> > > Is there scope to break it down into three simpler multiplications?
> > >
> > > (a+b)^2 = a^2 + b^2 + 2ab

> hmmmm ...methinks I need an intuition implant...
> my gut feel was that something like Andy W's "straight line"
> solution would be a bit quicker than a few smaller calculations
> whose results are combined. My gut feel isn't the most reliable
> indication of anything, though :-) I'm working on improving it.
>
> Thanks to Keith, Andy and Scott for excellent suggestions
> and examples... looks like I've got a few routines to
> write! If I'm brave enough, I'll post the fastest one I can
> come up with this week.

Something like this? [warning: untested]

        clrf    DstH
        clrf    DstM
        clrf    DstL
        movf    SrcL,w
        andlw   $0F
        btfss   Src,4
         addwf  DstM
        rrf     DstM
        rrf     DstL
        btfss   Src,5
         addwf  DstM
        rrf     DstM
        rrf     DstL
        btfss   Src,6
         addwf  DstM
        rrf     DstM
        rrf     DstL
        btfss   Src,7
         addwf  DstM
        call    Sqr4
        addwf   DstL
        swapf   SrcL
        andlw   $0F
        call    Sqr4
        addwf   DstM
                        ; At this point, 16-bit result is in DstM:DstH
                        ; 25 words of code prior to this point (plus a
                        ; 17-word table-lookup).  Total execution time:
                        ; 35 cycles up to this point.
        btfss   SrcH,0
         goto   NoBit8
        movf    SrcL,w
        btfsc   C
         incf   DstH
        addwf   DstM
        btfsc   C
         incf   DstH
        incf    DstH
                        ; Another 9 words for bit 8; 3 or 9 cycles to exec.
NoBit8:
        btfss   SrcH,1
         goto   NoBit9
        movlw   4
        btfss   SrcH,0
         movlw  8
        addwf   DstH
        rlf     SrcL,w
        btfsc   C
         incf   DstH
        btfsc   C
         incf   DstH
        addwf   DstM
        btfsc   C
         incf   DstH
        addwf   DstM
        btfsc   C
         incf   DstH
                        ; Another 17 words for bit 9; 3 or 17 cycles to execute
                        ; Total worst-case time: 35+26 = 61 cycles.
NoBit9:
        retlw   0       ; All done!
Sqr4:
        addwf   PC
        db      0,1,4,9,16,25,36,49,64,81,100,121,144,169,196,225