Improve error handling in fat bs

fat.asm

@@ -234,12 +234,17 @@ fopen:
 fread:  push ax
 	push cx
 	push dx
+	; abort if there is no file opened
 	cmp word [cs:cclus], 0
 	je .err
+	; get start sector for data area
 	call clusec
 	push ax
+	; get current cluster number
 	mov ax, [cs:cclus]
+	; clusters are 2-indexed
 	sub ax, 2
+	; multiply that with sectors per cluster
 	mov cl, [cs:fdc.sc]
 	xor ch, ch
 	mul word cx
@@ -251,13 +256,22 @@ fread:  push ax
 	; dx:ax now point to the sector that should be read next
 	call seek
 	call read
+	jc .err
+	inc word [cs:csec]
 	jmp .ret
-.err:	stc
+.err:	; in case of error, trash current state
+	call fclose
+	stc
 .ret:	pop dx
 	pop cx
 	pop ax
 	ret
 
+	; Reset open file info
+fclose: mov [cclus], bx
+	mov word [csec], 0
+	ret
+
 main:	mov [seek.dl], dl ; save drive number
 
 	call fopen