Jingle Bells

This program plays “Jingle Bells” using the TS2068’s AY-3-8912 sound chip, combining BEEP for the melody with SOUND commands that program the AY chip’s three tone channels to produce harmonic chords underneath. Song data is stored as triplets (duration, pitch, chord) in DATA statements beginning at line 1000, with each chord value dispatching via computed GO SUB to one of eight chord subroutines (D major, G major, F major, B♭ major, C major, C7, G7, A major) at lines 9010–9082. Negative duration values are used as PAUSE signals, and a sentinel value of -99 triggers a loop back to restart the song continuously. The AY chip registers are set directly: registers 0–5 control the three channel tone periods, register 7 sets the mixer, and registers 8–10 set channel amplitudes.

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Program Structure

The program is divided into four clearly delineated sections:

1. Main loop (lines 100–170): RESTOREs to line 1000 and iterates through DATA triplets, dispatching chord subroutines and issuing BEEP calls.
2. Song data (lines 1000–1120): Packed DATA statements encoding every note as a (duration, pitch, chord) triplet.
3. Chord subroutines (lines 9000–9084): Eight named subroutines each programming the AY-3-8912 via SOUND for a specific chord.
4. Stop-chords subroutine (line 9900): Silences the AY chip by setting register 7 to 63 (all channels off) and register 13 to 0.

Data Encoding

Each note is represented as a triplet read into duration, pitch, and chord. The encoding uses several conventions:

• A chord value of 0 means no chord change; values 10–90 (multiples of 10) select a chord subroutine via GO SUB 9000+chord.
• A negative duration that is not -99 triggers a PAUSE of ABS duration interrupt units — used here for rests.
• The sentinel triplet -99,-99,900 restarts the song (line 140 catches duration=-99) while the chord value of 900 calls the stop-chords subroutine at line 9900 first, cleanly silencing the AY before looping.

Computed GO SUB Dispatch

The idiom GO SUB 9000+chord at line 130 routes execution to the appropriate chord subroutine purely arithmetically, avoiding a lookup table or IF/THEN chain. Chord index values used in the data are multiples of 10 (10, 30, 40, 60, 70, 80, 90), each mapping directly to a subroutine at lines 9010, 9030, 9040, 9060, 9070, 9080, 9090 respectively. The value 900 dispatches to the stop-chords subroutine at line 9900.

AY-3-8912 Register Programming

Each chord subroutine uses SOUND to write directly to AY chip registers. The register assignments are consistent across all chord subroutines:

Registers	Function
0,1	Channel A tone period (low/high byte)
2,3	Channel B tone period (low/high byte)
4,5	Channel C tone period (low/high byte)
7	Mixer control — value 56 (binary 00111000) enables tone on all three channels, noise off
8,9,10	Amplitude for channels A, B, C — all set to 12 (fixed level, no envelope)

The stop-chords routine sets register 7 to 63 (binary 00111111), disabling all tone and noise output, and clears register 13.

Melody via BEEP

The melody is played through the built-in speaker using the standard BEEP duration,pitch statement. Pitch is expressed as semitones above middle C. While SOUND sustains the AY chord in the background, BEEP provides the foreground melodic line, producing a two-layer musical texture.

Notable Techniques and Idioms

• Using RESTORE 1000 at line 110 rather than a bare RESTORE targets the data section precisely, allowing the song loop to restart cleanly.
• Fractional duration constants (e.g., .125, .128) are used directly in DATA — BEEP accepts floating-point duration in seconds, so .128 ≈ one thirty-second note at a typical tempo and .125 = exactly one-eighth of a second.
• The chord value 900 in the terminal sentinel record cleverly reuses the dispatch mechanism to call the silence subroutine before the loop restarts, avoiding a separate cleanup step.