Multiboot.s

; MultiBoot.s -- Kernel start location. Also defines multiboot header.
; Based on Bran's kernel development tutorial file start.asm
;
; This work is licensed under the Creative Commons Attribution-NonCommercial-
; ShareAlike 3.0 Unported License. To view a copy of this license, visit
; http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to:
; Creative Commons
; 444 Castro Street, Suite 900
; Mountain View, California, 94041, USA.
;
; Author(s): Drew Cross <drew@ddcross.com>

MBOOT_PAGE_ALIGN    equ 1<<0    ; Load kernel and modules on a page boundary
MBOOT_MEM_INFO      equ 1<<1    ; Provide your kernel with memory info
MBOOT_HEADER_MAGIC  equ 0x1BADB002 ; Multiboot Magic value
; NOTE: We do not use MBOOT_AOUT_KLUDGE. It means that GRUB does not
; pass us a symbol table.
MBOOT_HEADER_FLAGS  equ MBOOT_PAGE_ALIGN | MBOOT_MEM_INFO
MBOOT_CHECKSUM      equ -(MBOOT_HEADER_MAGIC + MBOOT_HEADER_FLAGS)


[BITS 32]                        ; All instructions should be 32-bit.

[GLOBAL mboot]                   ; Make 'mboot' accessible from C.
[EXTERN code]                    ; Start of the '.text' section.
[EXTERN bss]                     ; Start of the .bss section.
[EXTERN end]                     ; End of the last loadable section.

mboot:
    dd  MBOOT_HEADER_MAGIC       ; GRUB will search for this value on each
                                 ; 4-byte boundary in your kernel file
    dd  MBOOT_HEADER_FLAGS       ; How GRUB should load your file / settings
    dd  MBOOT_CHECKSUM           ; To ensure that the above values are correct

    dd  mboot                    ; Location of this descriptor
    dd  code                     ; Start of kernel '.text' (code) section.
    dd  bss                      ; End of kernel '.data' section.
    dd  end                      ; End of kernel.
    dd  start                    ; Kernel entry point (initial EIP).

[GLOBAL start]                   ; Kernel entry point.
[EXTERN kmain]                    ; This is the entry point of our C code

start:
    push    ebx                  ; Load multiboot header location

; Execute the kernel:
    cli                          ; Disable interrupts.
    call kmain                   ; call our main() function.
    jmp $                        ; Enter an infinite loop, to stop the processor
                                 ; executing whatever rubbish is in the memory
                                 ; after our kernel!