A Music Engine | Pattern Library | Code Like It's 198x

Code status: illustrative code. Adapt the listing to your program and check it on your target machine. This page links no assembly or execution record for the listing itself.

Overview

One channel plays one note; a sound effect makes it move. Music is the step up: several channels at once, each handed a fresh note on a regular beat. On the NES that beat is already ticking — the NMI fires every frame, 60 times a second on NTSC. Count those frames into rows, and on each row look up a timer value per channel from a note table and write it. Pulse 1 takes the lead, pulse 2 the harmony, the triangle the bass; the noise channel drops in drums. That loop — a tick, a pattern, a note table — is every NES sound driver in miniature.

Code

6502 assembly

; =============================================================================
; A MUSIC ENGINE - NES (APU)
; music_tick, called once per NMI, steps a pattern and feeds three voices
; =============================================================================

APUSTATUS  = $4015
PULSE1_VOL = $4000
PULSE1_LO  = $4002
PULSE1_HI  = $4003
PULSE2_VOL = $4004
PULSE2_LO  = $4006
PULSE2_HI  = $4007
TRI_LINEAR = $4008
TRI_LO     = $400A
TRI_HI     = $400B

TEMPO       = 8         ; frames per row (60 / 8 ~ 7.5 rows a second)
PATTERN_LEN = 8

.segment "ZEROPAGE"
mus_row:   .res 1
mus_timer: .res 1

.segment "CODE"

music_init:
    lda #%00000111         ; pulse 1, pulse 2, triangle enabled
    sta APUSTATUS
    lda #$08               ; disabled sweep, negate selected on both pulses
    sta $4001
    sta $4005
    lda #%10111111         ; 50% duty, sustained, full volume
    sta PULSE1_VOL
    sta PULSE2_VOL
    lda #%11111111         ; triangle: control set -> plays continuously
    sta TRI_LINEAR
    lda #0
    sta mus_row
    lda #TEMPO
    sta mus_timer
    rts

; --- Call once per NMI; a new row lands only every TEMPO frames ---
music_tick:
    dec mus_timer
    beq @row
    rts                    ; between rows — hold the current notes
@row:
    lda #TEMPO
    sta mus_timer
    ldx mus_row

    ldy pat_p1,x           ; lead: note index for this row (0 = hold)
    beq @p2
    lda note_lo,y
    sta PULSE1_LO
    lda note_hi,y
    sta PULSE1_HI          ; writing $4003 retriggers the note
@p2:
    ldy pat_p2,x           ; harmony
    beq @tri
    lda note_lo,y
    sta PULSE2_LO
    lda note_hi,y
    sta PULSE2_HI
@tri:
    ldy pat_tri,x          ; bass — same table, an octave lower (triangle /32)
    beq @step
    lda note_lo,y
    sta TRI_LO
    lda note_hi,y
    sta TRI_HI
@step:
    inx
    cpx #PATTERN_LEN
    bne @save
    ldx #0                 ; loop the pattern
@save:
    stx mus_row
    rts

.segment "RODATA"
; Pulse timer values (NTSC), 11-bit split lo/hi. Index 0 = hold, not a rest.
;                 C-4   E-4   G-4
note_lo: .byte 0, $AA,  $52,  $1C
note_hi: .byte 0, $01,  $01,  $01

; The pattern: one note index per channel, per row.
pat_p1:  .byte 1, 2, 3, 2,  1, 2, 3, 2   ; arpeggiated lead
pat_p2:  .byte 3, 3, 1, 1,  3, 3, 1, 1   ; harmony
pat_tri: .byte 1, 0, 1, 0,  1, 0, 1, 0   ; bass on the beat

Trade-offs

Aspect	Cost
CPU	A few bytes of work most frames; a little more on a row change
Memory	The note table, the patterns, two zero-page bytes
Limitation	Five voices total, and an effect must borrow one from the song

When to use: Any in-game music. This is how NES music works — every driver is this shape with more on top.

When to avoid: A single reactive bleep; that’s a sound effect, not a sequencer.

The tick is the tempo

The engine keeps no clock of its own — it rides the frame. Each NMI decrements mus_timer; only when it hits zero does a new row land, and TEMPO sets how many frames that takes. TEMPO 8 gives ≈7.5 rows a second; drop it to 6 and the song speeds up. Real drivers count rows into a song through an order list and run effects — vibrato, slides, arpeggios — on the in-between frames, but the heartbeat is always this: one NMI, one tick.

One table, two octaves

The lead and harmony are pulse channels; the bass is the triangle — yet they share one note table. That’s a gift of the hardware: the triangle divides its timer by 32 where the pulse divides by 16, so the same timer value sounds exactly an octave lower on the triangle. Write $1AA to a pulse channel for C-4 and to the triangle for C-3, no second table needed. The 0 index is the other convention worth keeping — a row of 0 means “leave this channel alone”, so a voice holds its note across rows instead of restriking every beat.

From skeleton to driver

A shipping sound engine adds three things to this frame:

Instruments — a volume envelope and duty per note, rewritten each row, so a channel can pluck, swell or fade rather than blare one flat tone.
Drums — the noise channel triggered from its own pattern row; it takes a period index, not a note.
An order list — a sequence of pattern numbers, so a song reuses a handful of short patterns instead of storing every bar.

Add those and a sound-effect routine that borrows a channel from the music, and you have a driver like FamiTone or the ones behind the NES’s best scores.

Patterns: Sound Effects, A Square Wave, NMI Game Loop

Vault: APU | Nintendo Entertainment System

Initialise the APU frame/IRQ policy before music_init, own these three channels and leave DMC disabled unless its input/DMA interactions are handled elsewhere. The zero-page row/timer bytes must be initialised exactly once for a new phrase. music_tick changes A/X/Y and flags; an NMI wrapper preserves the interrupted program and returns with RTI. Zero holds the previous sound. To support an actual rest, add an explicit channel-specific mute operation rather than treating a held note as silence.