boot/x86/stage3/unreal.s

   1 ; Routine for enabling unreal mode, which allows using 32-bit offsets in real mode
   2
   3 bits 16
   4 section .text
   5
   6 enable_unreal:
   7         cli
   8         push eax
   9         push bx
  10         push ds
  11
  12         lgdt [gdt_info]
  13         mov eax, cr0
  14         or al, 1
  15         mov cr0, eax
  16
  17         jmp $+2
  18
  19         mov bx, 0x8
  20         mov ds, bx
  21
  22         and al, 0xfe
  23         mov cr0, eax
  24
  25         pop ds
  26         pop bx
  27         pop eax
  28         sti
  29         ret
  30
  31 section .rodata
  32
  33 ; GDT with 1 flat data segment descriptor
  34 gdt_info:
  35         dw gdt_end-gdt_start-1
  36         dd gdt_start
  37 gdt_start: dq 0 ; null entry
  38 gdt_flat:
  39         dw 0xffff
  40         dw 0
  41         db 0
  42         db 10010010b
  43         db 11001111b
  44         db 0
  45 gdt_end: