Re: HELP ME - porting to VLIW machine

[Nick Clifton <nickc at cambridge dot redhat dot com>]

Hi Fabrizio,

> I'm trying a porting for a VLIW machine (128-bit word and 11 bit
> address field).  In my system there's an ARM microcontroller embedded
> that load VLIW machine program memory.

> 1) Is it correct this definition architecture?
> 
>  const bfd_arch_info_type bfd_magic_arch =
>  {
>    32,     -32 bits in a word

You said you had a 128 bit word in your machine, so surely this field
should be 128 ?  Unless, that is, you pack multiple 32 bit
instructions into that 128 bit word, in which case 32 might be
correct.

>    11,     -11 bits in an address

That looks really strange.  Do you really only have 11 bit long
addresses ?

> 2) What's Elf32_Internal_Rela type?

It is a structure defined in include/elf/internal.h.  It is the
internal representation of an ELF style relocation.  (The ELF standard
specifies the layout of a relocation structure in a binary file.  This
is the external representation.  The BFD library, when it reads in a
binary ELF file, translate the external representation into its
internal representation).

> 3) Concerning the HOWTO macro: what's the relationship between the macro
> parameters and the architecture parameters? e. g. what  third and fourth
> fields have got to do with the first second and third fields of
> bfd_arch_info_type?

The third paramter of the HOWTO macro initialises the "size" field of
the reloc_howto_struct.  This is the number of bytes in the object
file that are going to be changed by the reloc.  For example if the
reloc is filling in missing fields in a machine instruction it might
have a size of 4 (bytes), assuming a 32 bit instruction, whereas if
the reloc is initialising the value of an unsigned short variable
in the .data section it might have a size of 2 (bytes).

The fourth parameter of the HOWTO macro initialises the "bitsize"
field, which is the number of bits that the reloc can modify.  When
the BFD library is processing a reloc is will read in "size" bytes,
examine them, modify "bitsize" bits inside these bytes, and then write
them back out again.

The fields in the bfd_arch_info structure refer to the architecture as
a whole, not to individual relocs.  Thus the first field,
"bits_per_word" specifies the word size of the architecture.  [This
field is not actually used very much inside the BFD library, but you
never know it may be important one day].  The other fields are
"bits_per_address" and "bits_per_byte" whose meanings should be self
explanatory.

Cheers
        Nick