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

[Michael Anburaj]

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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