Code Like It's 198x

Trigger a sound on Game Boy

Reuse a small real effect, understand who stops it, and compare its native output with an omitted-trigger control.

Give an accepted action a sound. The same caller changes its picture and triggers this effect; a second version performs the action with the trigger removed. Listen to both.

Working effect

Trigger omitted

Take the effect

The reusable effect

sound.inc
; DMG pulse channel1, both outputs; non-blocking eight-frame effect.
trigger_sound:
    ld a,$80
    ld [$ff26],a
    ld a,$77
    ld [$ff24],a
    ld a,$11
    ld [$ff25],a
    xor a
    ld [$ff10],a
    ld a,$80
    ld [$ff11],a
    ld a,$f0
    ld [$ff12],a
    xor a
    ld [$ff13],a
    ld a,$87
    ld [$ff14],a
    ld a,8
    ld [$c004],a
    ret
service_sound:
    ld a,[$c004]
    or a
    ret z
    dec a
    ld [$c004],a
    ret nz
    xor a
    ld [$ff12],a
    ret

Pulse channel 1, duration byte $C004. The trigger also writes shared NR50/NR51/NR52 routing, volume and power controls, so it is not safe beside an unrelated mixer. AF changes; B is preserved.

Include sound.inc once with the caller’s constants and duration byte. Initialise the duration byte to zero before the first service call. Call trigger_sound for an accepted action, then call service_sound exactly once per accepted display update. The eighth service call mutes the effect. Because the supplied caller services in the triggering iteration too, this is a count of service calls, not eight complete elapsed frame intervals. Triggering again restarts the duration; there is no queue.

The complete initialiser also mutes other channels. To reuse just this effect beside music, agree channel ownership and remove unrelated global setup only after providing a replacement. Device latches, power, routing and master volume may affect more than one voice.

Change it and recognise a mistake

Change pulse duty/frequency/envelope operands. An audio engine should initialise shared power/routing once and arbitrate channel use.

The omitted-trigger control still accepts the action but fails the same audio guard as the working caller. Keyboard clicks or other machine output are not enough: the BASIC checks compare against a separate rejected-command recording.

Deliberately omitted sound trigger

no-sound.asm
; Capability caller: original input/frame service, explicit drawing and sound.
; DMG cartridge: video keeps running; only a new A press advances the turn.
; Owns LCD/VRAM, JOYP and $C000-$C003; maskable interrupts stay disabled.
SECTION "Entry", ROM0[$100]
    nop
    jp start
    ds $134-@,0
    db "TURNPROBE"
    ds $150-@,0
SECTION "Program", ROM0[$150]
start:
    di
    ld sp,$fffe
wait_to_disable:
    ld a,[$ff44]
    cp 144
    jr c,wait_to_disable
    xor a
    ld [$ff40],a             ; disable LCD in VBlank before clearing VRAM
    ld hl,$8000
    ld bc,$2000
clear_vram:
    xor a
    ld [hl+],a
    dec bc
    ld a,b
    or c
    jr nz,clear_vram
    xor a
    ld [$c000],a             ; turn counter
    ld [$c001],a             ; previous A state
    ld [$c002],a             ; accepted frame counter, low
    ld [$c003],a             ; high
    call init_picture
    ld a,$e4
    ld [$ff47],a
    ld a,$91
    ld [$ff40],a             ; background enabled, tile/map zero initialised
main:
    call wait_frame
    ld a,[$c002]
    inc a
    ld [$c002],a
    jr nz,sample
    ld a,[$c003]
    inc a
    ld [$c003],a
sample:
    INCLUDE "../sample.inc"
    ld a,[$c001]
    cpl
    and b
    jr z,no_action
    ld a,[$c000]
    inc a
    ld [$c000],a
    nop
    nop
    nop
no_action:
    ld a,b
    ld [$c001],a
    call draw_picture
    call service_sound
    jp main

