Re: [ECOS] MIPS question: _gp = (. + 0x8000);

["Michael Anburaj" <embeddedeng at hotmail dot com>]

Hi Jonathan,

Thanks a lot for your response. That was very helpful.

I have a small question related to this.

> From YAMON's (mips's ROM monitor) linker scrip:

_gp = ABSOLUTE(. + 0x7ff0);



Is there a good reason to pick 0x7ff0 or any other offset instead of 0x8000?

Thanks & Cheers,
-Mike.



> From: Jonathan Larmour <jifl@eCosCentric.com>
> To: Michael Anburaj <embeddedeng@hotmail.com>
> CC: ecos-discuss@sources.redhat.com
> Subject: Re: [ECOS] MIPS question: _gp = (. + 0x8000);
> Date: Thu, 19 Feb 2004 06:19:44 +0000
>
> Michael Anburaj wrote:
> > Hi,
> >
> > I have a question regarding the following line found in a lot of MIPS 
> > linker scripts:
> >
> > _gp = (. + 0x8000);
> >
> > _gp gets loaded into $gp. How does this value ?_gp? relate with access to 
> > .sdata & .sbss session using $gp?
>
> Access to stuff in sdata/sbss uses different instructions that are more 
> efficient for small amounts of data. But the tradeoff is having to use a 
> fixed base register for accessing them. That is $gp. GCC/GAS generate 
> relocs relative to the _gp symbol, which the linker munges appropriately so 
> that as long as $gp points to _gp everything works.
>
> > Why is 0x8000 added (to the foot of .sdata section)? To Place $gp halfway 
> > in the 64KB $gp accessible space??
>
> Correct. The offsets that GCC uses are signed.

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