This is Appendix 5 of the SD-8516 Programmer's Reference Guide.

The SD-8516 is paired with the SD-450 sound subsystem; named for featuring 4 independent voices with 5 waveforms available, each with a programmable ADSR envelope. Let's dive in with an overview of the architecture and a quick look at INT 11h, the sound services library.

Each voice occupies 16 bytes of memory in Bank 1:

Voice base addresses:

• Voice 0: $1EF80
• Voice 1: $1EF90
• Voice 2: $1EFA0
• Voice 3: $1EFB0

Gate register values:

• 0: Silent (gate off)
• 1: Square wave
• 2: Triangle wave
• 3: Sawtooth wave
• 4: Sine wave
• 5: Pulse wave (variable width via DATA1)
• 6: White/pink/brown noise (type via DATA1)

The Attack-Decay-Sustain-Release envelope shapes each note:

• Attack: Time to reach peak volume (0-255 × 10ms)
• Decay: Time to decay to sustain level (0-255 × 10ms)
• Sustain: Held volume level (0.0-1.0 of peak)
• Release: Time to fade to silence after gate off (0-255 × 10ms)

Example:

; Play middle C on voice 0
    LDA $112B           ; C4 frequency (262 Hz / 0.0596)
    STA [$1ED00]        ; $01ED00 = FREQ_LO/MID
    LDA #0
    STA [$1ED02]        ; $01ED02 = FREQ_HI

    LDAL $4D            ; Initialize volume to ~30%
    STAL [$1ED03]       ; $1ED03 = VOLUME
    
    LDAL $01            ; Square wave
    STAL [$1ED02]       ; set GATE to square wave (i.e. turn on)

You can also use the sound services library (I

Also see Appendix 3 Memory Map.

The sound system memory map occupies ($01EF80–$01EFBF) – 64 bytes total

    ; ============================================================================
    ; INT 11h - SOUND & MUSIC SERVICES
    ; SD-450 Sound Interface Device + Music Player
    ; ============================================================================
    ;
    ; REGISTER CONVENTION:
    ;   AH = command
    ;   AL = voice (1-4 for music commands, 0-3 for hardware channel)
    ;   B  = data (BL=low, BH=high; or BL=single byte)
    ;   C  = additional data
    ;   D, E, F = extended data (for AH=30h "new instrument")
    ;
    ; FUNCTION MAP:
    ;   --- Music Player Control ---
    ;   AH=00h: Poll music player (check tick timer, advance if ready)
    ;   AH=01h: Execute next note group from song data
    ;   AH=02h: Set clock for next note group
    ;   AH=03h: Execute a single music command
    ;   AH=04h: Reset music player (stop + rewind to start)
    ;   AH=05h: Start / continue music player
    ;   AH=06h: Pause / unpause (AL=00 pause, AL=01+ unpause)
    ;   AH=07h: Load new song (pointer, tempo, calc tick length)
    ;   AH=08h: Insert note data by index
    ;   AH=09h: Delete note data by index
    ;   AH=0Ah: Ask JavaScript to LOAD a music file
    ;   AH=0Bh: Ask JavaScript to SAVE a music file
    ;
    ;   --- Direct Sound Commands (also used by music file data) ---
    ;   AH=10h: Turn off voice
    ;   AH=11h: Turn on as square   (+3 bytes freq: BL, BH, CL)
    ;   AH=12h: Turn on as triangle (+3 bytes freq)
    ;   AH=13h: Turn on as sawtooth (+3 bytes freq)
    ;   AH=14h: Turn on as sine     (+3 bytes freq)
    ;   AH=15h: Turn on as PWM      (+3 bytes freq)
    ;   AH=16h: Turn on as noise    (+3 bytes freq)
    ;   AH=17h: Turn on as waveform byte + note index
    ;            (BL=note index 1-88, BH=waveform 01-06)
    ;   AH=21h: Set GATE        (BL = gate byte)
    ;   AH=22h: Set FREQ        (BL=lo, BH=mid, CL=hi)
    ;   AH=23h: Set ATTACK      (BL = attack byte)
    ;   AH=24h: Set SUSTAIN     (BL = sustain byte)
    ;   AH=25h: Set RELEASE     (BL = release byte)
    ;   AH=26h: Set DATA1       (BL = data1 byte)
    ;   AH=27h: Set DATA2       (BL = data2 byte)
    ;   AH=28h: Set VOLUME      (BL = volume byte)
    ;   AH=29h: Set DECAY       (BL = decay byte)
    ;   AH=30h: Set new voice data at-once (10 bytes: BCDEF)
    ;
    ;   --- Legacy / Utility ---
    ;   AH=40h: Initialize sound system
    ;   AH=41h: Play note (blocking, with duration)
    ;   AH=42h: Stop channel
    ;   AH=43h: Stop all channels
    ;   AH=44h: Set master volume
    ;   AH=45h: Get channel status
    ;   AH=46h: Sound effect (bell, beep, buzz, etc.)
    ;   AH=47h: Play startup sound (GECF)
    ;   AH=48h: Wait milliseconds (C = ms)
    ;   AH=49h: Note index to frequency lookup (BH=index, returns B:CL)
    ;
    ; ============================================================================