8-bit Divide

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Revision as of 23:04, 9 October 2014 by Tepples (talk | contribs) (when the rol instruction itself produces a carry, subtract immediately; ring counter to save 15 deys; remainder)
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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
	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