I loved this reply.......can I annotate it with references to the linux kernel sources?
On Fri, May 13, 2011 at 9:42 AM, Dave Hylands <[email protected]> wrote: > 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 > yes: See ./arch/x86/boot/Makefile > This binary is then compressed to produce > arch/x86/boot/compressed/vmlinux.bin.gz > See ./arch/x86/boot/compressed/Makefile > 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 > ./arch/x86/boot/compressed/mkpiggy.c is compiled into a commandline binary - mkpiggy which will generate the piggy.o. > called arch/x86/boot/compressed/piggy.o > The linker then compiles a decompressor (misc.o) and piggy.o together > Yes, the routine is called "decompress_kernel", residing inside ./arch/x86/boot/compressed/misc.c. And this routine is called from ./arch/x86/boot/compressed/head_32.S (or head_64.S) and at runtime, the gzipped data is decompressed and immediately jumped into (perhaps after some relocation if needed): /* * Do the decompression, and jump to the new kernel.. */ leal z_extract_offset_negative(%ebx), %ebp /* push arguments for decompress_kernel: */ pushl %ebp /* output address */ pushl $z_input_len /* input_len */ leal input_data(%ebx), %eax pushl %eax /* input_data */ leal boot_heap(%ebx), %eax pushl %eax /* heap area */ pushl %esi /* real mode pointer */ call decompress_kernel addl $20, %esp 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. > Inside arch/x86/boot/Makefile: Creating the vmlinux.bin from vmlinux via objcopy (note that this operation will throw all relocation information): $(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE $(call if_changed,objcopy) And then packing together linearly to form the "bzImage" (output from make): make -f scripts/Makefile.build obj=arch/x86/boot arch/x86/boot/bzImage make -f scripts/Makefile.build obj=arch/x86/boot/compressed arch/x86/boot/compressed/vmlinux arch/x86/boot/tools/build arch/x86/boot/setup.bin arch/x86/boot/vmlinux.bin CURRENT > arch/x86/boot/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. > To be more precise, grub will load bzImage and jump into the startup_32 function located in arch/x86/boot/compressed/head_32.S at the following fixed address (from source code): /* * head.S contains the 32-bit startup code. * * NOTE!!! Startup happens at absolute address 0x00001000, which is also where * the page directory will exist. The startup code will be overwritten by * the page directory. [According to comments etc elsewhere on a compressed * kernel it will end up at 0x1000 + 1Mb I hope so as I assume this. - AC] * * Page 0 is deliberately kept safe, since System Management Mode code in * laptops may need to access the BIOS data stored there. This is also * useful for future device drivers that either access the BIOS via VM86 * mode. */ More info: http://books.google.com/books?id=e8BbHxVhzFAC&pg=PA1224&lpg=PA1224&dq=grub+head_32.S&source=bl&ots=0MSdKwBoM6&sig=2RyEpprl25zueiqi332TQHLIj0E&hl=en&ei=y5vQTY7eBNDNrQeI3bTCCg&sa=X&oi=book_result&ct=result&resnum=3&ved=0CCkQ6AEwAg#v=onepage&q=grub%20head_32.S&f=false > 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 > > _______________________________________________ > K > -- Regards, Peter Teoh
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