Interactive Sound in Assembly: The Real Deal

So far, we've made things move on screen. Now, let's make some noise! The Amiga's four 8-bit sound channels, collectively named "Paula," gave it an audio capability that was light-years ahead of its time. In the last tutorial, we used C to work with the OS, but you asked for the real deal: how to do it in pure 68k assembly. Let's dive in.

The Concept: OS Libraries from Assembly

This program will run as a proper AmigaDOS process. To do that, we need to talk to the Amiga's libraries. In assembly, this means loading the library into memory to get its "base address," and then calling its functions by jumping to specific negative offsets from that base address. We'll open a window, draw our button, and enter a loop to wait for user input (events). When we detect a mouse click inside our button's area, we'll trigger the sound and provide some visual feedback by changing the button's color.

Illustration of an Amiga playing sound from assembly

Preparing Your Sound Sample

For this assembly version, we need a raw sound file. The hardware plays raw, 8-bit signed audio samples. We will embed this data directly into our executable.

Find a short sound effect (`.wav` is a good starting point).
Use a modern audio editor like Audacity to convert it. Open the sound and export it:
- - Set the project rate to something reasonable, like 8363 Hz or 11025 Hz.
- - Go to `File > Export > Export Audio`.
- - Choose "Other uncompressed files" as the format.
- - Under "Header", select "RAW (header-less)".
- - Under "Encoding", select "Signed 8-bit PCM".
Save this new file as `sound.raw`. We will include it directly in our assembly code.

The Code (asm_sound_button.s):

;-----------------------------------------------------
;  Interactive Sound Button in 68k Assembly
;  - Opens an OS-friendly window
;  - Draws a button
;  - Plays an embedded sound on click
;-----------------------------------------------------
    SECTION code,CODE
start:
;--- Open Libraries ---
    move.l  #4,a6
    lea     IntuitionName(pc),a1
    moveq   #37,d0
    jsr     _LVOOpenLibrary(a6)
    move.l  d0,IntuitionBase
    beq.s   .fail
lea     GfxName(pc),a1
move.l  #4,a6
moveq   #37,d0
jsr     _LVOOpenLibrary(a6)
move.l  d0,GfxBase
beq.s   .fail_intuition

;— Open Window —
lea     NewWindow(pc),a0
move.l  IntuitionBase,a6
jsr     _LVOOpenWindow(a6)
move.l  d0,Window
beq.s   .fail_gfx
move.l  Window,d0
move.l  d0,a0
move.l  116(a0),a1 ; a1 = RastPort
move.l  a1,RastPort

;— Draw Button —
bsr.s   draw_button_up
;— Event Loop —
event_loop:
move.l  Window,d0
move.l  108(d0),d0 ; Get signal mask
move.l  IntuitionBase,a6
jsr     _LVOWait(a6)
move.l  Window,a0
move.l  104(a0),a1 ; UserPort
move.l  IntuitionBase,a6
jsr     _LVOGetMsg(a6)
move.l  d0,Message
beq.s   event_loop      ; No message? Loop again.

move.l  Message,a0
move.w  20(a0),d0      ; Get message Class
cmp.w   #$202,d0        ; IDCMP_CLOSEWINDOW
beq.s   .quit

cmp.w   #$401,d0        ; IDCMP_MOUSEBUTTONS
bne.s   .reply_msg

move.l  Message,a0
move.w  22(a0),d0      ; Get message Code
cmp.w   #$68,d0         ; SELECTDOWN
beq.s   .mouse_down
cmp.w   #$69,d0         ; SELECTUP
beq.s   .mouse_up

