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multiply_divide.asm
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; **_Multiply**
; 掛け算を行う関数
; - - -
; GBZ80 には mul命令 がないため関数として定義する必要がある
; すべての値は ビッグエンディアン として扱う
;
; c = a * b
;
; INPUT
; H_MULTIPLICAND, H_MULTIPLICAND+1, H_MULTIPLICAND+2 = 被乗数(a)
; H_MULTIPLIER = 乗数(b)
;
; OUTPUT
; FF95-FF98 = 掛け算の結果(c)
_Multiply:
ld a, $8
ld b, a
xor a
ld [H_PRODUCT], a
ld [H_MULTIPLYBUFFER], a
ld [H_MULTIPLYBUFFER+1], a
ld [H_MULTIPLYBUFFER+2], a
ld [H_MULTIPLYBUFFER+3], a
.loop
ld a, [H_MULTIPLIER]
srl a ; 1 >> a
ld [H_MULTIPLIER], a ; (aliases: H_DIVISOR, H_MULTIPLIER, H_POWEROFTEN)
jr nc, .smallMultiplier
; .notSmallMultiplier
ld a, [H_MULTIPLYBUFFER+3]
ld c, a
ld a, [H_MULTIPLICAND+2]
add c
ld [H_MULTIPLYBUFFER+3], a
ld a, [H_MULTIPLYBUFFER+2]
ld c, a
ld a, [H_MULTIPLICAND+1]
adc c
ld [H_MULTIPLYBUFFER+2], a
ld a, [H_MULTIPLYBUFFER+1]
ld c, a
ld a, [H_MULTIPLICAND] ; (aliases: H_MULTIPLICAND)
adc c
ld [H_MULTIPLYBUFFER+1], a
ld a, [H_MULTIPLYBUFFER]
ld c, a
ld a, [H_PRODUCT] ; (aliases: H_PRODUCT, H_PASTLEADINGZEROES, H_QUOTIENT)
adc c
ld [H_MULTIPLYBUFFER], a
.smallMultiplier
; b == 1 なら掛け算の必要なし
dec b
jr z, .done
ld a, [H_MULTIPLICAND+2]
sla a
ld [H_MULTIPLICAND+2], a
ld a, [H_MULTIPLICAND+1]
rl a
ld [H_MULTIPLICAND+1], a
ld a, [H_MULTIPLICAND]
rl a
ld [H_MULTIPLICAND], a
ld a, [H_PRODUCT]
rl a
ld [H_PRODUCT], a
jr .loop
.done
ld a, [H_MULTIPLYBUFFER+3]
ld [H_PRODUCT+3], a
ld a, [H_MULTIPLYBUFFER+2]
ld [H_PRODUCT+2], a
ld a, [H_MULTIPLYBUFFER+1]
ld [H_PRODUCT+1], a
ld a, [H_MULTIPLYBUFFER]
ld [H_PRODUCT], a
ret
_Divide:
xor a
ld [H_DIVIDEBUFFER], a
ld [H_DIVIDEBUFFER+1], a
ld [H_DIVIDEBUFFER+2], a
ld [H_DIVIDEBUFFER+3], a
ld [H_DIVIDEBUFFER+4], a
ld a, $9
ld e, a
.asm_37db3
ld a, [H_DIVIDEBUFFER]
ld c, a
ld a, [H_DIVIDEND+1] ; (aliases: H_MULTIPLICAND)
sub c
ld d, a
ld a, [H_DIVISOR] ; (aliases: H_DIVISOR, H_MULTIPLIER, H_POWEROFTEN)
ld c, a
ld a, [H_DIVIDEND] ; (aliases: H_PRODUCT, H_PASTLEADINGZEROES, H_QUOTIENT)
sbc c
jr c, .asm_37dce
ld [H_DIVIDEND], a ; (aliases: H_PRODUCT, H_PASTLEADINGZEROES, H_QUOTIENT)
ld a, d
ld [H_DIVIDEND+1], a ; (aliases: H_MULTIPLICAND)
ld a, [H_DIVIDEBUFFER+4]
inc a
ld [H_DIVIDEBUFFER+4], a
jr .asm_37db3
.asm_37dce
ld a, b
cp $1
jr z, .asm_37e18
ld a, [H_DIVIDEBUFFER+4]
sla a
ld [H_DIVIDEBUFFER+4], a
ld a, [H_DIVIDEBUFFER+3]
rl a
ld [H_DIVIDEBUFFER+3], a
ld a, [H_DIVIDEBUFFER+2]
rl a
ld [H_DIVIDEBUFFER+2], a
ld a, [H_DIVIDEBUFFER+1]
rl a
ld [H_DIVIDEBUFFER+1], a
dec e
jr nz, .asm_37e04
ld a, $8
ld e, a
ld a, [H_DIVIDEBUFFER]
ld [H_DIVISOR], a ; (aliases: H_DIVISOR, H_MULTIPLIER, H_POWEROFTEN)
xor a
ld [H_DIVIDEBUFFER], a
ld a, [H_DIVIDEND+1] ; (aliases: H_MULTIPLICAND)
ld [H_DIVIDEND], a ; (aliases: H_PRODUCT, H_PASTLEADINGZEROES, H_QUOTIENT)
ld a, [H_DIVIDEND+2]
ld [H_DIVIDEND+1], a ; (aliases: H_MULTIPLICAND)
ld a, [H_DIVIDEND+3]
ld [H_DIVIDEND+2], a
.asm_37e04
ld a, e
cp $1
jr nz, .asm_37e0a
dec b
.asm_37e0a
ld a, [H_DIVISOR] ; (aliases: H_DIVISOR, H_MULTIPLIER, H_POWEROFTEN)
srl a
ld [H_DIVISOR], a ; (aliases: H_DIVISOR, H_MULTIPLIER, H_POWEROFTEN)
ld a, [H_DIVIDEBUFFER]
rr a
ld [H_DIVIDEBUFFER], a
jr .asm_37db3
.asm_37e18
ld a, [H_DIVIDEND+1] ; (aliases: H_MULTIPLICAND)
ld [H_REMAINDER], a ; (aliases: H_DIVISOR, H_MULTIPLIER, H_POWEROFTEN)
ld a, [H_DIVIDEBUFFER+4]
ld [H_QUOTIENT+3], a
ld a, [H_DIVIDEBUFFER+3]
ld [H_QUOTIENT+2], a
ld a, [H_DIVIDEBUFFER+2]
ld [H_QUOTIENT+1], a ; (aliases: H_MULTIPLICAND)
ld a, [H_DIVIDEBUFFER+1]
ld [H_DIVIDEND], a ; (aliases: H_PRODUCT, H_PASTLEADINGZEROES, H_QUOTIENT)
ret