fox32rom/audio.asm

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; 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