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> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Caisson
> Sent: Tuesday, March 31, 1998 3:38 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: Subroutine locations
>
>
> > 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
>