When a character is received, if receive mode is enabled, RI flag is set. This leads to the interruption of the main program and the processor goes to the interrupt vector location, i.e. 0023H for serial port. The interrupt service routine at 0023H gets executed to read the character so that the next character can be received. The following program receives a character on interrupt basis and outputs the character to port-1, possibly for a display.
The crystal frequency is12MHz and baud rate is set at 1202 baud.