Code Like It's 198x

Trigger a sound on Commodore VIC-20

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.bas
2000 POKE 36878,15:POKE 36876,220
2010 FOR J=1 TO 200:NEXT J
2020 POKE 36876,0
2030 RETURN

POKE 36878,15 changes shared master volume; POKE 36876,220 enables the third tone voice. After a blocking J loop, POKE 36876,0 stops that voice.

Copy the 2000-range subroutine and call GOSUB 2000 when the game accepts an action. Keep its global scratch variables and any DATA/setup lines with it. A sound call is a consequence of an action, not a substitute for input validation.

Change it and recognise a mistake

Change the voice register value for another tone. Preserve or explicitly own master volume when mixing effects and music; this sample leaves volume at 15.

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.bas
10 N=0
20 PRINT CHR$(147);"TURN";N
21 GOSUB 1000
30 GOSUB 3000
40 IF A$="GO" THEN N=N+1
50 GOTO20
1000 PRINT "XXX"
1010 PRINT "X X"
1020 PRINT "XXX"
1030 RETURN
2000 POKE 36878,15:POKE 36876,220
2010 FOR J=1 TO 200:NEXT J
2020 POKE 36876,0
2030 RETURN
3000 INPUT A$
3010 RETURN

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

Enter the complete demo.bas in Commodore VIC-20 BASIC and RUN. Submit GO, then Return. One accepted command advances the displayed turn; another gives two.

Commodore VIC-20 native caller after accepted input.
Captured from the running caller in the configuration below.

Where setup, trigger and service belong

demo.bas
10 N=0
20 PRINT CHR$(147);"TURN";N
21 GOSUB 1000
30 GOSUB 3000
40 IF A$="GO" THEN N=N+1:GOSUB 2000
50 GOTO20
1000 PRINT "XXX"
1010 PRINT "X X"
1020 PRINT "XXX"
1030 RETURN
2000 POKE 36878,15:POKE 36876,220
2010 FOR J=1 TO 200:NEXT J
2020 POKE 36876,0
2030 RETURN
3000 INPUT A$
3010 RETURN

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

Verification and sources

Target: unexpanded PAL VIC-20, Commodore BASIC. 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.

Commodore, VIC-20 Programmer’s Reference Guide: BASIC INPUT/GOSUB and VIC sound registers $900A–$900E.