Hi,

Your 32 bit addition is incorrect. It should be:

        movf    twofour0,w
        addwf   prod0,f

        movf    twofour1,w
        btfsc   STATUS,c
         incfsz twofour1,w
          addwf prod1,f

        movf    twofour2,w
        btfsc   STATUS,c
         incfsz twofour2,w
          addwf prod2,f

        movf    twofour3,w
        btfsc   STATUS,c
         incfsz twofour3,w
          addwf prod3,f

regards,
Reggie

Saurabh Sinha wrote:
>
> Byron,
>
> Thank you for your code. I tried to run it, but I am having certain
> problems. Take for example,
> the following problem, in 2s complement.
>
> The following variable declerations are used:
>
> TWOFOUR2 B'00001000'  Equivalent: c0=534035 (dec)
> TWOFOUR1 B'00100110'
> TWOFOUR0 B'00010011'
>
> MULTIPLIER B'00000111'  Equivalent: u0=7 (dec)
>
> Result: (MPLAB)
>
> PROD2        B'00001000'   Equivalent: c0u0 (dec)
> PROD1        B'00100110'
> PROD0      B'00010011'
>
> This does not seem to be the correct answer.
>
> Any ideas on where the error could possibly be, or is it just me getting
> confused with 2's comp...?
>
> Thanks in advance for your help.
>
> Regards,
>
> Saurabh
> ======================================================CODE
> USED================================
>
> 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
>         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
>         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
>         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
>  return
> return