8-bit Divide: Difference between revisions

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m (clean up format)
(when the rol instruction itself produces a carry, subtract immediately; ring counter to save 15 deys; remainder)
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;by Bregalad
;by Bregalad
;Enter with A = Dividend, Y=Divisor
;Enter with A = Dividend, Y=Divisor
;Output with YA = (A/Y) << 8
;Output with YX = (A/Y) << 8, A = remainder
Division
Division
sta Dvd ;Stores dividend
sta Dvd ;Stores dividend
sty Dvs ;Stores divisor
sty Dvs ;Stores divisor
lda #$00
ldy #$00
sta Res ;Clear result
sty ResHi ;Clear result, setting up a ring counter
        iny            ;by initializing the result to $0001.
        sty Res        ;When the 1 gets shifted out, we're done


ldy #$10 ;The loop is for 16-bit result
ldy #$10 ;The loop is for 16-bit result
- asl Dvd
- asl Dvd
rol A ;Shift divisor in 1 bit
rol A ;Shift divisor in 1 bit
        bcs +          ;If carry is set, the dividend is already greater
cmp Dvs ;Check if fractional dividend is greater than divisor
cmp Dvs ;Check if fractional dividend is greater than divisor
bcc +
bcc ++
sbc Dvs ;Subtract (C is always set)
+ sbc Dvs ;Subtract (C is always set)
+ rol Res ;Shift result (1 if subtraction was done, 0 otherwise)
++ rol Res ;Shift result (1 if subtraction was done, 0 otherwise)
rol Res2
rol ResHi
dey
bcc -
bne -
ldx Res
lda Res
ldy ResHi
ldy Res2
rts
rts
</pre>
</pre>


[[Category:Arithmetic]]
[[Category:Arithmetic]]

Revision as of 23:04, 9 October 2014

The following code divide two 8-bit integers (range 0...255) and outputs a 16-bit result in fixed point 8.8 format. It is only using real calculations, no lookup table, thus the size of the code is very small.

;8-bit divide
;by Bregalad
;Enter with A = Dividend, Y=Divisor
;Output with YX = (A/Y) << 8, A = remainder
Division
	sta Dvd		;Stores dividend
	sty Dvs		;Stores divisor
	ldy #$00
	sty ResHi	;Clear result, setting up a ring counter
        iny             ;by initializing the result to $0001.
        sty Res         ;When the 1 gets shifted out, we're done

	ldy #$10	;The loop is for 16-bit result
-	asl Dvd
	rol A		;Shift divisor in 1 bit
        bcs +           ;If carry is set, the dividend is already greater
	cmp Dvs		;Check if fractional dividend is greater than divisor
	bcc ++
+	sbc Dvs 	;Subtract (C is always set)
++	rol Res		;Shift result (1 if subtraction was done, 0 otherwise)
	rol ResHi
	bcc -
	ldx Res
	ldy ResHi
	rts