On Wed, 2009-09-02 at 15:33 +0900, HongWoo Lee wrote:
> Hi everyone~
>
> In ther linux kernel code, I found the reloc_offset.
>
> {{{
> // file : misc.S
> /* Returns (address we are running at) - (address we were linked at)
> * for use before the text and data are mapped to KERNELBASE.
> */
> _GLOBAL(reloc_offset)
> }}}
>
> I couldn't understand the comment saying "Returns (address we are
> running at) - (address we were linked at)".
> For now, I'm studying each instruction.
>
> And below is best comment I can explain for each instruction.
>
> _GLOBAL(reloc_offset)
> mflr r0 // move from link register, save the return
> address
> bl 1f // bl 1f
> 1: mflr r3 // move from link register, r3 is just
> return address pointing itself At this point r3 contains the value of LR based on the branch we just did. So it's the address of the current instruction, based on where the code is _running_. > LOAD_REG_IMMEDIATE(r4,1b) // get the 1b address, r4 is the address Here we load into r4 the address of the previous instruction, but based on the label "1b". The address of the label is calculated by the linker, so r4 contains the address the instruction was linked at. > subf r3,r4,r3 // r3 = r3 – r4 So here we calculate any difference between the address the code was linked at and the address it's running at. > mtlr r0 // restore return address > blr > > After this, I still don't know why "r3-r4" is the offset. > And what does it mean ?? The offset is just the difference between the address the code was linked at and the address it's running it. It's used in places where the code might be (or is always) running at an address other than the address it was linked at. cheers
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