diff --git a/mbr/mbr.asm b/mbr/mbr.asm
index 410391e..9f8855e 100644
--- a/mbr/mbr.asm
+++ b/mbr/mbr.asm
@@ -93,11 +93,106 @@ print_loop:
 		RET						; return
 
 
+; function check_a20: get status of address line 20 (21st bit of any address)
+; Returns: 
+;	AX=0 if A20 off, AX=1 if A20 on
+check_a20:
+	PUSHF						; save FLAGS
+	PUSH 	DS					; save DS
+	PUSH 	ES					; save ES
+	PUSH	DI					; save DI
+	PUSH	SI					; save SI
+
+	CLI							; disable interrupts
+
+	XOR 	AX, AX				; AX = 0
+	MOV		ES, AX				; ES = 0
+
+	NOT 	AX					; AX = 0xFFFF
+	MOV		DS, AX				; DS = 0xFFFF
+
+	MOV		DI, 0x0500			; DI = 0x0500
+	MOV 	SI, 0x0510			; SI = 0x0510
+
+	MOV 	AL, [ES:DI] 		; AL = Memory[0x0500]
+	PUSH 	AX					; save original value at Memory[0x500]
+
+	MOV 	AL, [DS:SI] 		; AL = Memory[0xFFFF:0x0510]
+	PUSH 	AX					; save original value at Memory[0xFFFF:0x0510]
+
+	MOV 	BYTE [ES:DI], 0x00	; Memory[0x0500] = 0x00
+	MOV 	BYTE [DS:SI], 0xFF	; Memory[0xFFFF:0x0510] = 0xFF
+
+	CMP 	BYTE [ES:DI], 0xFF	; Compare Memory[0x0500] to 0xFF
+								; if the bytes match, the memory is wrapping around
+								; therefore, the A20 line is disabled
+
+	POP 	AX					; Restore memory at [0xFFFF:0x0510]
+	MOV 	[DS:SI], AL			; ^^^^
+
+	POP 	AX					; Restore memory at [0x0500]
+	MOV 	[ES:DI], AL			; ^^^^
+
+	MOV 	AX, 0				; AX (return value) = 0 (A20 disabled)
+
+	JE check_a20_exit			; If the last CMP was equal (A20 disabled), exit
+
+	MOV 	AX, 1				; AX (return value) = 1 (A20 enabled)
+
+check_a20_exit:
+	POP 	SI					; restore SI
+	POP		DI					; restore DI
+	POP		ES					; restore ES
+	POP		DS					; restore DS
+	POPF						; restore FLAGS
+
+	RET						; return
+
+enable_a20:
+	CALL	a20_wait			; wait for keyboard controller
+	MOV		AL, 0xAD			; 0xAD = disable keyboard
+	OUT		0x64, AL			; send command 0xAD to keyboard controller
+
+	CALL	a20_wait			; wait for keyboard controller
+	MOV		AL, 0xD0			; 0xD0 = read from input
+	OUT		0x64, AL			; send command 0xD0
+
+	CALL	a20_wait2			; wait for keyboard input
+	IN		AL, 0x60			; read input from keyboard controller
+	PUSH	AX					; save input
+
+	CALL	a20_wait			; wait for keybaord controller
+	MOV		AL, 0xD1			; 0xD1 = write to output
+	OUT		0x64, AL			; send command 0xD1
+
+	CALL	a20_wait			; wait for keyboard controller
+	POP		AX					; restore input value
+	OR		AL, 2				; enable bit 1
+	OUT		0x60, AL			; send output to keyboard
+
+	CALL	a20_wait			; wait for keyboard controller
+	MOV		AL, 0xAE			; 0xAE = enable keyboard
+	OUT		0x64, AL			; send command 0xAE
+	RET
+
+
+a20_wait:
+	IN		AL, 0x64			; read status of keyboard controller
+	TEST	AL, 2				; read bit 1 of status
+	JNZ		a20_wait			; if not zero, keyboard controller isn't ready. Retry
+	RET							; return
+
+a20_wait2:
+	IN		AL, 0x64			; read status of keyboard controller
+	TEST	AL, 1				; read bit 0 of status
+	JZ		a20_wait2			; if zero, input isn't ready
+	RET							; return
+
 entry:
 		; Set up stack and segments
 		MOV		AX, 0				; set up stack segment
 		MOV		SS, AX				; ^^^
-		MOV		SP, 0x7C00			; set up stack pointer (0x6c00-x7BFF) 
+		MOV		SP, 0x7C00			; set up stack pointer (0x6c00-0x7BFF) 
 									; Sp HAS TO be set in the instruction after settings SS 
 									; as setting SS disables interrupts for the next instruction
 									; Not doing this may cause an interrupt to use an old SP and new SS
@@ -106,8 +201,13 @@ entry:
 		MOV		FS, AX				; ^^^
 		MOV		GS, AX				; ^^^
 
+		PUSH	DX					; save drive number (DL)		
+
+		; Load drive parameters
+		MOV		AX,	4800h				; load AH=48h, AL=0h
+		MOV		SI, disk_info_packet	; load address of Disk_Info_Packet
+		INT		13H						; INT 14h AH=48h: get drive parameters
 
-		PUSH	DX					; save drive number (DL)
 
 		; Print message
 		MOV		AX, msg				; load address of msg
@@ -116,17 +216,31 @@ entry:
 		PUSH	AX					; push parameter 'size of string'
 		CALL	print				; call function 'print'
 
-over:
-		JMP		over				; loop forever
+		SUB		SP, 4				; clear previous call parameters
+
+		CALL 	check_a20			; call check_a20. Result in Ax
+		CMP		AX, 0				; compare AX to 0 (0 = A20 disabled)
+		JNE		after_a20			; if AX != 0, a20 is enabled, skip enabling it
+
+		CALL 	enable_a20			; enable A20 line via keyboard controller
+
+		CALL 	check_a20			; check status of A20 line (again)
+		CMP		AX, 0				; compare AX to 0 (0 = A20 disabled)
+		JNE		after_a20			; if AX != 0, a20 is enabled
+
+		MOV		AX, a20_msg			; load address of 'a20_msg'
+		PUSH	AX					; push parameter 'address of string'
+		MOV		AX, 6				; load size of 'a20_msg'
+		PUSH	AX					; push parameter 'size of string'
+		CALL	print				; print 'a20_msg'
+		JMP		endless_loop		; loop endlessly
 
 
-		; Load drive parameters
-		MOV		AX,	4800h				; load AH=48h, AL=0h
-		MOV		SI, disk_info_packet	; load address of Disk_Info_Packet
-		INT		13H						; INT 14h AH=48h: get drive parameters
-
+after_a20:
 		
 
+endless_loop:
+		JMP		endless_loop		; loop endlessly
 
 section .data
 disk_info_packet:
@@ -150,4 +264,5 @@ build_settings_magic_0		dd		0x49494949
 build_settings_bytes		dw		0x0
 build_settings_magic_1		dd		0x49494949
 
-msg				db			"OS3_BOOT_OK!"
\ No newline at end of file
+msg				db			"OS3_BOOT_OK!"
+a20_msg			db			"NO_A20"
\ No newline at end of file