QUOTE:

For a GOTO instruction, bits 8:0 of the PC are provided
by the GOTO instruction word. The PC Latch (PCL) is
mapped to PC<7:0>. Bit 5 of the STATUS register
provides page information to bit 9 of the PC (Figure 4-8).

For a CALL instruction, or any instruction where the
PCL is the destination, bits 7:0 of the PC again are
provided by the instruction word. However, PC<8>
does not come from the instruction word, but is always
cleared (Figure 4-8).

The STATUS register page preselect bits are cleared
upon a RESET, which means that page 0 is pre-
selected.

Therefore, upon a RESET, a GOTO instruction will
automatically cause the program to jump to page 0
until the value of the page bits is altered.

Because PC<8> is cleared in the CALL
instruction, or any Modify PCL instruction,
all subroutine calls or computed jumps are
limited to the first 256 locations of any pro-gram
memory page (512 words long).

:------------------------------------
The last paragraph is interesting.

Try making sure the sub-routine is in the PAGE1 but within the first 256
locations.

Pete



> -----Original Message-----
> From: EXT Geoff Wootton [mailto:woottong@isdugp.bham.ac.uk]
> Sent: Tuesday, July 11, 2000 2:03 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: 12C509 calls and gotos
>
>
> Pete,
>
> Thanks for the reply. You're right, if I move the subroutine
> closer it does work, however I'd rather be able to use the
> full address range as documented. The 12C508 has 512 program
> memory locations, so setting bit 5 of the status register
> will (should) give full addressabilty. The datasheet is
> ambiguous on addressing the 12C509. There are 1024 locations
> and it says to set bits 5 and 6 of the status register but
> later on says that bit 6 is not implemented. If I move the
> subroutines closer to the calls then they will be embedded
> in my main code which makes it less readable. Also I'd then
> have to put gotos in to jump over the subroutines. I've thought
> of using nested subroutines but the stack on the 12C509 only
> has 2 levels.
>
> Regards
>
> Geoff.
>
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