----- Original Message ----- 
From: "Jens M. Guessregen / Mailinglists" <jmg_mailinglists@mcm-it.de>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Tuesday, May 01, 2007 2:05 PM
Subject: RE: [PIC] Using PCL for relative jumps


>> >  I did some sim-debugging and had to find out, that RETLW
>> is not restoring PCLATH correctly after returning from the
>> computed goto.
>> > A normal RETURN does :-(
>> >
>> >
>> You are wrong. The datalist is not so clear but the PCLATH is
>> <B>NEVER</B> restored by a return.
>> If you want to call a subroutine in another page:
>>         PAGESEL    MySub               ;PAGESEL is an internal MPASM
>> macro that load the correct value
>>         call      MySub
>>         PAGESEL   CurrentPage           ;the choice of CurrentPage is
>> important!
>
> I was not sure, if a RETURN restores the PCLATH, but all the doc's I
> found in the web said, it does.
> I made a small sample code for a 16F877A, jumping between the pages all
> over the 8Kbyte ROM.
> I used the ICD2 on real hardware with MPLAB7.50 and also the Software
> Simulator in 7.50.
>
> And ... after the RETURN, the PCLATH was restored to the value, the CALL
> was made from ...
> But only with a RETURN. RETLW was not restoring, but RETLW is the one to
> use for tables ...
>
> And, the datasheet is absolute clear about the PC stack:
>
> This is from the 16F877A datasheet:
>
> 2.3.2 STACK
> The PIC16F87XA family has an 8-level deep x 13-bit
> wide hardware stack. The stack space is not part of
> either program or data space and the stack pointer is not
> readable or writable. The PC is PUSHed onto the stack
> when a CALL instruction is executed, or an interrupt
> causes a branch. The stack is POP'ed in the event of a
> RETURN, RETLW or a RETFIE instruction execution.
> PCLATH is not affected by a PUSH or POP operation.
>
> That means, that a RETURN, RETLW and RETFIE will cause a POP of PC (=
> PCLATH and PCL) back to where the CALL was made from, or?
>
> Best Jens
>
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Caveat: I have only a few months exposure to Microchip PICs and, then, only 
the mid-range (primarily 16F84 and 16F88). However, that doesn't stop this 
old fool from posting.

According to the PICMicro Mid-Range MCU family Reference Manual (DS33023A) , 
page 6-5, Table 6-2, and text:

1. The high byte of the PC can be modified ONLY through PCLATH (6.2.4).
2. The 13-bit address in the PC (PCH:PCL) is PUSHed onto the stack.
3. Any return results in the 13-bit address at the top of the stack being 
put back into the PC (PCH:PCL).
4. Explicitly stated at bottom of table that "PCLATH is never updated with 
the contents of PCH", which brings us back to #1 (also supported by Figure 
6-1 showing PCLATH -> PC, but not the reverse

Have a wonderful Wednesday.

Richard Seriani, Sr.


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