80 lines
1.4 KiB
Plaintext
80 lines
1.4 KiB
Plaintext
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.include "vcs-ca65.inc"
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.export _sound_update
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.export _sound_play
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.import _sound_duration
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.import _sound_data
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.zeropage
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_sndchan_sfx: .res 2
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_sndchan_timer: .res 2
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.exportzp _sndchan_sfx
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.exportzp _sndchan_timer
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.importzp ptr3
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sounddata = ptr3
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.segment "ROM3"
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; 00-1F: set frequency
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; 81-9F odd: set volume
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; 80-9E even: set control register
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.proc _sound_update
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ldx #0
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jsr _sound_update_channel
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ldx #1
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.endproc
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.proc _sound_update_channel
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; decrement timer
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lda _sndchan_timer,x
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beq @done
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dec _sndchan_timer,x
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beq @killsound
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; get sound data address
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lda _sndchan_sfx,x
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asl
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tay
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lda _sound_data+0,y
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sta sounddata+0
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lda _sound_data+1,y
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sta sounddata+1
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; get next sound data byte
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ldy _sndchan_timer,x
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lda (sounddata),y ; get sound data
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bpl @setfreq ; hi bit clear = just freq
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lsr ; right shift (/ 2)
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bcs @setvol ; lo bit set = volume
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sta AUDC0,x ; lo bit clear = control
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lsr ; also set freq (/ 2)
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@setfreq:
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sta AUDF0,x ; set frequency
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rts
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@killsound:
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lda #0
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@setvol:
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sta AUDV0,x ; set volume
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@done:
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rts
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.endproc
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.proc _sound_play
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; but which channel to use?
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; whichever has the lower timer counter
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ldx #0
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ldy _sndchan_timer+0
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cpy _sndchan_timer+1
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bcc @Chan0Free ; timer0 < timer 1?
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inx
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@Chan0Free:
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sta _sndchan_sfx,x
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tay
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lda _sound_duration,y
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sta _sndchan_timer,x
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rts
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.endproc
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