LIST          P=PIC16F84A
             __CONFIG      3FF2H
PORTB        EQU           06H
W            EQU           0
F            EQU           1
PORTA        EQU           05H
TMR0         EQU           01H
TRISA        EQU           05H
TRISB        EQU           06H
INTCON       EQU           0BH
REG_1        EQU           0CH
OPTION_REG   EQU           01H
STATUS       EQU           03H
RP0          EQU           5
Z            EQU           2
             ORG 0
             GOTO          START
             ORG 4
PRERIV       MOVF          STATUS,W
             INCF          REG_1,F
             MOVWF         STATUS             
             BCF           INTCON,2
             RETFIE
START        CLRF          PORTA
             MOVLW         .255
             MOVWF         PORTB
             CLRF          INTCON
             CLRF          REG_1
             BSF           STATUS,RP0
             CLRF          TRISA
             CLRF          TRISB
             MOVLW         B'11010111'
             MOVWF         OPTION_REG
             MOVLW         B'10100000'
             BCF           STATUS,RP0
             MOVWF         INTCON
             CLRF          TMR0
RAB          MOVLW         .255             
             BCF           STATUS,Z
             SUBWF         REG_1,W
             BTFSC         STATUS,Z
             CLRF          PORTB
             GOTO          RAB
             END