The program is supposed to display OPEN on pressing O and CLOSE on pressing C on the terminal screen

$mod51
ORG 0000H ;Origin address
LJMP MAIN ; Jump to intialisation
ORG 000BH

ORG 0023H ;Serial interupt
LJMP SERIAL ; Jump to serial action routine


ORG 30H
MAIN: MOV P1,#0FFH ;Make P1 input port
MOV TMOD,#20H ; Timer1 8 bit auto reload
MOV TH1,#0FDH ;Set TH1 for 9600 bps
MOV SCON,#50H ;8 bit data mode with 1 start and stop bit and receive enable
MOV IE,#90H ;Global interrupt, serial interrupt enable,TIMER OVERFLOW ET0 INTERRUPT
CLR P1.0 ;led Glows
SETB TR1 ; Start timer for serial transmission
CLR TI


LOOP:SJMP LOOP

;-----------------END OF MAIN------------------------
ORG 350H

SERIAL: JB TI,CLEAR

MOV A,SBUF

CJNE A,#'S',RET_OPEN
MOV SBUF,#'K'
SJMP RET_END

RET_OPEN:CJNE A,#'O',RET_CLOSE
CLR P1.0
CLR P1.1
MOV SBUF,#'O'
MOV SBUF,#'P'
MOV SBUF,#'E'
MOV SBUF,#'N'
SJMP RET_END

RET_CLOSE:CJNE A,#'C',RET_END
SETB P1.0
SETB P1.1
MOV SBUF,#'C'
MOV SBUF,#'L'
MOV SBUF,#'O'
MOV SBUF,#'S'
MOV SBUF,#'E'

RET_END:
CLR RI
RETI

CLEAR: CLR TI
RETI

END