Chase H.Q.: Routine at F7D6

F7D6: Starts tune A and sets up its sound-effect trigger table

Plays tune A (via start_tune), looks up a pointer in the table at $FA75 (indexed by A*2) into a per-tune SFX script, and clears the 3 SFX "busy" flags at $F837/$F895/$F8A2. Falls into the script reader below, whose byte-code and interaction with titlescr_music's SFX dispatch is documented at titlescr_drum_advance. Used by the routines at insert_high_score_entry, name_entry_input, run_title_screen, play_success_music, options_menu_driver and run_title_tune.
titlescr_start_tune	F7D6	PUSH AF
	F7D7	CALL start_tune
	F7DA	POP AF
This entry point is used by the routine at run_title_screen.
load_drum_script	F7DB	RLCA
	F7DC	LD C,A
	F7DD	LD B,$00
	F7DF	LD HL,$FA75
	F7E2	ADD HL,BC
	F7E3	XOR A
	F7E4	LD ($F8A2),A
	F7E7	LD ($F895),A
	F7EA	LD ($F837),A
	F7ED	LD A,(HL)
	F7EE	INC HL
	F7EF	LD H,(HL)
	F7F0	LD L,A
	F7F1	JP load_drum_op
SFX script byte-code reader: reads opcode bytes from the script pointer at $F7FC, with $FE meaning "jump to a new script pointer read from the following 2 bytes" ($F823) and $FF meaning "end of script, disable interrupts and stop the tune" (stop_music_and_silence via $F829). Any other byte is a delay/repeat value (stored to $F7F5 and used to throttle titlescr_drum_advance's own re-entry -- it decrements $F7F5 and returns early via RET NZ until it counts down to 0). The following script byte is a raw byte offset (not a scaled index) into the table at $FAA4: each entry there is 3 bytes -- a selector byte (copied to both $F84E and $F842, read back by titlescr_music) followed by a 2-byte pointer (stored to $F85E, the reload source for titlescr_music's sample-selector-stream pointer at $F853). This entry point is used by the routine at titlescr_music.
titlescr_drum_advance	F7F4	LD A,$00
	F7F6	DEC A
	F7F7	LD ($F7F5),A
	F7FA	RET NZ
	F7FB	LD HL,$0000
load_drum_op	F7FE	LD A,(HL)
	F7FF	INC HL
	F800	CP $FE
	F802	JP Z,titlescr_start_tune_1
	F805	CP $FF
	F807	JR Z,titlescr_start_tune_0
	F809	LD ($F7F5),A
	F80C	LD C,(HL)
	F80D	INC HL
	F80E	LD ($F7FC),HL
	F811	LD B,$00
	F813	LD HL,$FAA4
	F816	ADD HL,BC
	F817	LD A,(HL)
	F818	INC HL
	F819	LD ($F84E),A
	F81C	LD ($F842),A
	F81F	LD ($F85E),HL
	F822	RET
titlescr_start_tune_0	F823	LD A,(HL)
	F824	INC HL
	F825	LD H,(HL)
	F826	LD L,A
	F827	JR load_drum_op
titlescr_start_tune_1	F829	POP HL
	F82A	POP HL
	F82B	DI
	F82C	JP stop_music_and_silence