Platoon: music

01 · Music

Music: three tunes for each section of the film

The player below runs the game's own music driver from $E000 on the kit's 6502 simulator, one call a frame, and plays what it writes to the SID.

The driver's bytes come from this game's Source tab: the first load's for the jungle and village, the third load's for the tunnels and the dugout, and the fourth load's for the jungle and Barnes's bunker. Every tune was checked against the same code run in the kit's 6502 simulator (test_music.js): every write to the sound chip, in order, on each of 3,000 frames, with no difference. The game calls the driver once a frame from its raster interrupt.

Each load carries its own copy of the driver at $E000 and its own three tunes. A tune is a ten-byte record: three sequence addresses, one for each voice, then its speed and three bytes for the voices' start. The game picks one with Y = 0, 10 or 20 and calls start_tune: $E20F in the first two loads, $E222 in the third and fourth. Every frame play_music reads the next note for one voice, in turn, and steps the pitch, pulse and waveform tables of all three.

In every section, M turns the music on and O turns it off, which hands the voices to the sound effects; the music key restarts the section's tune.