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