; AF clobbered. Requires LCD enabled. Waits beyond any current VBlank,
; then accepts the next VBlank. Does not count missed announcements.
wait_frame:
    ld a,[$ff44]
    cp 144
    jr nc,wait_frame
wait_vblank:
    ld a,[$ff44]
    cp 144
    jr c,wait_vblank
    ret
    INCLUDE "../picture.inc"
    INCLUDE "../sound.inc"
    ds $8000-@,0

The working implementation above repairs that omission. If an effect instead drones on, check the mute/service path and that its state is initialised. Servicing twice makes a counted effect shorter; servicing only after input can leave it running while the player waits. Those are lifecycle decisions to test when adapting it.

Run the caller

Build the complete caller with make -C nintendo-game-boy/patterns/assembly/input/read-action. Press A once, keep it held, then release and press again. The piece moves on each new press and returns to its starting position after two; holding does not repeat the action. Load the cartridge produced by the Makefile in the named configuration; retain its reset/header setup.

Game Boy native caller after accepted input.
Captured from the running caller in the configuration below.

Where setup, trigger and service belong

demo.asm
; Capability caller: original input/frame service, explicit drawing and sound.
; DMG cartridge: video keeps running; only a new A press advances the turn.
; Owns LCD/VRAM, JOYP and $C000-$C003; maskable interrupts stay disabled.
SECTION "Entry", ROM0[$100]
    nop
    jp start
    ds $134-@,0
    db "TURNPROBE"
    ds $150-@,0
SECTION "Program", ROM0[$150]
start:
    di
    ld sp,$fffe
wait_to_disable:
    ld a,[$ff44]
    cp 144
    jr c,wait_to_disable
    xor a
    ld [$ff40],a             ; disable LCD in VBlank before clearing VRAM
    ld hl,$8000
    ld bc,$2000
clear_vram:
    xor a
    ld [hl+],a
    dec bc
    ld a,b
    or c
    jr nz,clear_vram
    xor a
    ld [$c000],a             ; turn counter
    ld [$c001],a             ; previous A state
    ld [$c002],a             ; accepted frame counter, low
    ld [$c003],a             ; high
    call init_picture
    ld a,$e4
    ld [$ff47],a
    ld a,$91
    ld [$ff40],a             ; background enabled, tile/map zero initialised
main:
    call wait_frame
    ld a,[$c002]
    inc a
    ld [$c002],a
    jr nz,sample
    ld a,[$c003]
    inc a
    ld [$c003],a
sample:
    INCLUDE "sample.inc"
    ld a,[$c001]
    cpl
    and b
    jr z,no_action
    ld a,[$c000]
    inc a
    ld [$c000],a
    call trigger_sound
no_action:
    ld a,b
    ld [$c001],a
    call draw_picture
    call service_sound
    jp main

; AF clobbered. Requires LCD enabled. Waits beyond any current VBlank,
; then accepts the next VBlank. Does not count missed announcements.
wait_frame:
    ld a,[$ff44]
    cp 144
    jr nc,wait_frame
wait_vblank:
    ld a,[$ff44]
    cp 144
    jr c,wait_vblank
    ret
    INCLUDE "picture.inc"
    INCLUDE "sound.inc"
    ds $8000-@,0

Compare blocking, serviced and queued sound when choosing the next step.

Verification and sources

Target: DMG default reset without boot ROM, 32 KiB cartridge. These callers were executed with released Emu198x 0.44.1; assembly uses Asm198x 0.0.71. The sample folder contains reproduction instructions and retained observations. The independent checker binds source, native captures and actual output; its rejection tests exercise wrong state, missing files and false audio claims.

This qualifies the named emulator/firmware path. Pitch accuracy, original hardware and other peripherals require their own observations.

Nintendo, Game Boy Programming Manual v1.1: P1/JOYP scan example (six action-button reads), LCDC/LY access rules and NR10–NR14/NR50–NR52.