> Van: Tim Crist <tjcrist@CCGATE.HAC.COM>
> Aan: PICLIST@MITVMA.MIT.EDU
> Onderwerp: Subroutine locations
> Datum: maandag 30 maart 1998 13:29
>
>      I have a question about subroutines that are located above and
>      below 255H in the 16F84 and the 16C54A.  I understand that tables
>      that use the program counter will not work if they cross 255H
>      because of PCLATH.

I assume you mean 255 Decimal (or FF HexaDecimal).

You are right, but for the wrong reasons.  It's not because of PCLATH, but
because the register PCL consists outof only 8 bits.  this means that you
can't put a number higher than FF (or 255 decimal) into PCL.  Said
otherwise,
if you add a Index of 0x05 to a PCL of 0xFE the result will become 0x03,
and
NOT 0x0103.  Ah. before I forget :  The low 5 bits of PCLATH will be
prepended
to PCL to obtain the full 13 -bit adres on a add-to-PCL -jump.  Not that
all
PIC's have that much Code-memory, but it seems to be a sort of standard for
Mid-range PIC's.

>      My question is:  Can I have a subroutine below 255H successfully
>      call a subroutine above 255H without incident?  If not, how would
>      one go about making it work.

Yes, they can.  A Goto or Call has a 11 Bit adres, wich means you can jump
from 0x0000 to 0x07FF.  If you would like to be able to jump to an adres
beyond this, you would have to (besides finding a PIC with more
Code-memory)
set bits 3 & 4 in the PCLATH register. these will be prepended to your
Jump/
Call adres to obtain (again) the full 13 -bit adres.

>      TIA,
>
>      Tim Crist

Greetz,
  Rudy Wieser