[ros-dev] Endianess

[art yerkes]

On Tue, 13 Mar 2007 07:31:14 -0400
"Tristan Miller" <trismandev at gmail.com> wrote:

> >Sorry for waiting until now to reply.
> It's all good.  : )
> 
> >What do you think?
> I think we should convert to BE, but not away from PE-COFF.  There are too
> many differences between them, and while it would be less work in the short
> term to switch to ELF, in the long term, implementing all of the features of
> PE would be very troublesome.  Not the least concern is that information
> such as import data and file headers aren't even mapped into a process's
> address space with ELF.  As I understand it, when you get an IHANDLE to a
> linked in DLL it's just a pointer to the mapped in PE file header.  I'm sure
> there are programs that require this on a source level to run (which, for
> quite some time will be the only thing that affects us on the PowerPC side
> of things).
> 
> That being said, a proper pe-powerpcbe target in gcc should then be the
> first priority.  Binutils already more or less properly creates pe-powercbe
> files (with what I assume are your patches on the wiki, and a little
> monkeying to make sure that the MZ header and PE signature are always
> written as little-endian regardless of the target architecture).  What kind
> of issues had you had on the EABI side of things, and what was your plan in
> more detail for emulating the features of PE in ELF?
> 
> Also how much progress had you made in the port up to now?
> 
>    ~Tristan Miller
>      (monocasa)
> 

The port to this point can load ntoskrnl and hal and use the OFw framebuffer
to display text.  Freeldr has been perverted to use openfirmware to allocate
memory and access the disk, but uses its own implementation of fat32 to load
files.  We allocate a new PTEG area, fill it with a mapping of freeldr and
the kernel area and reset SDR1 just before jumping to kernel land.  I haven't
yet written the code that either communicates our new page arrangement in the
existing page table format or hacks what I do store for ntoskrnl's use.

The method used to write to the display is effective but slow.  It could be
improved.

ABI issues have been a significant sticking point at points, mainly due to
the mismatch between relocation types in the ELF we're generating to be linked
and the ultimate result PE.  Admittedly I hadn't considered BE PE-COFF because
I had mostly given up on reactivating COFF powerpc in binutils.  I'd like to
know what target you're using to produce pe-powerpcbe.  My experience has been
that the out of date portions of the coff infrastructure in binutils, combined
with all the code for dead variants of coff-powerpc makes generating sensible
coff-powerpc output from ld hard.

As to your question about how I'd emulate PE-coff features in elf, for dynamic
linking I'd follow the example of mcore in dlltool, for resources I'd leave it
as a specially named section and for the PE header, I'd make space for it at
link time and have the loader construct it based on the elf info.  We'd only
be missing irreplaceable information like subsystem id and memory use (stack
commit/reserve etc), which we could bury in a well named section with some 
implementation specific type.  The main thing we'd have to look out for is that
once tranformed, the resulting image won't be relocatable, so the loader would
have to find the right aperture and use elf relocation to place everything.
-- 
The way out is via the door. Why is it that no one will use this method? - Confucius