strncat.s

;
; Ullrich von Bassewitz, 31.05.1998
;
; char* strncat (char* dest, const char* src, size_t n);
;

       	.export		_strncat
	.import		popax
	.importzp	ptr1, ptr2, ptr3, tmp1, tmp2

_strncat:
	eor	#$FF		; one's complement to count upwards
	sta	tmp1
	txa
	eor	#$FF
	sta	tmp2
       	jsr	popax 	  	; get src
       	sta	ptr1
       	stx	ptr1+1
       	jsr	popax 	  	; get dest
  	sta	ptr2
  	stx	ptr2+1
  	sta	ptr3  	  	; remember for function return
  	stx    	ptr3+1
  	ldy	#0

; find end of dest

L1:	lda	(ptr2),y
  	beq	L2
  	iny
  	bne	L1
  	inc	ptr2+1
  	bne	L1

; end found, get offset in y into pointer

L2:	tya
  	clc
  	adc	ptr2
  	sta	ptr2
  	bcc	L3
  	inc	ptr2+1

; copy src. We've put the ones complement of the count into the counter, so
; we'll increment the counter on top of the loop

L3:	ldy	#0
	ldx	tmp1		; low counter byte

L4:	inx
	bne	L5
   	inc	tmp2
       	beq	L6 		; jump if done
L5:  	lda	(ptr1),y
   	sta	(ptr2),y
   	beq	L7
   	iny
   	bne	L4
   	inc	ptr1+1
   	inc	ptr2+1
   	bne	L4

; done, set the trailing zero and return pointer to dest

L6:    	lda	#0
   	sta	(ptr2),y
L7:	lda	ptr3
  	ldx	ptr3+1
  	rts

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