;
; adv ptr hl^ by 128 modulo bc, de := unadvanced ptr, hl:= hl+1
; Wrap around signalled by no carry on exit with nz flag, when
; c returns the bytes to be wrapped.
; a,f,c,d,e,h,l
.advance::
	mov a,m	! mov e,a;		get read (from buffer) pointer 
	adi 128	! mov m,a ! inx h
	mov a,m	! mov d,a
	aci 0	! mov m,a;		and advance for next time
	dcx h	! mov a,m ! sub c
	mov	c,a
	inx h	! mov a,m ! sbb b
	rc;				advanced < size
	mov m,a	! dcx h ! mov m,c;	else reset to start
	inx h	! inr c ! dcr c;	check for wrap around
	ret