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61 lines
853 B
ArmAsm
61 lines
853 B
ArmAsm
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; approx sine
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; uses two parabolas to approximate sine
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; see https://codebase64.org/doku.php?id=base:generating_approximate_sines_in_assembly
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VALUE_L = $F0
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VALUE_H = $F1
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DELTA_L = $F2
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DELTA_H = $F3
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approx_sine:
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initSineTable:
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ldy #$3f
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ldx #$00
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stx VALUE_L
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stx VALUE_H
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stx DELTA_L
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stx DELTA_H
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; Accumulate the delta (normal 16-bit addition)
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outer_loop:
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lda VALUE_L
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clc
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adc DELTA_L
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sta VALUE_L
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lda VALUE_H
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adc DELTA_H
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sta VALUE_H
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; Reflect the value around for a sine wave
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sta sinetable+$c0,X
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sta sinetable+$80,Y
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eor #$ff
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sta sinetable+$40,X
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sta sinetable+$00,Y
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; Increase the delta, which creates the "acceleration" for a parabola
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lda DELTA_L
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adc #$10 ; this value adds up to the proper amplitude
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sta DELTA_L
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bcc skip_oflo
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inc DELTA_H
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skip_oflo:
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; Loop
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inx
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dey
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bpl outer_loop
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end:
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jmp end
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sinetable=$6000
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