Implement calculation for FAT position

kernel/fat.asm

@@ -0,0 +1,68 @@
+	section .bss
+
+	; the sector numbers have the partition offset already included
+
+fat_offset:
+	resd 1
+fat_size:
+	resd 1
+fat_num: ; a byte would be enough, but i want to work with cx
+	resw 1
+
+rootdir_offset:
+	resd 1
+rootdir_size:
+	resw 1
+
+clusters_offset:
+	resd 1
+clusters_num:
+	resd 1
+
+	section .text
+
+	; invoked after disk select
+load_bpb:
+	; read word for "sectors per fat"
+	mov ax, [disk_buffer+0x016]
+	xor dx, dx
+	; if zero, we need to check dword in BPB 7.1
+	test ax, ax
+	jnz .short_sf
+	; fail if BPB 7.1 signature is not present
+	mov cl, [disk_buffer+0x042]
+	or cl, 1
+	cmp cl, 0x29
+	jne .err
+	; load dword
+	mov ax, [disk_buffer+0x024]
+	mov dx, [disk_buffer+0x026]
+.short_sf:
+	; store local value
+	mov [fat_size], ax
+	mov [fat_size+2], dx
+
+	; copy number of fat's
+	xor ah, ah
+	mov al, [disk_buffer+0x010]
+	mov [fat_num], ax
+
+	mov ax, [part_offset]
+	mov dx, [part_offset+2]
+	add ax, [disk_buffer+0x0B+bpb_rsc]
+	adc ax, 0
+	mov [fat_offset], ax
+	mov [fat_offset+2], dx
+
+	int 3
+
+	; add FAT size * FAT number
+	mov cx, [fat_num]
+.floop:	add ax, [fat_size]
+	adc dx, [fat_size+2]
+	loop .floop
+
+	int 3
+	ret
+.err:	stc
+	ret