Accesing the USART registers by Arduino
It is handed the addresses when the object is constrcted:

 HardwareSerial Serial(&amp;UBRRH, &amp;UBRRL, &amp;UCSRA, &amp;UCSRB, &amp;UCSRC, &amp;UDR);


&amp;UDR is passed as *udr. That is then assigned to _udr in the initialiser list.