;********************************************
;	Enable A20 address line
;
;	OS Development Series
;********************************************

%ifndef __A20_INC_67343546FDCC56AAB872_INCLUDED__
%define __A20_INC_67343546FDCC56AAB872_INCLUDED__

bits	16

;----------------------------------------------
; Enables a20 line through keyboard controller
;----------------------------------------------

EnableA20_KKbrd:

	cli
	push	ax
	mov	al, 0xdd	; send enable a20 address line command to controller
	out	0x64, al
	pop	ax
	ret

;--------------------------------------------
; Enables a20 line through output port
;--------------------------------------------

EnableA20_KKbrd_Out:

	cli
	pusha

        call    wait_input
        mov     al,0xAD
        out     0x64,al		; disable keyboard
        call    wait_input

        mov     al,0xD0
        out     0x64,al		; tell controller to read output port
        call    wait_output

        in      al,0x60
        push    eax		; get output port data and store it
        call    wait_input

        mov     al,0xD1
        out     0x64,al		; tell controller to write output port
        call    wait_input

        pop     eax
        or      al,2		; set bit 1 (enable a20)
        out     0x60,al		; write out data back to the output port

        call    wait_input
        mov     al,0xAE		; enable keyboard
        out     0x64,al

        call    wait_input
	popa
        sti
        ret

	; wait for input buffer to be clear

wait_input:
        in      al,0x64
        test    al,2
        jnz     wait_input
        ret

	; wait for output buffer to be clear

wait_output:
        in      al,0x64
        test    al,1
        jz      wait_output
        ret

;--------------------------------------
; Enables a20 line through bios
;--------------------------------------

EnableA20_Bios:
	pusha
	mov	ax, 0x2401
	int	0x15
	popa
	ret

;-------------------------------------------------
; Enables a20 line through system control port A
;-------------------------------------------------

EnableA20_SysControlA:
	push	ax
	mov	al, 2
	out	0x92, al
	pop	ax
	ret

%endif