cc65/libsrc/runtime/udiv.s

;
; Ullrich von Bassewitz, 07.08.1998
;
; CC65 runtime: division for unsigned ints
;

       	.export		tosudiva0, tosudivax, udiv16
 	.import		popsreg
 	.importzp	sreg, ptr1, ptr4


tosudiva0:
        ldx     #$00            ; Clear high byte
tosudivax:
       	sta	ptr4
       	stx	ptr4+1 	       	; Save right operand
       	jsr	popsreg		; Get left operand

; Do the division

 	jsr	udiv16

; Result is in sreg, remainder in ptr1

 	lda	sreg
 	ldx	sreg+1
 	rts

;---------------------------------------------------------------------------
; 16by16 division. Divide sreg by ptr4. Result is in sreg, remainder in ptr1
; (see mult-div.s from "The Fridge").
; This is also the entry point for the signed division

udiv16:	lda	#0
 	sta	ptr1+1
 	ldy	#16
        ldx     sreg+1
        beq     udiv16by8a

L0:	asl	sreg
 	rol	sreg+1
 	rol	a
 	rol	ptr1+1

 	pha
 	cmp	ptr4
 	lda	ptr1+1
 	sbc	ptr4+1
 	bcc	L1

 	sta	ptr1+1
 	pla
 	sbc	ptr4
 	pha
 	inc	sreg

L1:	pla
 	dey
 	bne	L0
 	sta	ptr1
 	rts


;---------------------------------------------------------------------------
; 16by8 division

udiv16by8:
        lda	#0
 	sta	ptr1+1
 	ldy	#16
udiv16by8a:
@L0:	asl	sreg
  	rol	sreg+1
  	rol	a
        bcs     @L1

   	cmp	ptr4
    	bcc	@L2
@L1:    sbc	ptr4
    	inc	sreg

@L2:	dey
    	bne    	@L0
    	sta    	ptr1
    	rts