ORG 0
          LJMP MAIN
          ORG 23H
          LJMP SERIAL    ;jump to serial ISR
          ORG 30H
MAIN:     MOV P1,#0FFH   ;make P1 an input port
          MOV TMOD,#20H  ;timer 1, mode 2 (auto reload)
          MOV TH1,#0FDH  ;9600 baud rate
          MOV SCON,#50H  ;8-bit,1 stop, REN enabled
          MOV IE,#10010000B   ;enable serial interrupt
          SETB TR1            ;start timer 1
BACK:     MOV  A,P1           ;read data from port 1
          MOV  P2,A           ;send it to P2
          SJMP BACK           ;stay in loop indefinitely
;------------------SERIAL PORT ISR
          ORG 100H
SERIAL:   JB  TI,TRANS        ;jump if TI is high
          MOV A,SBUF          ;otherwise due to receive
          MOV P0,A            ;send incoming data to P0
          CLR RI              ;clear RI since CPU doesn't
          RETI           ;return from ISR
TRANS:    CLR TI              ;clear TI since CPU doesn't
          RETI                ;return from ISR
          END