The serial port included in the 8051 can operate in full-duplex mode (send and receive simultaneously). The serial port interrupt is generated by TI OR RI. So your ISR (or your polling mechanism alternatively), must test TI and RI separately, execute the suitable code, and clear the corresponding flag (TI, RI). Take into account that both TI and RI might be active when the ISR enters.

Alfredo del Rio.