Thanks Scott, I think I only need to shift a max of 4 times, with the multiplier
being only 4 bits, I need to do this longhand on paper to get my head round it!
Thanks for the help:-)

mike.




scott@DATTALO.COM on 09/01/2002 23:56:58
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Subject:        Re: [PIC]: 12x4 multiply needed. (16f8XX)

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On Tue, 8 Jan 2002, Mike Blakey wrote:

> I am looking for a *smart* 12bit by 4bit unsigned multiply. I have seen several
> methods, some of which I can follow and some that I don't yet fully understand as
> they are un commented I would prefer the execution time to be the same length for
> all number combinations (including zero x zero). any help in understanding this
> would be most appreciated.

A simple unrolled multiplication is probably what you want:

;multiply x(12bits) times y (bits)

     clrf  resH      ; result goes here
;    clrf  resL      ; Don't need to clear resL

     movf   xL,w     ; First multiply lower 8 bits
     btfsc  y,0
      addwf resH,F

     clrc            ;in case we didn't add

     rrf    resH,F
     rrf    resH,F

     btfsc  y,1
      addwf resH,F

     rrf    resH,F
     rrf    resL,F

     btfsc  y,2
      addwf resH,F

     rrf    resH,F
     rrf    resL,F

     btfsc  y,3
      addwf resH,F

     rrf    resH,F
     rrf    resL,F

  ; Now for the high nibble

     swapf  xH,w      ;Shift xH "right" 4 bits to match
     andlw  0xf0      ;the four shifts above.

     btfsc  y,0
      addwf resH

     clrc            ;in case we didn't add

     rrf    resH,F
     rrf    resL,F

     btfsc  y,1
      addwf resH,F

     rrf    resH,F
     rrf    resL,F

     btfsc  y,2
      addwf resH,F

     rrf    resH,F
     rrf    resL,F

     btfsc  y,3
      addwf resH,F

     rrf    resH,F
     rrf    resL,F

This, of course, is untested.
The only way I can think of explaining it is by saying that it's just a
clasic shift-and-add multiply routine that takes advantage of the weird
bit sizes of the operands.

Scott

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