On Mon, 12 Jul 1999 16:10:36 +0100 peterc <peterc@HMGCC.GOV.UK> writes:
>Can anyone improve on the following for copying a variable number of
>bytes from one area of memory to another. Both destination and source
>locations are variable.

The best optimization is to place the data strategically in RAM so a less
general-purpose routine can be used.  If you can arrange it so the
address in FSR can be changed between the source and destination by
setting and clearing bits, then it isn't necessary to use a temporary
location to save the data.  To do that the address of the source and
destination should be seperated by a power of two bytes.

The routine originally posted can be optimized a little by realizing the
two bytes to be accessed are always a fixed distance away in RAM.  So you
could add and subtract a constant offset to FSR to change between source
and destination.  This way the address can be kept in FSR.

; dest = address of first destination byte
; offset = distance between source and dest  (source - dest)
; count = number of bytes to move

        decf    dest,w          ;Correct for incrementing
later.
        movwf   FSR             ;Set FSR to start
move_loop
        incf    offset,w        ;Offset + 1 to W
        addwf   FSR,f           ;point to next source byte
        movfw   INDF
        movwf   temp
        movfw   offset
        subwf   FSR,f           ;Point back at destination byte
        movfw   temp            ;Get data back
        movwf   INDF            ;Store.
        decfsz  count,f         ;Do another?
        goto    move_loop       ;Yes.

This takes 11 cycles per loop to move a byte, the original one takes 12
(if you use a decfsz for the count).  So no big savings.  I think it is
possible to use a swap trick to not need a temporary byte, but it would
take more cycles.  The routine is easiest to understand if the source
data is at a higher address than the destination data.  I think it will
work properly if it is not though.

Hopefully the PIC18 will have dual FSRs to make this sort of stuff easy.


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