boot/x86/stage3/unreal.s

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