cc65/libsrc/common/_swap.s

;
; Ullrich von Bassewitz, 1998-12-09, 2004-11-30
;
; void __fastcall__ _swap (void* p, void* q, size_t size);
;

      	.export	       	__swap
      	.import		popax
      	.importzp	ptr1, ptr2, ptr3


__swap:	eor     #$FF
        sta	ptr3
        txa
        eor     #$FF
       	sta	ptr3+1          ; Save -(size+1) into ptr3

      	jsr	popax		; Get q
      	sta	ptr2
      	stx	ptr2+1

      	jsr	popax		; Get p
      	sta	ptr1
      	stx	ptr1+1

; Prepare for swap

      	ldy	#$00

; Swap loop

@L1:    inc     ptr3            ; Bump counter low byte
        beq     @L3             ; Branch on overflow

@L2:    lda	(ptr1),y
      	tax
      	lda	(ptr2),y
      	sta	(ptr1),y
      	txa
      	sta	(ptr2),y
      	iny
      	bne	@L1
        inc     ptr1+1
        inc     ptr2+1
        bne     @L1             ; Branch always (hopefully)

; Bump the high counter byte

@L3:    inc     ptr3+1
        bne     @L2

; Done

        rts