REG_STATUS
while (!(mbox_read_4(sc, REG_STATUS) & STATUS_EMPTY))
while ((mbox_read_4(sc, REG_STATUS) & STATUS_EMPTY) == 0)
while ((mbox_read_4(sc, REG_STATUS) & STATUS_FULL) && --limit)
while ((mbox_read_4(sc, REG_STATUS) & STATUS_EMPTY))