boot/x86/stage3/a20.s

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