Re: [microblaze-uclinux] Setting up for Microblaze work.

[John Williams <jwilliams@xxxxxxxxxxxxxx>]

David,

David H. Lynch Jr wrote:

>           I am still trying to sort out whether and why  MMUless cpu 
> can not use an ELF file.

ELF application binaries have no relocation info - All relocations are 
resolved to a base address of zero.  The final link phase of building an 
ELF app strips out all relocation data - gone.

Try this:

[jwilliams@g512-9423 Core]$ objdump -r /bin/sh

/bin/sh:     file format elf32-i386


No relocations.

MMU-full CPUs map applications into their own zero-based virtual address 
space, and the MMU + kernel page fault handlers do the rest.

On NOMMU, we can't do that.  Each app runs in a physical address space, 
based at whatever address is given by the kernel when memory is 
allocated for the text segment.

At load time, the binfmt_flat loader must instead go through the binary, 
and relocate all the jumps, address references - anything that needs 
relocation.

So, a flat binary is sort of a binary dump of the object code, plus a 
relocation table to allow the kernel to do its job, relocating the 
binary to whatever address the text, data and bss segments ultimately 
get placed at.

As an aside, this is why PIC compilers are so useful for NOMMU targts - 
if you can generate fully position independent code, you can get away 
with few, or no relocations.  That speeds app load time, and lets you do 
great things like directly mmap()ing the filesystem backing store into 
the app's text segment - load time is zero, just start executing 
straight off the "disk" (or rather flash, or mem file system).

See fs/binfmt_flat.c for the gory details.

Regards,

John
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