di
        ld  e,1
        ld  hl,$c000
loop:   ld  (hl),e
bank:   ld  bc,$7ffd
        ld  a,16
        or  e
        out (c),a
        ld  a,(hl)
        sub e
        jr  nz, exit
        dec e
        jr  z, loop
        ld  c,a
exit:   dec e
        jr  z, bank
        ei
        ret