;; boot.stage2.a20.asm
_enable_a20:    
    cli

    ;; Disable keyboard
    call .wait_io1
    mov al, 0xad
    out 0x64, al

    ;; Read from keyboard controller
    call .wait_io1
    mov al, 0xd0
    out 0x64, al

    call .wait_io2
    in al, 0x60
    push eax ; Save current value


    ;; Write back with the A20 bit set
    call .wait_io1
    mov al, 0xd1
    out 0x64, al

    call .wait_io1
    pop eax
    or al, 2 ; Set A20 bit

    ;; Enable keyboard
    call .wait_io1
    mov al, 0xae
    out 0x64, al

    call .wait_io1
    
    .wait_io1:
    in al, 0x64
    test al, 2
    jnz .wait_io1
    ret

    .wait_io2:
    in al, 0x64
    test al, 1
    jz .wait_io2
    ret