_cleanstack:
    or a
    ld hl, sp
    ex de, hl
    ld hl, (sp)
    sbc hl, de      ; bytes to 0 in hl
    ex de, hl       ; bytes to 0 in de
    ld hl, sp       ; addr to start in hl
    ex de, hl       ; addr to start in de, bytes to 0 in hl
.zero:
    ld (de), a
    dec hl
    inc de
    sbc hl, 0
    jr nz, .zero
    ex de, hl       ; bottom of stack in hl
    ld sp, hl