;_-Upon wake-up go to main, avoid using memory space ;allocated to Interrupt Vector Table 
          ORG  0000H
          LJMP MAIN      ;by-pass interrupt vector table
;
;_-ISR for Timer 0 to generate square wave 
          ORG  000BH     ;Timer 0 interrupt vector table
          CPL  P2.1      ;toggle P2.1 pin
          RETI           ;return from ISR
;
;_-The main program for initialization 
          ORG  0030H     ;after vector table space
MAIN:     MOV  TMOD,#02H ;Timer 0,mode 2(auto reload)
          MOV  P0,#0FFH  ;make P0 an input port 
          MOV  TH0,#-92  ;TH0=A4H for -92  
          MOV  IE,#82H  ;IE=10000010 (bin) enable Timer 0
          SETB TR0       ;Start Timer 0
BACK:     MOV  A,P0      ;get data from P0
          MOV  P1,A      ;issue it to P1
          SJMP BACK      ;keep doing it
                         ;loop unless interrupted by TF0
          END