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cc65/libsrc/atari/sound.s

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;
; Mariano Domínguez
; 2022-12-4
;
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; this file provides an equivalent to the BASIC SOUND function
;
; void __fastcall__ _sound (unsigned char voice, unsigned char frequency, unsigned char distortion, unsigned char volume);
;
.include "atari.inc"
.export __sound
.import popa
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.importzp tmp1,tmp2
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; play sound, arguments: voice, pitch, distortion, volume
.proc __sound
sta tmp2 ;save volume
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jsr popa ;get distortion
sta tmp1 ;save distortion
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jsr popa ;get pitch
pha ;save in stack
jsr popa ;get voice
asl a ;adjust voice *2 for offset in x
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tax
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pla ;get pitch from stack
sta AUDF1,x ;store pitch
lda #0
sta AUDCTL
lda #3
sta SKCTL ;init sound
lda tmp1 ;get distortion
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asl a ;ignore the high nibble
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asl a
asl a
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asl a
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clc ;setup for adding volume
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adc tmp2 ;add volume
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sta AUDC1,x ;volume + distortion in control channel
rts
.endproc