Brans Kernel Development : Grub Multiboot header?

Question

I am following Bran's kernel development tutorial. In his assembly code, he has the following block of code that he describes to be not so important but something that is do with GRUB.

; This part MUST be 4byte aligned, so we solve that issue using 'ALIGN 4'
ALIGN 4
mboot:
    ; Multiboot macros to make a few lines later more readable
    MULTIBOOT_PAGE_ALIGN    equ 1<<0
    MULTIBOOT_MEMORY_INFO   equ 1<<1
    MULTIBOOT_AOUT_KLUDGE   equ 1<<16
    MULTIBOOT_HEADER_MAGIC  equ 0x1BADB002
    MULTIBOOT_HEADER_FLAGS  equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO |        MULTIBOOT_AOUT_KLUDGE
    MULTIBOOT_CHECKSUM  equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)
    EXTERN code, bss, end

    ; This is the GRUB Multiboot header. A boot signature
    dd MULTIBOOT_HEADER_MAGIC
    dd MULTIBOOT_HEADER_FLAGS
    dd MULTIBOOT_CHECKSUM

    ; AOUT kludge - must be physical addresses. Make a note of these:
    ; The linker script fills in the data for these ones!
    dd mboot
    dd code
    dd bss
    dd end
    dd start

I would still like to know what this code is doing? Can someone give me or point me to where I can find a description about this. Second, why does this piece of code needs to be 4-byte aligned? Is this for efficiency reasons or has some special significance in context of GRUB?

Answer 1

This bit of assembly is not code, but a multiboot header. When you link your kernel this header will be placed at the beginning of the binary, so GRUB can see that your kernel is multiboot-compliant. This header contains a magic number (0x1badb002), a list of flags (in your case, that the kernel should be loaded on a page-aligned address, and that you would like GRUB to give you a memory map), and a checksum. If one of these entries does not follow the multiboot specification, GRUB will not load your kernel.

You can find more information about multiboot in GRUB's multiboot manual:

http://www.gnu.org/software/grub/manual/multiboot/multiboot.html

Quoting from the manual:

An OS image must contain an additional header called Multiboot header, besides the headers of the format used by the OS image. The Multiboot header must be contained completely within the first 8192 bytes of the OS image, and must be longword (32-bit) aligned. In general, it should come as early as possible, and may be embedded in the beginning of the text segment after the real executable header.

I'm not sure what the AOUT kludge does. I'm only using ELF in my OS. I followed this guide: http://wiki.osdev.org/Bare_Bones