (128K)

Lives at $8045 when relocated. Presets the AY register soft copies for the two-tone siren: channel A fine pitch 140, channel A volume 14 and channel B volume 12. $AA seeds the rotating pattern that play_siren_sfx_128k steps the pitch by, and doubles as the "siren on" flag in $A239.
start_siren_128k	F251	LD A,$8C	Store 140 to channel A fine pitch (~792Hz)
	F253	LD ($A213),A	Store 140 to channel A fine pitch (~792Hz)
	F256	LD A,$0E	Store 14 to channel A volume (4-bit)
	F258	LD ($A21B),A	Store 14 to channel A volume (4-bit)
	F25B	SUB $02	Store 12 to channel B volume
	F25D	LD ($A21C),A	Store 12 to channel B volume
Initialise alternating pattern for siren tone.
	F260	LD A,$AA	Self modify 'LD A,x' @ F271 below
	F262	LD ($8066),A	Self modify 'LD A,x' @ F271 below
	F265	LD ($A239),A	Enable siren (storing $AA for bool)
	F268	RET	Return