How vmlinux is recognized?

[Dave Hylands]

Hi Vikram,

...snip...
> So when compiling the kernel, what is the purpose of the other
> files(mentioned below)
> linux-2.6/vmlinux - ELF executable, not stripped
> linux-2.6/arch/x86/boot/vmlinux.bin - Raw binary (Guess this is the
> one which is inside the bzImage)
> linux-2.6/arch/x86/boot/compressed/vmlinux.bin - ELF executable, stripped
> linux-2.6/arch/x86/boot/compressed/vmlinux - ELF executable, not stripped

Take luca's email and start at the bottom working towards the top.

linux-2.6/vmlinux is the output of the linker. As such, it is an ELF file.
A binary is then extracted from this to create
arch/x86/boot/compressed/vmlinux.bin
This binary is then compressed to produce
arch/x86/boot/compressed/vmlinux.bin.gz
This gzipped binary is then converted into an object file (which just
contains the gzipped data) but now we're back to having an ELF file
called arch/x86/boot/compressed/piggy.o
The linker then compiles a decompressor (misc.o) and piggy.o together
to produce arch/x86/boot/compressed/vmlinux (an ELF file).
objcopy is used again to convert this ELF into a binary:
arch/x86/boot/compressed/vmlinux arch/x86/boot/vmlinux.bin
Finally, the binary is compressed to produce bzImage.

So what you get is a compressed binary which contains a decompressor
and another compressed binary, this inner compressed binary being the
kernel.

GRUB loads bzImage into memory and decompresses it and then executes
the resulting binary.
This binary starts with a decompressor which then decompresses the
kernel, and executes the resulting binary.
This binary may relocate itself (probably depends on the architecture)
to a different spot in memory, and then runs.
The kernel is now running.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com