asm {
start:
	; mov [boot_drive], dl
	du8 0h88
	du8 0h16
	du16 boot_drive

	; mov si, kdap
	du8 0hBE
	du16 kdap

	; call lddap
	du8 0hE8
	du16 &lddap

	; mov si, pdap
	du8 0hBE
	du16 pdap

	; call lddap
	du8 0hE8
	du16 &lddap

	;; Is A20 supported
	; mov ax, 0h2403
	du8 0hB8
	du16 0h2403

	int 0h15

	; jc a20ns
	du8 0h0F
	du8 0h82
	du16 &a20ns

	test ah, ah

	; jnz a20ns
	du8 0h0F
	du8 0h85
	du16 &a20ns

	;; Check A20 status
	; mov ax, 0h2402
	du8 0hB8
	du16 0h2402

	int 0h15

	; jc a20scfail
	du8 0h0F
	du8 0h82
	du16 &a20scfail

	test ah, ah

	; jnz a20scfail
	du8 0h0F
	du8 0h85
	du16 &a20scfail

	test al, al

	; jnz pm_phase
	du8 0h0F
	du8 0h85
	du16 &pm_phase

	;; Try activating
	; mov ax, 0h2401
	du8 0hB8
	du16 0h2401
	int 0h15

	; jc a20acfail
	du8 0h0F
	du8 0h82
	du16 &a20acfail

	test ah, ah

	; jnz a20acfail
	du8 0h0F
	du8 0h85
	du16 &a20acfail
}

asm {
pm_phase:
	cli ; disable interrupts

	; lgdt [GDT_DESC]
	du8 0h0F
	du8 0h01
	du8 0h16
	du16 GDT_DESC

	; mov eax, cr0
	du8 0h0F
	du8 0h20
	du8 0hC0

	or al, 1

	; mov cr0, eax
	du8 0h0F
	du8 0h22
	du8 0hC0

	; jmp CODE_DESC:pm_jump
	du8 0h66
	du8 0hEA
	du32 pm_jump
	du16 GDT_CODE - GDT_START
}

asm {
pm_jump:
	; mov eax, DATA_DESC
	du8 0hC7
	du8 0hC0
	du32 GDT_DATA - GDT_START

	; mov ds, ax
	du8 0h8E
	du8 0hD8

	; mov es, ax
	du8 0h8E
	du8 0hC0

	; mov fs, ax
	du8 0h8E
	du8 0hE0

	; mov gs, ax
	du8 0h8E
	du8 0hE8

	; mov ss, ax
	du8 0h8E
	du8 0hD0

	; jmpt to PBOOT
	du8 0hEA
	du32 0h8400
	du16 GDT_CODE - GDT_START
}

asm {
lddap:
	; mov dl, [boot_drive]
	du8 0h8A
	du8 0h16
	du16 boot_drive

	mov ah, 0h42
	int 0h13

	; jc lddapfail
	du8 0h0F
	du8 0h82
	du16 &lddapfail

	ret
}

asm {
print:
	mov ah, 0h0e
print_loop:
	du8 0h8A ; mov al, [si]
	du8 0h04

	cmp al, 0h00

	; jz print_done
	du8 0h0F
	du8 0h84
	du16 &print_done

	int 0h10

	; inc si
	du8 0hFF
	du8 0hC6

	; jmp print_loop
	du8 0hE9
	du16 &print_loop
print_done:
	ret
}

asm {
busyloop:
	; jmp busyloop
	du8 0hE9
	du16 &busyloop
}

asm {
a20ns:
	; mov si, a20nsmg
	du8 0hBE
	du16 a20nsmsg

	; call print
	du8 0hE8
	du16 &print

	; jmp busyloop
	du8 0hE9
	du16 &busyloop
}

asm {
a20scfail:
	; mov si, a20scfailmsg
	du8 0hBE
	du16 a20scfailmsg

	; call print
	du8 0hE8
	du16 &print

	; jmp busyloop
	du8 0hE9
	du16 &busyloop
}

asm {
a20acfail:
	; mov si, a20scfailmsg
	du8 0hBE
	du16 a20acfailmsg

	; call print
	du8 0hE8
	du16 &print

	; jmp busyloop
	du8 0hE9
	du16 &busyloop
}

asm {
lddapfail:
	; mov si, a20nsmg
	du8 0hBE
	du16 lddapfailmsg

	; call print
	du8 0hE8
	du16 &print

	; jmp busyloop
	du8 0hE9
	du16 &busyloop
} 

asm {
; PROTECTED DAP
pdap:
	du8 0h10
	du8 0
	du16 1
	du16 0h8400
	du16 0
	du64 2
}

asm {
; Kernel DAP
kdap:
	du8 0h10
	du8 0
	du16 1
	du16 0
	du16 0h4000 ; Temporary address
	du64 3
}

asm {
GDT_START:
	du64 0 ; null desc
	GDT_CODE:
		du16 0hFFFF
		du16 0
		du8 0
		du8 0h9A
		du8 0hCF
		du8 0
	GDT_DATA:
		du16 0hFFFF
		du16 0
		du8 0
		du8 0h92
		du8 0hCF
		du8 0
GDT_END:
GDT_DESC: 
	du16 GDT_END - GDT_START - 1
	du32 GDT_START
}

asm {
boot_drive: du8 0
a20nsmsg: du8 "Boot failed! A20 not supported!", 13, 10, 0
a20scfailmsg: du8 "Boot failed! Failed to check a20 status!", 13, 10, 0
a20acfailmsg: du8 "Boot failed! Failed to activate A20!", 13, 10, 0
lddapfailmsg: du8 "Boot failed! Could not load dap", 13, 10, 0
}