AW: position independent code

[FALE@skidata.com]

thankyou for your detailed information. I think in the moment
we can live with absolute code/data addressing.
My intent was to load pieces of user code dynamically as required,
but this is not very important for us at the moment.


> ----------
> Von: 	Olivier Carmona[SMTP:Olivier.Carmona@epfl.ch]
> Gesendet: 	Montag, 22. Februar 1999 17:45
> An: 	crossgcc@cygnus.com
> Betreff: 	Re: position independent code
> 
> At 05:24 PM 2/5/99 -0800, Scott Howard wrote:
> >Gnu CC does support position independant code with the -fpic and -fPIC
> >options.  It is only supported when the compiler & assembler use the ELF
> >object file format, it does not work when using COFF (as you have
> >already discovered, judging from the listing you enclosed).  ELF for the
> >m68k will be supported in a forthcoming release of EGCS.
> >
> >There is an additional complication: the gnu implementation is designed
> >with shared libraries in mind; it creates an address translation table
> >which must be 'fixed up' at run time to point to the absolute address of
> >functions and variables.  This is not really useful for an embedded
> >(diskless, OS-less) system.  This is a more complicated issue, and it
> >would take some work to implement for a low-end system (essentially you
> >would need a run-time loader of some sort).
> 
> Even if this is not intentionally built for embedded system, GNU PIC
> could be used for it. The loader could be left to the designer, for
> instance on a m68k :
> 
> start:
> 
> ....
> 
>         /* Load the address relative to the PC */
>         lea     (_GLOBAL_OFFSET_TABLE_,pc),a5
>         /* Load the address given by the linker */
>         movea.l #_GLOBAL_OFFSET_TABLE_,a2
>         /* Then find the offset */
>         suba.l  a5,a2
>         beq     L1                  /* Null Offset ? */
> 
>         /* Add the offset to all of the Global Offset Table */
>         move.l  #gotsize,d1         /* gostize : linker variable */
> L0:     subq.l  #4,d1
>         blt.b   L1                  /* End of GOT ? */
>         movea.l (a5,d1.l),a3        /* Read the GOT */
>         adda.l  a2,a3               /* Add the offset */
>         move.l  a3,(a5,d1.l)        /* Write back in the GOT */
>         bra     L0
> 
> ....
> 
> BUT there is a major concern, what if everything is in ROM ? Alas, THE
> GOT NEED TO BE IN RAM or, at least, the _GLOBAL_OFFSET_TABLE_ pointer.
> 
> A solution is to set the _GLOBAL_OFFSET_TABLE_ in a fixed position in
> RAM via the linker (e.g.: at the top of the stack) and to memorize the
> position of the offset table in another variable like
> __GLOBAL_OFFSET_TABLE_. You then need to write another loader.
> 
> Hope it helps,
> 
> -- 
> Olivier Carmona
> Research Assistant
> IN-F-132, LAMI-EPFL, 1015 Lausanne, Switzerland
> Tel: ++41(21).693.3908, Fax: ++41(21).693.5263
> http://diwww.epfl.ch/lami/team/carmona
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