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%define ROOT_DIRECTORY_POS 0x7e00

org 0 ; we start everything at zero
	jmp short start
	nop     
MANU_DESC 			db 'mkdosfs '
BLOCK_SIZE	 		dw 512
BLOCKS_PER_ALLOCATION_UNIT 	db 128
RESERVED_BLOCKS			dw 1
TOTAL_FATS			db 1
TOTAL_ROOT_ENTRIES		dw 512
TOTAL_BLOCKS			dw 0xffff
MEDIA_DESCRIPTOR		db 0xf8
BLOCKS_PER_FAT			dw 0x01
BLOCKS_PER_TRACK		dw 18
TOTAL_HEADS			dw 0x02
HIDDEN_BLOCKS			dd 0x00
TOTAL_BLOCKS_2			dd 0x00
DRIVE_NUMBER			dw 0x00
EXTENDED_BOOT_SIGNATURE		db 0x29
VOLUME_SERIAL_NUMBER		dd 0x9d86f18c
VOLUME_LABEL			db 'SEAGULL    '
FILE_SYSTEM_IDENTIFIER		db 'FAT16   '

; Calculated in memory
ROOT_DIRECTORY		dw 0x00

DAPACK:
	db 0x10
	db 0
.len:	dw 1
.loc:	dd ROOT_DIRECTORY_POS
.sec:	dd 0
	dd 0
	
start:
	cli
	mov ax, 0x7c0
	mov ds, ax
	mov es, ax
	mov sp, 0x1000
	mov bp, 0
	mov ss, bp
	sti


	mov byte [DRIVE], dl

; Root Directory Logical Sector =  (TOTAL_FATS * BLOCKS_PER_FAT) + (RESERVED_BLOCKS)

	mov ax, [TOTAL_FATS]
	mul word [BLOCKS_PER_FAT]
	add ax, [RESERVED_BLOCKS]
	mov word [ROOT_DIRECTORY], ax


; Load root directory


	mov word [DAPACK.sec], ax

	mov si, DAPACK		; address of "disk address packet"
	mov ah, 0x42		; AL is unused
	mov dl, [DRIVE]		; drive number 0 (OR the drive # with 0x80)
	int 0x13
	jc err


FIND_KERNEL:

	cli			; Turn interrupts off when editing segment registers
	mov ax, 0x7e0
	mov es, ax
	sti

	mov bx, 0

; Calculate root directory data start - (ROOT_DIRECTORY + (TOTAL_ROOT_ENTRIES/16))
	mov ax, [ROOT_DIRECTORY]
	mov dx, 16
	div dl				; (TOTAL_ROOT_ENTRIES/16), AL = AX / DL
	add bx, ax
	mov ax, 1
	
.find:
	mov cx, 0
	mov si, KERNEL
.repeat:
	cmp cx, 11
	je .load_clusters
	lodsb
	cmp al, [es:bx]
	jne .next
	inc cx
	inc bx
	jmp .repeat
.next:
	mov ax, 32
	sub ax, cx
	add bx, ax
	jmp .find

.load_clusters:
	mov ax, 15	; 15 bytes away from start of cluster number
	add bx, ax	; Seek to cluster number
	mov ax, [es:bx]	; Grab cluster number
	
; Time to calculate the LBA of the kernel

; LBA = (cluster-2)*BLOCKS_PER_ALLOCATION_UNIT+(TOTAL_ROOT_ENTRIES/16)+(HIDDEN_BLOCKS+RESERVED_BLOCKS+TOTAL_FATS*BLOCKS_PER_FAT)


	mov bx, 2		
	sub ax, bx				; Subtract 2 from cluster number
	
	mov dl, [BLOCKS_PER_ALLOCATION_UNIT]
	mul dl					; AX = AL * BLOCKS_PER_ALLOCATION_UNIT

	push ax					; Push AX on to the stack
	mov dx, 0	
	mov ax, [TOTAL_ROOT_ENTRIES]
	mov bx, 16
	div bx					; DX:AX = TOTAL_ROOT_ENTRIES / 16
	mov bx, ax
	pop ax					; Pop AX off the stack
	add ax, bx				; Add the result on to AX

	mov bx, [HIDDEN_BLOCKS]			; NOTE: Hidden blocks is 32 bit. 16 bit will be fine.
	mov dx, [RESERVED_BLOCKS]
	add bx, dx				; BX = HIDDEN_BLOCKS + RESERVED_BLOCKS
	xor dx, dx
	mov dl, [TOTAL_FATS]			
	add bx, dx				; BX += TOTAL_FATS
	push ax					; Push AX on to the stack
	mov ax, bx
	mov dx, [BLOCKS_PER_FAT]
	mul dx					; DX:AX = AX * BLOCKS_PER_FAT
	mov bx, ax
	pop ax					; Pop AX off the stack
	add ax, bx				; AX += BX

LOAD_KERNEL:
						; Time to load the kernel!
	mov word [DAPACK.sec], ax
	mov si, DAPACK		; address of "disk address packet"
	mov ah, 0x42		; AL is unused
	mov dl, [DRIVE]		; drive number 0 (OR the drive # with 0x80)
	int 0x13
	jc err
	
	jmp 0x000:0x7e00				

err:
	mov si, ERROR
	call print
	


print:
	pusha
	mov ah, 0eh
.repeat:
	lodsb
	cmp al, 0
	je .done
	int 10h
	jmp .repeat
.done:
	popa
	ret


KERNEL db "KERNEL  BIN", 0
ERROR db "Failed to load sector", 10, 13, 0
DRIVE db 0

times 510-($-$$) db 0 ; fill in the remaining space with zero
dw 0xaa55 ; legacy boot signature