boot/x86/stage3/a20.s

   1 bits 16
   2 section .text
   3
   4 %macro in_wait 0
   5         push ax
   6 %%loop:
   7         in al, 0x64
   8         test al, 2
   9         jnz %%loop
  10         pop ax
  11 %endmacro
  12
  13 %macro out_wait 0
  14         push ax
  15 %%loop:
  16         in al, 0x64
  17         test al, 1
  18         jz %%loop
  19         pop ax
  20 %endmacro
  21
  22 ; CF set = A20 enabled
  23 check_a20:
  24         push esi
  25         push edi
  26         push ax
  27         mov esi, 0x500
  28         mov edi, 0x100500
  29
  30         mov al, [esi]
  31         mov ah, [edi]
  32
  33         mov [esi], byte 0
  34         mov [edi], byte 0xff
  35         cmp [esi], byte 0xff
  36
  37         mov [esi], al
  38         mov [edi], ah
  39
  40         pop ax
  41         pop edi
  42         pop esi
  43         ret
  44
  45 enable_a20:
  46         push ax
  47         mov ax, 0x2401
  48         int 0x15
  49         jc .i8042_method
  50         call check_a20
  51         jnc .i8042_method
  52         pop ax
  53         ret
  54         .i8042_method:
  55                 in_wait
  56                 mov al, 0xad
  57                 out 0x64, al
  58                 in_wait
  59                 mov al, 0xd0
  60                 out 0x64, al
  61                 out_wait
  62                 in al, 0x60
  63                 or al, 2
  64                 push ax
  65                 in_wait
  66                 mov al, 0xd1
  67                 out 0x64, al
  68                 in_wait
  69                 pop ax
  70                 out 0x60, al
  71                 in_wait
  72                 mov al, 0xae
  73                 out 0x60, al
  74                 call check_a20
  75                 pop ax
  76                 ret
  77