; Bootsector for FAT32 filesystems
	org 0x0800
	jmp near init

	; WORD reserved sector count
rsc:    equ ( $$ + 0x00E )
	; BYTE number of FATs
fn:	equ ( $$ + 0x010 )
	; DWORD hidden sector count (partition offset)
po:	equ ( $$ + 0x01C )
	; DWORD sectors per FAT
sf:	equ ( $$ + 0x024 )
	; BYTE drive number (we set it from dl)
dn:	equ ( $$ + 0x040 )

	; Area for BPB
	times (0x5A - ($-$$)) db 0

init:	xor ax, ax
	mov ds, ax
	mov es, ax
	mov ss, ax
	mov sp, 0x7C00

	mov si, sp
	mov di, 0x0800
	mov cx, 0x0100
	rep movsw
	jmp 0x0:main

msg:	pop si
mloop:	lodsb
	test al, al
	jz mend
	mov ah, 0x0e
	mov bx, 7
	int 0x10
	jmp mloop
mend:	push si
	ret

err:	call msg
	db "DISKERR", 0x0A, 0x0D, 0
hlt:	hlt
	jmp hlt

	; Requires DWORD ptr and QWORD sector on the stack
read:	pop bp
	push cx
	mov ax, 0x10
	push ax
	mov ah, 0x42
	mov dl, 0x80
	mov si, sp

	int 0x13
	jc err

	pop ax
	pop cx
	push bp
	ret

	; Advances cluster number to next entry
next:	; Upper two words for sector num
	xor ax, ax
	push ax
	push ax

	; Lower two words for sector num
	mov ax, [clus]
	mov dx, [clus+2]

	; 32-bit >> 7
	mov cl, 7
shftl:	clc
	rcr dx, 1
	rcr ax, 1
	loop shftl

	; Add reserved sector count
	add ax, word [rsc]
	adc dx, 0

	push dx
	push ax

	; target buffer
	push cs
	mov ax, buf
	push ax

	mov cx, 1
	call read

        ; Stow off offset in FAT sector
        mov si, [clus]
        sal si, 1
        sal si, 1
        and si, 0x1FF
	add si, buf
	mov di, clus
	movsw
	movsw

	; 1x QWORD, 1x DWORD = 12 bytes
	add sp, 12
	ret

main:	call next
	mov ax, [clus]
	mov dx, [clus+2]
	int 3

	xor ah,ah
	int 0x16
	int 0x19
	jmp hlt

clus:	dd 2

	; Padding and signature
	times (0x1FE - ($-$$)) db 0
	dw 0xAA55
buf: