E1000_TDT
KASSERT(EM_READ_REG(&sc->hw, E1000_TDT(que->me)) == que->tx.sc_tx_desc_head);
EM_WRITE_REG(&sc->hw, E1000_TDT(que->me), que->tx.sc_tx_desc_head);
case E1000_TDT(0):