Or, if you know you have 256 entries in your table:

my_table_lookup:
    movlw    high(my_table_data)
    movwf    PCLATH
    movf       table_index,W
    movwf    PCL

    org        X'300'        ; use last 'page' for table

my_table_data:
    retlw    X'??'
    254 more entries
    retlw    X'??'

----- Original Message -----
From: "Olin Lathrop" <olin_piclist@EMBEDINC.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, February 08, 2003 9:19 AM
Subject: Re: [PIC]: AD 12F675 lockup


> > Please ignore the question, I discovered that if the a/d value was 255,
> the
> > program would go off the end of my lookup table - because the table only
> > has 254 entrys due to the addwf PCL,f at the start, therefore it
> exceeeds
> > the page when at 255. As my output is interrupt driven, the output
> looked
> > normal but wouldn't respond.
> > ------------------------------------------
> >                 org 0x0200
> > MSB_Period
> >                    addwf  PCL,f
> >                    retlw  0xXX
> >                    (252 other retlw 0xXX) retlw  0xXX
> >                    (0x300 page)
>
> This comes up periodically.  Now that you've been penny wise and pound
> foolish, do youself a favor and spend the little extra effort to do table
> lookups the right way.  The only excuse is the rather rare case where you
> can't afford the few extra instructions or cycles.  This table lookup code
> has no restriction as to where the code or table resides in program
> memory:
>
> ;
> ;   Return to the caller with the table value in W for the
> ;   0-255 table entry number in OFFSET.  The bank must
> ;   already be set for access to OFFSET.
> ;
>     movlw   high table   ;get table start address high byte
>     movwf   pclath       ;init jump address high byte
>     movf    offset, w    ;get the 0-255 table entry offset
>     addlw   low table    ;make jump address low byte
>     skip_nc              ;no carry into high byte ?
>     incf    pclath       ;propagate the carry
>     movwf   pcl          ;jump to the selected table entry
>
> ...
>
> table
>     retlw ...
>     ...
>
>
> *****************************************************************
> Embed Inc, embedded system specialists in Littleton Massachusetts
> (978) 742-9014, http://www.embedinc.com
>
> --
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>

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