Implement calculation of number of clusters

kernel/fat.asm

@@ -21,6 +21,15 @@ clusters_num:
 
 	section .text
 
+load_ts: ; load total number of sectors into DX:AX
+	xor dx, dx
+	mov ax, [disk_buffer+0x013]
+	test ax, ax
+	jnz .ret
+	mov ax, [disk_buffer+0x020]
+	mov dx, [disk_buffer+0x022]
+.ret:	ret
+
 	; invoked after disk select
 load_bpb:
 	; read word for "sectors per fat"
@@ -47,22 +56,57 @@ load_bpb:
 	mov al, [disk_buffer+0x010]
 	mov [fat_num], ax
 
+	; copy number of root directory entries
+	mov ax, [disk_buffer+0x011]
+	mov cl, 4
+	shr ax, cl
+	mov [rootdir_size], ax
+
+	; calculate offsets for everything
 	mov ax, [part_offset]
 	mov dx, [part_offset+2]
+	; add reserved sectors
 	add ax, [disk_buffer+0x0B+bpb_rsc]
 	adc ax, 0
+	; save begin of FAT tables
 	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
+	; save begin of root directories
+	mov [rootdir_offset], ax
+	mov [rootdir_offset+2], dx
+	; add root directory size (might be zero)
+	add ax, [rootdir_size]
+	adc dx, 0
+	; save start of data area
+	mov [clusters_offset], ax
+	mov [clusters_offset+2], dx
+
+	call load_ts
+	add ax, [part_offset]
+	adc dx, [part_offset+2]
+	sub ax, [clusters_offset]
+	sbb dx, [clusters_offset+2]
+
+	; get "sectors per cluster"
+	mov cl, [disk_buffer+0x00D]
+	; this turns dx:ax from data sectors to number of clus
+.sloop:	shr cl, 1
+	test cl, cl
+	jz .send
+	clc
+	rcr dx, 1
+	rcr ax, 1
+	jmp .sloop
+.send:  mov [clusters_num], ax
+	mov [clusters_num+2], dx
 
 	int 3
+
 	ret
 .err:	stc
 	ret