;
; advance de by 1, modulo bc
; a,f,d,e
.advptr::
	inx	d
	mov	a,d
	cmp	b
	rnz;				not at buffer end
	mov	a,e
	sub	c
	rnz;				not at buffer end
	mov	e,a;			else zero the pointer
	mov	d,a
	ret