AY Music

Overview

One channel plays one tone; envelopes make it move. Music is the step up: all three channels at once, each handed a fresh note on a regular beat. And here the AY earns its keep — where beeper music seizes the whole CPU, an AY tune plays from the interrupt: step the pattern once a frame, and between those steps the chip holds all three notes on its own while your game runs. Count the interrupts into rows, look up a period per channel from a note table, write it, and you have the skeleton of every AY player — and, with only the port addresses changed, of CPC and Atari ST players too.

Code

; =============================================================================
; AY MUSIC - ZX SPECTRUM 128K
; ay_music_tick, called once per 50 Hz interrupt, feeds all three channels
; =============================================================================

TEMPO equ 6                ; interrupts per row
ROWS  equ 8

ay_music_init:
    ld   a,7               ; mixer: all three tones ON, noise off
    ld   e,%00111000
    call ay_out
    ld   d,3               ; set channels A, B, C to full fixed volume
.vol:
    ld   a,7
    add  a,d               ; volume registers are 8, 9, 10
    ld   e,15
    call ay_out
    dec  d
    jr   nz,.vol
    ld   a,TEMPO
    ld   (mtimer),a
    xor  a
    ld   (mrow),a
    ret

; --- Call once per interrupt; a new row lands every TEMPO frames ---
ay_music_tick:
    ld   hl,mtimer
    dec  (hl)
    ret  nz                ; between rows — the chip holds the notes, no work
    ld   (hl),TEMPO

    ld   a,(mrow)          ; channel A's note this row
    ld   c,a
    ld   b,0
    ld   hl,pat_a
    add  hl,bc
    ld   a,(hl)
    ld   d,0               ; A's tone registers are 0 / 1
    call set_channel

    ld   a,(mrow)          ; reload row (ay_out clobbers BC)
    ld   c,a
    ld   b,0
    ld   hl,pat_b
    add  hl,bc
    ld   a,(hl)
    ld   d,2               ; B -> registers 2 / 3
    call set_channel

    ld   a,(mrow)
    ld   c,a
    ld   b,0
    ld   hl,pat_c
    add  hl,bc
    ld   a,(hl)
    ld   d,4               ; C -> registers 4 / 5
    call set_channel

    ld   a,(mrow)          ; step the pattern, loop at ROWS
    inc  a
    cp   ROWS
    jr   c,.save
    xor  a
.save:
    ld   (mrow),a
    ret

; A = note index (0 = rest/hold), D = channel's tone-LO register (0, 2 or 4)
set_channel:
    or   a
    ret  z                 ; 0 = leave the channel playing its last note
    add  a,a               ; note index * 2 (two bytes per entry)
    ld   l,a
    ld   h,0
    ld   bc,note_tab
    add  hl,bc             ; HL -> {period lo, period hi}
    ld   a,d
    ld   e,(hl)
    call ay_out            ; period low  -> register D
    inc  hl
    ld   a,d
    inc  a
    ld   e,(hl)
    call ay_out            ; period high -> register D+1
    ret

; A = register, E = value
ay_out:
    ld   bc,$fffd
    out  (c),a
    ld   b,$bf
    out  (c),e
    ret

mtimer: defb 0
mrow:   defb 0
; AY tone periods (128K clock). Index 0 = rest.
;              C-3     C-4     E-4     G-4
note_tab: defw 0, $034F,  $01A8,  $0150,  $011B
pat_a:    defb 2,3,4,3, 2,3,4,3   ; lead
pat_b:    defb 4,4,2,2, 4,4,2,2   ; harmony
pat_c:    defb 1,0,1,0, 1,0,1,0   ; bass

Trade-offs

Aspect	Cost
CPU	A few bytes of work most interrupts; a little more on a row change
Memory	The period table, the patterns, two bytes of state
Limitation	Three channels, and one shared envelope generator across them

When to use: Any 128K game that wants music during play — which is most of them.

When to avoid: Nothing about music; for a single reactive blip, a one-shot envelope effect is lighter.

It plays from the interrupt

This is the whole reason the AY beats the beeper for games. The Spectrum raises an interrupt 50 times a second; hook it (the HALT game loop already waits on it) and call ay_music_tick there. Each call is tiny — usually just a counter decrement — and only every TEMPO-th one writes a row. In between, the chip sustains all three notes with no help at all, so the same frame that nudges the music also moves the sprites and reads the keys. TEMPO sets the speed; 6 interrupts a row is a common default.

Three channels, one envelope

The player above gives every channel a fixed volume, so all three ring flat. Bring in the envelope generator and you meet its one hard limit: a single envelope, shared. Real AY drivers get around it by writing volumes by hand every interrupt — a software envelope table per channel, stepped alongside the note table — so each voice can pluck or swell independently. That per-frame volume work, plus ornaments (fast note wobbles that fake vibrato and chords), is most of what separates this skeleton from a tracker replay.

The same chip, other machines

The AY-3-8912 and its near-twins sit in the Amstrad CPC, the Oric, the MSX, and — as the YM2149 — the Atari ST. The register map is identical, so this player ports almost verbatim: change the two port addresses (the CPC reaches the AY through its PPI, not $FFFD/$BFFD) and rebuild the period table for that machine’s clock, and the same pattern, the same tick, the same three voices play on. It’s why AY tracker formats like PT3 and the Vortex Tracker lineage span so many machines at once.

Patterns: AY Envelopes and Noise, AY Tone, Game Loop (HALT)

Vault: AY-3-8912 | ZX Spectrum