xv6/multiboot.S

# Multiboot header, for multiboot boot loaders like GNU Grub.
# http://www.gnu.org/software/grub/manual/multiboot/multiboot.html
#
# Using GRUB 2, you can boot xv6 from a file stored in a
# Linux file system by copying kernel or kernelmemfs to /boot
# and then adding this menu entry:
#
# menuentry "xv6" {
# 	insmod ext2
# 	set root='(hd0,msdos1)'
# 	set kernel='/boot/kernel'
# 	echo "Loading ${kernel}..."
# 	multiboot ${kernel} ${kernel}
# 	boot
# }

#include "asm.h"
#include "memlayout.h"
#include "mmu.h"

#define STACK 4096

# Multiboot header.  Data to direct multiboot loader.
.p2align 2
.text
.globl multiboot_header
multiboot_header:
  #define magic 0x1badb002
  #define flags (1<<16 | 1<<0)
  .long magic
  .long flags
  .long (-magic-flags)
  .long multiboot_header  # beginning of image
  .long multiboot_header
  .long edata
  .long end
  .long multiboot_entry

# Multiboot entry point.  Machine is mostly set up.
# Configure the GDT to match the environment that our usual
# boot loader - bootasm.S - sets up.
.globl multiboot_entry
multiboot_entry:
  lgdt V2P_WO(gdtdesc)
  ljmp $(SEG_KCODE<<3), $mbstart32

mbstart32:
  # Set up the protected-mode data segment registers
  movw    $(SEG_KDATA<<3), %ax    # Our data segment selector
  movw    %ax, %ds                # -> DS: Data Segment
  movw    %ax, %es                # -> ES: Extra Segment
  movw    %ax, %ss                # -> SS: Stack Segment
  movw    $0, %ax                 # Zero segments not ready for use
  movw    %ax, %fs                # -> FS
  movw    %ax, %gs                # -> GS

  # Set up the stack pointer and call into C.
  movl $(stack + STACK), %esp
  call main
spin:
  jmp spin

# Bootstrap GDT
.p2align 2                                # force 4 byte alignment
gdt:
  SEG_NULLASM                             # null seg
  SEG_ASM(STA_X|STA_R, -KERNBASE, 0xffffffff)   # code seg
  SEG_ASM(STA_W, -KERNBASE, 0xffffffff)         # data seg

gdtdesc:
  .word   (gdtdesc - gdt - 1)             # sizeof(gdt) - 1
  .long   V2P_WO(gdt)                      # address gdt

.comm stack, STACK