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Enable_A20:
; Enables A20 gate using:the BIOS, keyboard controller, or Fast A20
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	pushad									;Preserve registers
	pushfd									;Preserve EFLAGS because we might disable interrupts
	mov ax,0x2402							;INT 15h AX=2402h: Query A20 status
	int 0x15								;Check to see if A20 gate is enabled
	jc A20_No_BIOS							;Error? Don't use BIOS to enable gate
	test al,1								;Test Bit 0 of AL
	je A20_Enabled							;A20 gate already enabled
	mov ax,0x2401							;INT 15h AX=2401h: Enable A20 gate
	int 0x15								;Use BIOS to enable A20 gate
	jc A20_No_BIOS							;Error? Don't use BIOS to enable gate
	or ah,ah								;Test AH
	jnz A20_No_BIOS							;Non-zero? A20 gate may not be enabled
A20_Enabled:
	popfd									;Restore EFLAGS
	popad									;Restore registers
	ret										;Return
A20_No_BIOS:
	mov ax,0x2403							;INT 15h AX=2403h: Query A20 support
	int 0x15								;Call BIOS to find out how the A20 gate can be enabled
	jc A20_KBD								;Error? Assume that keyboard controller is the only option
	test bx,1								;Bit 0:Keyboard controller supported
	je A20_KBD								;BIOS indicates that keyboard controller is supported
	test bx,2								;Bit 1:Fast A20 supported
	je FAST_A20								;BIOS indicated that Fast A20 is supported
	call Check_A20							;Test A20 gate manually
	jc A20_Enabled							;A20 gate already enabled
A20_Fail:
	lea si,[A20_ERROR]						;"Unable to enable A20 gate!"
	call PrintString					
	call A20_OUT_Wait						;Print string
	cli										;Disable interrupts
	hlt										;Halt machine
FAST_A20:
	in al,0x92								;Read System Control Port A
	test al,2								;Test Fast A20 bit
	jnz A20_Fail							;Bit already set, failed to enable A20 gate
	or al,2									;Set Bit 1:Enable Fast A20
	and al,0xFE								;Always clear Bit 0 so that machine does not reboot
	out 0x92,al								;Write to port 92h
	call Check_A20							;Check A20 gate
	jc A20_Enabled							;Success?
	cli										;No? something must have gone wrong
	hlt										;Clear interrupts and halt machine
A20_KBD:
	cli										;Disable interrupts
	mov cx,50								;Number of attempts to enable gate
Use_KBD:
	call A20_OUT_Wait						;Wait for keyboard
	mov al,0xAD								;Disable Keyboard
	out 0x64,al								;Wait for Keyboard
	mov al,0xD0								;Read Controller Output Port 
	out 0x64,al								
	call A20_IN_Wait						;Wait for Keyboard
	in al,0x60								;Read Data Port 
	push ax									;Save
	call A20_OUT_Wait						;Wait for Keyboard
	mov al,0xD1								;Write Controller Output Port
	out 0x64,al								
	call A20_OUT_Wait						;Wait for Keyboard
	pop ax									;Get port data back
	or al,10b								;Bit 1: Enable A20 gate
	out 0x60,al								;Write to data port
	call A20_OUT_Wait						;Wait for Keyboard
	mov al,0xAE								;Enable Keyboard
	out 0x64,al
	call A20_OUT_Wait						;Wait for Keyboard
	call Check_A20							;Verify that gate is enabled
	jc A20_Enabled							;Yes? Return
	loop Use_KBD							;No? Keep trying
	jmp A20_Fail							;Could not enable gate after 50 tries
A20_OUT_Wait:
	in al,0x64								;Read status register
	test al,10b								;Is port ready?
	jnz A20_OUT_Wait						;Nope, wait
	ret										;Yep, continue
A20_IN_Wait:	
	in al,0x64								;Read status register
	test al,1b								;Is port ready?
	jz A20_IN_Wait							;Nope, wait
	ret										;Yep, continue

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Check_A20:
; Test if A20 gate is enabled by comaparing the value in FFFF:7E0E
; 	with the VBR boot signature at 0000:7DFE
; Out: Carry flag set if A20 gate is enabled
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	pushad									;Preserve registers
	push es
	mov ax, 0xFFFF
	mov es, ax								;Extra segment:FFFF
	cmp WORD [es:0x7DFE + 16], 0xAAFF		;See if boot signature is wrapped at FFFF:7E0E (Meaning FFFF:7E0E = 0:7DFE)
	stc										; Assume we didn't wrap, yes A20
	jne .End
	clc										; We did wrap, no A20
.End:
	pop es
	popad
	ret
	
A20_ERROR db "Unable to enable A20 gate!",0