8051 is half duplex or full duplex while communicating serially..if its half duplex then can we configure it to function as a full duplex
How else would you do it?
Think about how the hardware - any hardware - has to work:
You need one register to supply data to the transmitter; You need one register to retreive data from the receiver.
That's inherently two registers, isn't it?!
Some products give them distinct names - so one is read-only and the other is write-only; The 8051 gives them both the same name - they are distinguished by the read or write operation.
In processors with a limited address space, it would be a waste to give them separate addresses!