fox32rom: Add play_audio and stop_audio routines

audio.asm

@@ -0,0 +1,97 @@
+; audio routines
+
+; use ffmpeg to convert an audio file for playback:
+; ffmpeg -i input.mp3 -f s16le -ac 1 -ar 22050 audio.raw
+
+; fox32's audio output works by swapping between two buffers
+; interrupt 0xFE (vector 0x000003F8) is fired when it's time to swap buffers
+; when that interrupt fires, it calls `refill_buffer` which fills up the buffer
+
+const CURRENT_BUFFER: 0x01FFFF00
+const AUDIO_POINTER: 0x01FFFF01
+const AUDIO_LENGTH: 0x01FFFF05
+const OLD_BUFFER_SWAP_VECTOR: 0x01FFFF09
+
+; play an audio clip (does not block)
+; inputs:
+; r0: pointer to audio clip
+; r1: length of audio clip in bytes (must be a multiple of 32768 bytes)
+; outputs:
+; none
+play_audio:
+    push r0
+    push r1
+
+    ; set the interrupt vector for interrupt 0xFE and save the old one
+    mov [OLD_BUFFER_SWAP_VECTOR], [0x000003F8]
+    mov [0x000003F8], refill_buffer
+
+    ; store audio pointer
+    mov [AUDIO_POINTER], r0
+
+    ; store audio length
+    ; floor it to the nearest multiple of 32768
+    and r1, 0xFFFF8000
+    mov [AUDIO_LENGTH], r1
+
+    ; reset the current buffer to zero
+    ; fox32 resets its current buffer to zero internally, so reset it to zero here to stay in sync
+    ; refill_buffer will bitwise not this before checking it,
+    ; so set it to 0xFF so that it becomes zero when checked for the first time
+    mov.8 [CURRENT_BUFFER], 0xFF
+
+    ; enable audio playback
+    mov r0, 0x80000600
+    out r0, 1
+
+    pop r1
+    pop r0
+    ret
+refill_buffer:
+    push r0
+    push r1
+    push r31
+
+    not.8 [CURRENT_BUFFER]
+    cmp.8 [CURRENT_BUFFER], 0
+    ifnz jmp refill_buffer_1
+refill_buffer_0:
+    mov r31, 8192 ; 32768 bytes = 8192 words
+    mov r0, [AUDIO_POINTER]
+    mov r1, 0x0212C000 ; buffer 0 address
+refill_buffer_loop:
+    mov [r1], [r0]
+    add r0, 4
+    add r1, 4
+    loop refill_buffer_loop
+    mov [AUDIO_POINTER], r0
+    sub [AUDIO_LENGTH], 32768
+    ifz call stop_audio
+
+    pop r31
+    pop r1
+    pop r0
+    reti
+refill_buffer_1:
+    mov r31, 8192 ; 32768 bytes = 8192 words
+    mov r0, [AUDIO_POINTER]
+    mov r1, 0x02134000 ; buffer 1 address
+    jmp refill_buffer_loop
+
+; stop audio playback
+; inputs:
+; none
+; outputs:
+; none
+stop_audio:
+    push r0
+
+    ; disable audio playback
+    mov r0, 0x80000600
+    out r0, 0
+
+    ; restore the old buffer swap vector
+    mov [0x000003F8], [OLD_BUFFER_SWAP_VECTOR]
+
+    pop r0
+    ret

fox32rom.def

@@ -71,6 +71,10 @@ string_length:                            jmp [0xF0046018]
 ; integer jump table
 string_to_int:                            jmp [0xF0047000]
 
+; audio jump table
+play_audio:                               jmp [0xF0048000]
+stop_audio:                               jmp [0xF0048004]
+
 ; event types
 const EVENT_TYPE_MOUSE_CLICK:    0x00000000
 const EVENT_TYPE_MOUSE_RELEASE:  0x00000001

main.asm

@@ -112,6 +112,7 @@ get_rom_version:
     ret
 
     ; code
+    #include "audio.asm"
     #include "background.asm"
     #include "boot.asm"
     #include "cursor.asm"
@@ -218,6 +219,11 @@ get_rom_version:
     org.pad 0xF0047000
     data.32 string_to_int
 
+    ; audio jump table
+    org.pad 0xF0048000
+    data.32 play_audio
+    data.32 stop_audio
+
     org.pad 0xF004F000
 standard_font_width:
     data.16 8