Actually, in this case, the links Erik cited constitute the "old testament" description.

For a description of the Enhanced UART facilities - including automatic address recognition - you need Intel Application Note AP-410, "Enhanced Serial Port on the 83C51FA"
http://www.intel.com/design/mc...270490.htm.
It includes sample code.

Automatic address recognition makes the software much simpler, and greatly reduces the loading on the processor.

It was discussed at length on the Keil forum:
http://www.keil.com/discuss/docs/thread2882.htm

See also
http://www.8052.com/forum/read.phtml?id=60664
http://www.keil.com/discuss/docs/thread2849.htm