Here is the way I do it for 16 bit in assembler.  There are some simple
macros I use based on the CARRY bit too.

        MOVFW   A3
        ADDWF   B3,F
        IFCARRY
        INCF            B2,F
        MOVFW   A2
        ADDWF   B2,F
        IFCARRY
        INCF            B1,F
        MOVFW   A1
        ADDWF   B1,F
        IFCARRY
        INCF            B0,F
        MOVFW   A0
        ADDWF   B0,F
        IFCARRY
        GOTO            OVERFLOW

  <edtodd@sni.net>    Ed Todd