Tony,
That would be great! I think I understand it now, but a little
reinforcement would be great. Feel free to send it to this address.

Thanks again!

Josh

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>
> Hi Josh,
>
> When the PCL is changed by any instruction, the PCLATH is used to make
> up the ROM address bits that the instruction can't.
>
> For example, a GOTO instruction can only fit 11 bits of address
> information in it. As the 14 bit PICs are 13 bit addressable, there
> needs to be two other bits specified somewhere. These 2 bits come from
> the PCLATH bits <4,3> and are placed into the 13 bit ROM address (PC) in
> bit positions 12 and 11. This is done automatically whenever a GOTO or
> CALL instruction is processed. This equates to 2K ROM page boundaries
> for these two instructions.
>
> If the PCL is changed by something like ADDWF PCL, then only 8 bits of
> address information is available from the instruction itself. So in this
> case, 5 extra bits need to come from somewhere to make up 13 bits total.
> These 5 bits also come from the PCLATH in bits <4-0>, and are placed
> into the ROM address (PC) in bit positions 8 to 12. This is also done
> automatically whenever an instruction such as ADDWF PCL, SUBWF PCL is
> processed. This equates to the 256 word ROM boundaries for these type of
> instructions.
>
> RETURN, RETFIE and RETLW instructions pull a complete 13 bit word from
> the stack and place it directly into the Program Counter, and the PCLATH
> value 'is not' changed.
>
> > As an aside, how do you know if your program crosses a page boundary? I
> > have never checked, and I know for this program it should be safe to let
> > my table start on page 3, but how do I know for sure?
>
> Warning messages are places in the *.lst file and at the end of the
> assembly information file providing that these messages are not turned
> off by a compiler directive.
>
> If you want, I can forward you an animated turorial (Windows 95 up) on
> this subject to help make it a bit clearer. It will be about 600K.
>
> --
> Best regards
>
> Tony

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