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51 lines
643 B
NASM
51 lines
643 B
NASM
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_Sine:: ; 84d9
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; A simple sine function.
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; Return d * sin(e) in hl.
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; e is a signed 6-bit value.
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ld a, e
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and %111111
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cp %100000
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jr nc, .negative
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call .ApplySineWave
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ld a, h
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ret
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.negative
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and %011111
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call .ApplySineWave
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ld a, h
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xor -1
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inc a
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ret
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.ApplySineWave: ; 84ef
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ld e, a
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ld a, d
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ld d, 0
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ld hl, .sinewave
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add hl, de
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add hl, de
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ld e, [hl]
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inc hl
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ld d, [hl]
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ld hl, 0
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; Factor amplitude
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.multiply
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srl a
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jr nc, .even
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add hl, de
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.even
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sla e
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rl d
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and a
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jr nz, .multiply
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ret
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.sinewave: ; 850b
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; A $20-word table representing a sine wave.
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; 90 degrees is index $10 at a base amplitude of $100.
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sine_wave $100
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