kernel/realmode/a20.inc

   1 ;----------------------------------------------------------;
   2 ; BOS 0.04                       Christoffer Bubach, 2005. ;
   3 ;----------------------------------------------------------;
   4 ;                                                          ;
   5 ;      Realmode functions to set a20-gate.                 ;
   6 ;                                                          ;
   7 ;----------------------------------------------------------;
   8
   9 ;----------------------;
  10 ;   enable a20 gate    ;
  11 ;----------------------;
  12 enable_a20:
  13           in      al, 0x64                            ; first try..
  14           test    al, 2
  15           jnz     enable_a20
  16           mov     al, 0xD1
  17           out     0x64, al
  18      .d6:
  19           in      al, 0x64
  20           and     ax, 2
  21           jnz     .d6
  22           mov     al, 0xDF
  23           out     0x60, al
  24
  25           call    a20_test
  26           jz      .ok
  27
  28           in      al, 0x92                            ; try again, diffrent method.
  29           or      al, 0x02
  30           out     0x92, al
  31
  32           call    a20_test
  33           jz      .ok
  34
  35           ;    ERROR!
  36           ; a20-gate not set! Do something about it here... ;)
  37
  38      .ok:
  39           ret
  40
  41 ;------------------------;
  42 ;  test if A20 is set    ;
  43 ;------------------------;
  44 a20_test:
  45           mov     al, byte [fs:0]
  46           mov     ah, al
  47           not     al
  48           xchg    al, byte [gs:0x10]
  49           cmp     ah, byte [fs:0]
  50           mov     [gs:0x10], al
  51           ret