w_roodir_remaining	dw	ROOTDIR_TOT_SECTORS	
w_cur_sector_num		dw	0			

;
; fat12_read_file(u16 segment, u16 offset, uintptr (ds:)filename)
;

fat12_read_file:
	push bp
	mov bp,sp
	push bp
	
    ; reset the drive
	xor	ah, ah
	xor	dl, dl
	int	13h
	
	mov	word [w_cur_sector_num], ROOTDIR_SEC_NUM
.loop_rootdir:
	cmp	word [w_roodir_remaining], 0	
	jz	.not_found	
	dec	word [w_roodir_remaining]	
	mov	ax, [ss:bp+4]
	mov	es, ax
	mov	bx, [ss:bp+6]	
	mov	ax, [w_cur_sector_num]	
	mov	cl, 1

	call	read_sector
	
	mov	si, [ss:bp+8]
	mov	di, [ss:bp+6]
	cld
	mov	dx, 10h

.loop_find_file:
	cmp	dx, 0										
	jnz	.loop_find_file_ctn	
    ; We traversed all entires in the root dir
    add word [w_cur_sector_num], 1
    jmp .loop_rootdir

.loop_find_file_ctn:
	dec	dx										
	mov	cx, 11
.loop_strcmp:
	cmp	cx, 0
	jz	.found
dec	cx
	lodsb			
	cmp	al, byte [es:di]
	jnz	.loop_strcmp_diff		
	inc	di
	jmp	.loop_strcmp	

.loop_strcmp_diff:
	and	di, 0FFE0h					
	add	di, 20h					
	mov	si, [ss:bp+8]				
	jmp	.loop_find_file;   

.found:			
	mov	ax, ROOTDIR_TOT_SECTORS
	and	di, 0FFE0h		
	add	di, 01Ah		
	mov	cx, word [es:di]
	push	cx		
	add	cx, ax
	add	cx, SECTOR_DELTA	
	mov	ax, [ss:bp+4]
	mov	es, ax		
	mov	bx, [ss:bp+6]
	mov	ax, cx		

.loop_readfile:
	mov	cl, 1
	call	read_sector
	pop	ax	
	push word [ss:bp+4]
	call	read_fat_entry
	add sp,2
	cmp	ax, 0FFFh
	jz	.end
	push	ax			
	mov	dx, ROOTDIR_TOT_SECTORS
	add	ax, dx
	add	ax, SECTOR_DELTA
	add	bx, [BPB_BytsPerSec]
	jmp	.loop_readfile
.not_found:
    mov ax, 1
.end:
	mov sp,[bp-2]
	pop bp
	ret

read_sector:
	push bp
	mov	bp, sp
	sub	esp, 2		
	mov	byte [bp-2], cl
	push bx			
	mov	bl, [BPB_SecPerTrk]	
	div	bl			
	inc	ah			
	mov	cl, ah		
	mov	dh, al		
	shr	al, 1		
	mov	ch, al		
	and	dh, 1			
	pop	bx			
	mov	dl, [BS_DrvNum]		
.retry:
	mov	ah, 2		
	mov	al, byte [bp-2]	
	int	13h
	jc	.retry		

	add	sp, 2
	pop	bp

	ret

b_is_odd_cluster			db	0	
read_fat_entry:
	push	bp
	mov bp, sp
	push	es
	push	bx
	push	ax
	
	mov	ax, [ss:bp+4]
	sub	ax, 0100h	
	mov	es, ax
	pop	ax
	
	mov	byte [b_is_odd_cluster], 0
	mov	bx, 3
	mul	bx			
	
	mov	bx, 2
	div	bx			
	cmp	dx, 0

	jz	.even
	mov	byte [b_is_odd_cluster], 1
.even:
	xor	dx, dx			
	mov	bx, [BPB_BytsPerSec]
	div	bx
	push	dx
	mov	bx, 0 
	add	ax, FAT1_SECTOR_NUM 
	mov	cl, 2
	call	read_sector
			  
	pop	dx
	add	bx, dx
	mov	ax, [es:bx]
	cmp	byte [b_is_odd_cluster], 1
	jnz	.end
	shr	ax, 4
.end:
	and	ax, 0FFFh
	pop	bx
	pop	es
	pop bp
	ret