HandleNoiseMusic

static void
HandleNoiseMusic()
{
    u8 a, y, x;

    // The underground and castle music do not use the noise channel. This bitmask is used
    // to check if either of these are the active music
    u8 const NO_NOISE_DATA_MASK = MUSIC_Underground | MUSIC_Castle;

    if ( !( AreaMusicBuffer & (u8)~NO_NOISE_DATA_MASK ) )
        return;

    if ( --Noise_BeatLenCounter == 0 )
    {
        for ( ; ; )
        {
            y = MusicOffset_Noise++;
            a = MusicData[y];

            if ( a != 0 ) // If not end of noise data
                break;

            MusicOffset_Noise = NoiseDataLoopbackOfs;
        }

        x = a;
        Noise_BeatLenCounter = AlternateLengthHandler( a );
        a &= 0b00111110;

        // Hihat
        if ( a == 0x30 )
        {
            a = 0x1c;
            x = 0x03;
            y = 0x58;
        }
        // Kick
        else if ( a == 0x20 )
        {
            a = 0x1c;
            x = 0x0c;
            y = 0x18;
        }
        // Snare
        else if ( a & 0x10 )
        {
            a = 0x1c;
            x = 0x03;
            y = 0x18;
        }
        // Rest (X and Y are not written to since the value in A will silence the channel)
        else
            a = 0x10;

        write_apu_reg( NOIVOL, a );
        write_apu_reg( NOIFREQ, x );
        write_apu_reg( NOILEN, y );
    }
}