KEY  BIT   P3.3
SW1  BIT   P2.2         ;89s51 pin link to key
      ORG  000H
START: MOV sp,#40h      ;configure the stack register
       MOV  p2,#0ffh    
       MOV  p3,#0ffh
       MOV  P1,#0ffh;
       CLR   KEY
 WAIT: JB    KEY,$       ;wait for push key!
         JB   SW1,$ 
LCALL  DELAY     ;call the delay sub procedure
        CPL   P1.0          ;opposite  the status of the p1.0
        JNB   SW1,$
        JMP   wait
 DELAY: MOV  R3,#300
 D1:    MOV  R4,#300
         DJNZ  R4,$
         DJNZ  R3,D1
         RET
    END