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>, slightly modified
   5 check_a20
   6         pushf ; save EFLAGS
   7         ; save segment and index registers
   8         push ds
   9         push es
  10         push di
  11         push si
  12
  13         cli ; disable interrupts
  14         xor ax, ax ; ax = 0
  15         mov es, ax ; es = 0
  16
  17         not ax ; ax = FFFFh
  18         mov ds, ax ; ds = FFFFh
  19
  20         mov di, 500h
  21         mov si, 510h
  22
  23         mov al, [es:di] ; preserve contents of 0:500h
  24         mov ah, [ds:si] ; preserve contents of FFFFh:510h
  25         push ax ; push all that to stack
  26
  27         mov byte [es:di], 0 ; set 0:500h to 0
  28         mov byte [ds:si], 0FFh ; set FFFFh:510h to FFh
  29
  30         cmp byte [es:di], 0FFh ; compare 0:500h with FFh
  31                                           ; if A20 is enabled, this will not be equal, but if A20 is disabled it will be
  32
  33         pop ax ; pop old contents of the 2 addresses
  34         mov [ds:si], ah ; restore contents of FFFFh:510h
  35         mov [es:di], al ; restore contents of 0:500h
  36
  37         xor ax, ax ; clear ax
  38         je .exit ; if A20 is disabled, return 0
  39         mov ax, 1 ; otherwise return 1
  40
  41 .exit
  42         pop si
  43         pop di
  44         pop es
  45         pop ds
  46         popf
  47         sti
  48         ret
  49