Failed to use USART transmit interrupt (solved)

Hardware Arduino Due
11

I could not find an extra bit for clearing interrupt. Normally TxD-Interrupt is cleared by writing THR and RxD-Interrupt is cleared by reading RHR.
I am happy with my new interrupt handled serial communication class now, everything works perfectly.
Here is my final IRQ-Handler:

void SerialCom::IrqHandler()
{
  status = usartPtr->US_CSR;

  // --------------------------------------------------------------------------
  // Transmit-Interrupt (Sendepuffer ist frei)
  // --------------------------------------------------------------------------
  //
  if(status & US_CSR_TXRDY)
  {
    // Der Sendepuffer ist frei
    //
    if(restSend > 0)
    {
      // es sind noch Zeichen zum Senden da
      //
      usartPtr->US_THR = *ptrSend;
      ptrSend++;
      restSend--;
    }

    // Der Transmit-Interrupt wird gesperrt,
    // wenn das letzte Zeichen ausgegeben wurde.
    if(restSend < 1)
      usartPtr->US_IDR = US_IDR_TXRDY;
  }

  // --------------------------------------------------------------------------
  // Receive-Interrupt (Zeichen wurde empfangen)
  // --------------------------------------------------------------------------
  //
  if((status & US_CSR_RXRDY) && (restRec > 0))
  {
    // Ein Byte wurde empfangen und
    // es sind noch Speicherplätze frei
    //
    *ptrRec = usartPtr->US_RHR;
    ptrRec++;
    restRec--;
  }
}

And here is the sending function:

void SerialCom::write(uint8_t *wrPtr, int nrOfBytes)
{
  restSend  = nrOfBytes - 1;
  usartPtr->US_THR = *wrPtr;

  if(nrOfBytes > 1)
  {
    ptrSend   = wrPtr + 1;
    usartPtr->US_IER = US_IER_TXRDY;
  }
}

Thank You again for Your interest and valuable comments.
For me this issue is solved.