.reply_msg:
move.l  Message,a1
move.l  IntuitionBase,a6
jsr     _LVOReplyMsg(a6)
bra.s   event_loop
.mouse_down:
bsr.s   draw_button_down
bsr.s   play_sound
bra.s   .reply_msg
.mouse_up:
bsr.s   draw_button_up
bra.s   .reply_msg
.quit:
move.l  Message,a1
move.l  IntuitionBase,a6
jsr     _LVOReplyMsg(a6)
;— Cleanup —
.fail_window:
move.l  Window,a0
move.l  IntuitionBase,a6
jsr     _LVOCloseWindow(a6)
.fail_gfx:
move.l  GfxBase,a1
move.l  #4,a6
jsr     _LVOCloseLibrary(a6)
.fail_intuition:
move.l  IntuitionBase,a1
move.l  #4,a6
jsr     _LVOCloseLibrary(a6)
.fail:
moveq   #0,d0
rts
;— Subroutine to draw button (up state) —
draw_button_up:
movem.l d1-d2/a1,-(sp)
move.l  RastPort,a1
move.l  GfxBase,a6
move.w  #1,d0           ; Color 1
jsr     _LVOSetAPen(a6)
move.w  #100,d0
move.w  #40,d1
move.w  #220,d2
move.w  #60,d3
jsr     _LVORectFill(a6)
move.w  #2,d0           ; Color 2
jsr     _LVOSetAPen(a6)
move.w  #140,d0
move.w  #50,d1
jsr     _LVOMove(a6)
lea     button_text(pc),a0
move.w  #10,d0
jsr     _LVOText(a6)
movem.l (sp)+,d1-d2/a1
rts
;— Subroutine to draw button (down state) —
draw_button_down:
movem.l d1-d2/a1,-(sp)
move.l  RastPort,a1
move.l  GfxBase,a6
move.w  #2,d0           ; Color 2
jsr     _LVOSetAPen(a6)
move.w  #100,d0
move.w  #40,d1
move.w  #220,d2
move.w  #60,d3
jsr     _LVORectFill(a6)
move.w  #1,d0           ; Color 1
jsr     _LVOSetAPen(a6)
move.w  #140,d0
move.w  #50,d1
jsr     _LVOMove(a6)
lea     button_text(pc),a0
move.w  #10,d0
jsr     _LVOText(a6)
movem.l (sp)+,d1-d2/a1
rts
;— Subroutine to play sound —
play_sound:
lea     $DFF000,a5
lea     sound_data(pc),a0
move.l  a0,$DFF0A0    ; AUD0LCH/LCL - Audio channel 0 pointer
move.w  #sound_len,$DFF0A4 ; AUD0LEN - Length of sample in words
move.w  #15000/8363,d0  ; Period for 8363Hz playback (adjust for your sample rate)
move.w  d0,$DFF0A8    ; AUD0PER - Playback period
move.w  #64,$DFF0A6     ; AUD0VOL - Max volume
move.w  #%0000000000000001,DMACON(a5) ; Enable audio channel 0 DMA
rts
;— Data section —
SECTION data,DATA
IntuitionName: dc.b ‘intuition.library’,0
GfxName:       dc.b ‘graphics.library’,0
button_text:   dc.b ‘Play Sound’,0
even
IntuitionBase: dc.l 0
GfxBase:       dc.l 0
Window:        dc.l 0
RastPort:      dc.l 0
Message:       dc.l 0
NewWindow:
dc.w 0,0,320,100  ; Left, Top, Width, Height
dc.w 1,2          ; DetailPen, BlockPen
dc.l $10001203    ; IDCMP Flags
dc.l $0007001F    ; Flags (WFLG_DRAGBAR, etc)
dc.l 0,0          ; FirstGadget, CheckMark
dc.l .title,.screen,0,0,0 ; Title, Screen, BitMap, Min/Max size
dc.w 0            ; Type (custom screen)
.title: dc.b ‘Sound Player’,0
.screen: dc.l 0 ; Use default screen
even
SECTION chip,BSS_C ; Sound data must be in Chip RAM

sound_data:
incbin  “sound.raw” ; Include your raw sound file here
sound_end:
sound_len       equ (sound_end-sound_data)/2 ; Length in words
even
;— Library function offsets —
_LVOOpenLibrary equ -552
_LVOCloseLibrary equ -414
_LVOOpenWindow equ -222
_LVOCloseWindow equ -72
_LVOWait equ -468
_LVOGetMsg equ -372
_LVOReplyMsg equ -378
_LVOSetAPen equ -330
_LVOMove equ -294
_LVOText equ -342
_LVORectFill equ -318

How to Compile and Run

Save the Code: Save the assembly code into a file named `asm_sound_button.s`.
Prepare the Sound: Convert a sound to `sound.raw` (8-bit signed mono) and place it in the same folder.
Assemble: Open your Terminal and run the command: `vasm -Fhunk -o asm_sound_button asm_sound_button.s`
Run in Emulator: Mount the folder containing your new executable and `sound.raw`. Boot into Workbench, open the Shell, and run `asm_sound_button`.
See the Result: A window will appear with a button. Clicking it will animate the button and play your sound!