boot.s

;
; boot.s -- Kernel start location. Also defines multiboot header.
; Based on Bran's kernel development tutorial file start.asm
;

MBOOT_PAGE_ALIGN    equ 1<<0       ; Load kernel and modules on a page boundary
MBOOT_MEM_INFO      equ 1<<1       ; Provide the kernel with memory info
MBOOT_HEADER_MAGIC  equ 0x1BADB002 ; Multiboot Magic Value
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.

section .mbheader

mboot:
  dd  MBOOT_HEADER_MAGIC  ; GRUB will search for this value on each
                          ; 4-byte boundary in the 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)

section .text

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

start:
  push ebx                ; Load multiboot header location

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