What sort of problem are you having?
What happens when you run this?
Code:
    BAUD    CON 16572  '4800 INVERTED, PC
    TX       VAR PORTB.0
    SUM     VAR BYTE
    NUM     VAR BYTE
    NUM = 0

    BOOT:
    SEROUT2 TX,BAUD,[$d,$a]
    SEROUT2 TX,BAUD,["NUM= ",DEC NUM,$d ,$a]
    SEROUT2 TX,BAUD,["SUM= ",DEC SUM,$d,$a]
    PAUSE 1000
    WRITE 300,NUM
    PAUSE 100
    READ 300,SUM
    PAUSE 100
    NUM = NUM + 1
    GOTO BOOT