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Re: [avr-libc-dev] __PC__ or _PC_ ???

From: Joerg Wunsch
Subject: Re: [avr-libc-dev] __PC__ or _PC_ ???
Date: Wed, 21 Aug 2002 21:39:03 +0200
User-agent: Mutt/1.2.5i

As Theodore A. Roth wrote:

> I've also been thinking about this and am beginning to question wether it
> should even be documented. My experience has been that the developer
> really has no way to directly access the PC (program counter) on the AVR.

> Can someone give me a concrete example of using the _PC_ symbol which
> justifies it's existence in the docs? How does gcc/gas handle this symbol?

If you look into the compiler-generated code, it's obviously part of the
standard preamble inserted into each compiled program.  It's indeed called
_PC_, and has a fixed value of 2.

Occasionally, the compiler generates jumps that jump just over a jump :)
using this special name:

        lds r24,measuring_mode
        lds r25,(measuring_mode)+1
        sbiw r24,0
        brne _PC_+2
        rjmp .L70
        sbiw r24,1
        breq .L77

My guess from a first glance is that this happens in situations where the
directly reachable offset for the conditional branch instructions is
exhausted, so a larger rjmp (or even jump on the large devices) is needed.
_PC_ is then used as sort of a `location counter', although it actually
just seems to be the offset relative to the current location.

I don't understand all this.  Instead, gas' location counter named `.'
should IMHO have been used.  Both branches in the following code assemble
into the same instruction (just send it through the compiler, and
disassemble it using avr-objdump -d):

% cat foo.s
_PC_ = 2
.global main
        brne    _PC_ + 2
        rjmp    foo
        brne    . + 2
        rjmp    bar
foo:    nop
bar:    nop

Maybe Denis or Marek could give us some more enlightenment...

IMHO, this symbol shouldn't be documented at all.
J"org Wunsch                                           Unix support engineer
address@hidden        http://www.interface-systems.de/~j/

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