> "Tables, for instance, have to be placed in the first 128 (or 256?)
> words of your current page..."
>
> Really?  I didn't know this!  I thought I can place the table anywhere in
> the memory.  Good to know

Be careful.  That statement in general is not true.  On the 12 series parts,
you can only call into the first 256 locations of each page.  On other parts
there is no restriction.

Another source of confusion comes from the fact that some people think a
"table" means code like:

    addwf  pcl
    retlw  <value for table entry 0>
    retlw  <value for table entry 1>
    .
    .
    retlw  <value for table entry N>

This type of table is also not restricted to being in the first 256
locations of the current page.  It IS however restricted to being all on the
same block of 256 locations.  That's because the table index code (ADDWF
PCL) only adjusts the low byte of the address.  Any overflow from that ADDWF
is lost, causing the table to wrap back to the start of that block of 256.

You can have a RETLW table anywhere and of any size if your code adjusts
PCLATH properly.  On the F series parts you can read the program memory
directly, which allows for totaly different table structures.  These kind of
tables take a few more cycles to access and will use the EECON hardware, but
store 14 bits of data per program memory location instead of just 8.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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