>     I'm porting a software that is running ok on a PIC16F877 to a
PIC18F452
> and now I'm having problems with the computed goto's.
>
>     On the PIC16:
>
>     MOVLW  0x01
>     MOVWF  PCLATH
>     MOVF   state,W_
>     ADDWF  PCL,F_
>     GOTO   case0
>     GOTO   case1
>     GOTO   case2
>     etc
>
>     On the PIC18 strange things happen:
>     MOVLW  0x01
>     MOVWF  PCLATH
>     RLCF   dkstate, W_
>     ADDWF  PCL
>     GOTO   case0
>     GOTO   case1
>     GOTO   case2
>     etc

First, DOCUMENT YOUR CODE!!!  If you had forced yourself to explain what
each instruction was doing, you might have found the bug yourself.

Second, why are you hard setting PCLATH to 1?  This is very bad programming
practise and a bug waiting to happen (maybe it just did).  Your code will
only work if all the GOTO instructions are located from 100h - 1FFh.  This
should be derived symbolically, or at least the same symbolic constant used
to force the address of the table as derive the value to stick into PCLATH.

Note that the PIC16 addresses program memory as whole words, with each
instruction taking up one word.  The PIC18 addresses program memory in
bytes, with instructions taking up either 2 or 4 bytes.  The PIC18 GOTO
instruction takes up 4 bytes, so the DKSTATE needs to be multiplied be 4,
not 2, before used to index into the table.  Also, this type of GOTO table
can only support up to 64 entries because that are all that fit into a 256
byte region.  Larger tables require computation of the PCLATH value, since a
single PCLATH value addresses a 256 byte region.

Note also that the PIC18 has a PCLATU register in addition to PCLATH.  You
can get away with ignoring PCLATU on the 18F452 because it only has 32Kbytes
of program memory, but this is another bug waiting to happen.  I would use
assembly conditionals to insert the proper code if this is ever ported to a
machine with more than 64Kbytes.  Or, at least generate an assembly error in
that case so that someone is forced to inspect the code.  It's easy to do
now, a real pain 4 years from now when you've forgotten all about this
little section of code.  While you're at it, you can generate an assembly
error automatically is someone tries to grow the table beyond 64 entries.


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