Andrew Hall <PICLIST@MITVMA.MIT.EDU> wrote:

> I ran the following (simplified) code expecting it to fail:
>
>         goto  too_far ; Shouldn't work ?
>
>         org 0x100
> too_far ; PCL will contain 0
>
>         nop
>
> When I ran the simulation I was surprised to see the "goto" jump to
> the memory address  0x100 even though PCLATH contained zero :( I had
> expected it to end up at address 0x0 ! In the register display PCL
> contained 0 !!
>
> What am I doing wrong  ? Have I misunderstood something about the
> 84's architecture or simulator ?

Andy:

You've misunderstood how the GOTO command encodes the destination
address.  If you check the "PCLATH" section of the data book again,
you'll see that the GOTO command can directly jump to ANY location in
the 16C84's program space... Manipulation of the PCLATH register for
GOTOs isn't necessary unless you're using parts with more than 2K of
program memory.

On the 16C84, you only need to use PCLATH when you're DIRECTLY
modifying the PCL register (by executing a "MOVWF PCL" or "ADDWF
PCL" or whatever).

-Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499