boot/x86/a20.s

   1 ; Some routines for managing the A20 line
   2
   3 ; Check if A20 is enabled
   4 ; Based on <https://wiki.osdev.org/A20_Line#Testing_the_A20_line>
   5 check_a20:
   6         pushf
   7         push ds
   8         push es
   9         push di
  10         push si
  11
  12         cli
  13
  14         xor ax, ax
  15         mov es, ax
  16
  17         not ax
  18         mov ds, ax
  19
  20         mov di, 500h
  21         mov si, 510h
  22
  23         mov al, byte [es:di]
  24         push ax
  25
  26         mov al, byte [ds:si]
  27         push ax
  28
  29         mov byte [es:di], 0
  30         mov byte [ds:si], 0FFh
  31
  32         cmp byte [es:di], 0FFh
  33
  34         pop ax
  35         mov byte [ds:si], al
  36
  37         pop ax
  38         mov byte [es:di], al
  39
  40         mov ax, 0
  41         je .exit
  42         mov ax, 1
  43
  44 .exit
  45         pop si
  46         pop di
  47         pop es
  48         pop ds
  49         popf
  50         ret
  51
  52 a20_i8042_wait
  53         in al, 64h
  54         test al, 2
  55         jnz a20_i8042_wait
  56         ret
  57
  58 a20_i8042_wait2
  59         in al, 64h
  60         test al, 1
  61         jnz a20_i8042_wait2
  62         ret
  63
  64 enable_a20
  65         mov ax, 2401h
  66         int 15h
  67         call check_a20
  68         cmp ax, 1
  69         jne .fail
  70 .success
  71         ret
  72 .fail
  73         cli ; disable interrupts
  74
  75         call a20_i8042_wait
  76         mov al, 0ADh ; disable 1st PS/2 port
  77         out 64h, al
  78
  79         call a20_i8042_wait
  80         mov al, 0D0h ; read controller output port
  81         out 64h, al
  82
  83         call a20_i8042_wait2
  84         in al, 60h ; do the actual read
  85         push ax ; save the register
  86
  87         call a20_i8042_wait
  88         mov al, 0D1h ; write byte to output port
  89         out 64h, al
  90
  91         call a20_i8042_wait
  92         pop ax ; restore the register
  93         or al, 2 ; toggle A20 bit
  94         out 60h, al
  95
  96         call a20_i8042_wait
  97         mov al, 0AEh ; reenable 1st PS/2 port
  98         out 64h, al
  99
 100         call a20_i8042_wait
 101         sti ; enable interrupts
 102         ret
 103
 104         call check_a20
 105         cmp ax, 1
 106         jne .fail2
 107         ret
 108 .fail2
 109         mov si, A20_FAIL
 110         call print
 111         jmp $
 112
 113 A20_FAIL db "A20 enable fail, not booting!", 0