>
>Hi Byron,
>
>There is a generic multibyte addition routine that is on the Microchip
>docs.  I don't know who made it but works like a charm.
>
>;A = A + B
>
>        movf    B0,w
>        addwf   A0,f            ; add LSB
>
>        movf    B1,w            ; W = next byte
>        skpnc                   ; skip next instr if there's a carry
>         incfsz B1,w            ; inc W and skip next if B1 = 0xFF, C is still set
>          addwf A1,f            ; add it except if C and B1 = 0xFF
>
>        movf    B2,w            ; carry is propagated here if B1 was 0xFF
>        skpnc                   ; and C was set
>         incfsz B2,w            ; or from the last addition
>          addwf A1,f
>
>        movf    B3,w
>        skpnc
>         incfsz B3,w
>          addwf A3,f

I see. It basically does the same carry propogation I did in line instead
of out of sequence.

BAJ

>>>>>>mult24x8:
>>>>>>        clrf    prod0           ; Clear the product
>>>>>>        clrf    prod1
>>>>>>        clrf    prod2
>>>>>>        clrf    prod3
>>>>>>        clrf    twofour3        ; Clear the top byte of the multiplicand
>>>>>>return
>>>>>>
>>>>>>multloop:
>>>>>>        movf    mulitiplier,W   ; Check to see if eight bit is empty
>>>>>>        btfsc   STATUS,Z        ; Done if zero
>>>>>>        return
>>>>>>        bcf     STATUS,C        ; clear carry
>>>>>>        rrf     mulitiplier,F   ; Get the next bit
>>>>>>        btfss   STATUS,C        ; Add if carry is set
>>>>>>        goto    multshift       ; shift only if 0
>>>>>>        movf    twofour0,W      ; Add the 32 bits of multiplicand to
>>>>product
>>>>>>        addwf   prod0,F         ; each addwf sets the carry
>>>>>>        btfsc   STATUS,C        ; Don't increment if no carry
>>>>>>        incf    prod1,F         ; Add one to next byte if carry occured
>>;Replace the line above with a call to the carry propogation routine.
>>                call    carryprop1
>>>>>>        movf    twofour1,W      ;
>>>>>>        addwf   prod1,F         ;
>>>>>>        btfsc   STATUS,C        ; Don't increment if no carry
>>>>>>        incf    prod2,F         ; Add one to next byte if carry occured
>>;Replace the line above with a call to the carry propogation routine.
>>                call    carryprop2
>>>>>>        movf    twofour2,W      ;
>>>>>>        addwf   prod2,F         ;
>>>>>>        btfsc   STATUS,C        ; Don't increment if no carry
>>>>>>        incf    prod3,F         ; Add one to next byte if carry occured
>>;Propogation unnecessary for the last byte. Leave as incf above.
>>>>>>        movf    twofour3,W      ;
>>>>>>        addwf   prod3,F         ;
>>>>>>
>>>>>>multshift:
>>>>>>        bcf     STATUS,C        ; clear carry
>>>>>>        rlf     twofour0,F      ; Shift the 24 bits one bit to the left
>>>>>>        rlf     twofour1,F
>>>>>>        rlf     twofour2,F
>>>>>>        rlf     twofour3,F
>>>>>
>>>>>          goto    multloop ; need to add all partial products.
>>
>>carryprop1      incfsz  prod1,F
>>                return
>>carryprop2      incfsz  prod2,F
>>                return
>>carryprop3      incf    prod3,F
>>                return
>>
>>>>>
>>>>>> return ; Neither of these returns are required.
>>>>>>return ; The routine will return when the multiplier is 0.
>>>>>
>>>>>>
>>>
>