Start:
  banksel  ANSELA
  bcf   ANSELA,3     ; set RA3 as digital         
  banksel TRISA     
  bsf  TRISA3           ; set RA3 as an input pin
Loop:
  banksel PORTA
  btfsc  PORTA,3  ;  "test" RA3
  goto  Exception_Routnine  ; if 1 do this
  goto Loop    

Exception_Routine 
 ;  exception code goes here