_cleanstack:
    ld hl, (sp)
    ld bc, sp
    or a
    sbc hl, bc
    ld (_stacklen), hl
    ld bc, (_stacklen)
    ld de, sp
    ld hl, sp
    ld (de), 0
    inc de
    dec bc
    ldir
    ex de, hl
    ld sp, hl
    ret
_stacklen:  rb 3