Self modified by EBB3 (start_tune) and EE71 (in-song tempo command); the operand byte at $EC9A is tune_tempo. Confirmed against a real-hardware trace: tune 0's tempo byte (2) gives a period-2 fire/skip alternation.

Helper: write a single AY register (D = register number, E = value). Not directly referenced elsewhere in this bank -- may be dead code or called from another bank.

Silence the AY chip: write 0 to every register 13 downto 0 (in particular the 3 volume registers 8-10), then write register 7 (mixer) a second time for good measure.

Stop any playing tune and silence the AY chip. This entry point is used by the routines at name_entry_input, run_title_screen, titlescr_start_tune and run_title_tune.

Not traced (structural pass) -- nothing in this bank jumps or calls into $ED1D-$ED32; the instruction stream here does not obviously cohere (an SBC/ADD/ADC/SUB chain feeding register loads with no clear purpose before falling into a POP HL/JP). Likely either dead code or a misdisassembled data table. Ruled out as related to advance_channel_phrase: that routine is a self-contained phrase-pointer table walker, fully traced, and does not fall through from or into this range.

Pattern command/effect opcode handlers ($ED36-$EDD1), reached only via the computed jump at dispatch_pattern_command (never by a direct CALL/JP visible to the disassembler, hence no entry-point markers below). Each handler consumes a fixed number of further bytes from the pattern stream (DE) and stores them into fields of the current channel's 37-byte tracker record (see EC01), then rejoins the main interpreter at acp_read_pattern_byte or acp_reset_row_counter. Traced mechanically below; the AY-level meaning of $EF7A and +$1D/+$1F is inferred (marked "likely"), not confirmed against a working build. Handlers 1-3 all bit-merge P = (IX+$24) into the shared byte $EF7A using the "A XOR M; A AND mask; A XOR M" replace-bits-under-mask idiom; $EF7A is likely a per-channel AY mixer (tone/noise enable) bit cache, given $EEA0 onward outputs a mixer byte to the AY.

Advance one channel's pattern by one tracker row. Decrements the per-row wait counter (+$10); while it is still counting down, only channel_slide_upkeep (envelope/portamento upkeep) runs. Once it reaches zero, reads and processes the next byte(s) from the pattern stream via acp_read_pattern_byte.

This entry point is used by the routine at advance_channel_phrase.

Between pattern rows: nudge the current note (+$12) up or down by one per frame if a portamento/slide effect (channel flag bit 3) is active.

Command/effect byte decode: range-tests the byte against a cascade of thresholds ($B0, then +$20, +$10, +$18) to find which opcode group it falls in, then either handles it directly (note-table/pointer effects) or falls through to the jump table at dispatch_pattern_command for the fixed-length opcode handlers at pcmd_set_mixer_bits_low3-$EDD1.

In-song tempo command: self modifies 'LD A,x' at EC99 to set the tempo-counter reload value

Jump-table dispatch into the fixed-length opcode handlers at pcmd_set_mixer_bits_low3-$EDD1, indexed by the remaining low bits of the command byte (A, with B assumed 0 as throughout this driver). Rather than storing a full 2-byte address per entry, the table at $EC9D stores a 1-byte displacement from the table slot itself to the handler: HL is first set to the table slot ($EC9D + A), then the displacement byte at that slot is added back onto HL, giving the handler's address. This works because every handler lies within 256 bytes of its table slot.

Recompute this channel's AY tone-period and volume/envelope register values from its current tracker state (note, envelope index, effect flags) for the per-frame register cache write at ts_music_service. Returns the tone period in HL and the volume/mixer byte in A (see the callers at ECA2/$ECAF/$ECBC). Five phases, in order: 1. EEA1-$EED0: envelope amplitude advance. If status bit 5 (+$00) is set, decrements the envelope-step counter (+$19); on underflow, reloads it from the speed value (+$0F) and reads the next byte from the envelope-shape table pointer (+$16/+$17, advanced by 1 unless the byte read has bit 7 set, an end-of-table marker that halts the pointer), storing that byte as the current amplitude (+$18). Either way, +$18 is copied into +$13, the volume value this call returns. 2. ts_music_service_6-$EEFC: tone-period lookup. Note index = (+$12) [set by EDEA from the pattern's raw note + transpose (+$20)] plus a pitch-offset byte read from a per-channel offset sequence at +$0B/+$0C (default/loop start +$09/+$0A): if the byte's bit 7 is clear it is used as-is and the pointer advances; if set, it is a 7-bit end-of-sequence delta and the pointer resets to +$09/+$0A for next time. (note + offset) indexes the 2-byte-per-note tone-period table at EFBC, giving the base tone period in DE. 3. EEFC-$EF4B: vibrato. Only runs if +$1D bit 6 is set (skips to phase 4 otherwise). +$1C is a triangle-wave phase counter that ping-pongs between 0 and (+$1A)*2 by +/-(+$1B) each call, direction tracked in +$1D bit 5 (bit 7 and status bit 0 gate whether the phase updates this call at all, at EF09-$EF12 -- not fully resolved). The phase is centred (-depth..+depth), sign-extended, then left-shifted a note-dependent number of times (the $EF3D-$EF47 loop, incrementing a counter seeded from the note index by $18 until it overflows) to scale a linear vibrato depth into the tone-period domain, where pitch and period are inversely related. The scaled offset is added onto the phase-2 tone period. 4. ts_music_service_15-$EF76: portamento/slide. Status bit 0 (+$00) is unconditionally toggled every call. If status bit 2 was set (slide active, pcmd_set_slide_target), a countdown in +$0E ticks down; when it reaches zero it reloads and the signed step (+$0D) is sign-extended into BC and accumulated into +$07/+$08, which is then added onto the tone period. 5. ts_music_service_18-$EFAE: mixer/volume finalisation. Every 4th call (via the bit-0 toggle from phase 4), XORs bit 3 of a global byte ($EC79) into $ECC6 (purpose not established elsewhere in this bank). Merges this channel's tone-enable bits into the shared mixer cache at $EFB6 using the same replace-bits-under-mask idiom as pcmd_set_mixer_bits_low3 (mask = +$24). If +$1F was just set to $FF (pcmd_mute_channel, "mute"), handles the one-shot mute transition: clears bit 7 of +$1F (so this only fires once), clears this channel's noise-enable bits in the $EFB6 cache, and forces $ECC6 to $41. Returns the phase-3/4 tone period in HL and the phase-1 volume (+$13) in A.