> The following code will do a computed branch to any address in the
> address space.
>
>     MOVLW    HIGH FARLABEL
>     MOVWF    PCLATH
>     MOVLW    LOW FARLABEL
>     ADDWF    OFFSET,W
>     MOVWF    PCL
>     .
>     .
>     .
>
> FARLABEL

Not exactly.  This assumes that LOW FARLABEL plus the contents of offset
does not exceed 255.  This is only guaranteed to be true if the table at
FARLABEL starts at a 256 word boundary (FARLABEL is multiple of 256).  A
more general approach is:

     movlw    high table  ;get table start address high byte
     movwf    pclath      ;init jump address high byte
     movf     offset, w   ;get table entry number in W
     addlw    low table   ;make table entry address low byte
     skip_nc              ;no carry into high address byte ?
     incf     pclath      ;propagate carry into high adr byte
     movwf    pcl         ;jump to the selected table entry

This is the approach I recommend you always use except in rare cases where
the few extra instructions matter for speed or memory size.  Note that
this code needs a slight tweak for the 18 family, since each instruction
takes up 2 memory addresses, and GOTOs take up 4.

> Another approach I have used is to put all of my lookup tables or
> subroutines into specific pages, then if I need to do multiple accesses
> I set the PCLATH register once at the beginning.

That sort of manual management of PCLATH will sooner or later cause some
nasty bugs.  It is also a maintenence nightmare in the making.

I use more automated PCLATH handling.  See the GCALL and related macros in
STD.INS.ASPIC at http://www.embedinc.com/pic.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics