Format of FCB reserved field for DOS 1.0:
Offset	Size	Description	)
 16h	WORD	location in directory (if high byte = FFh, low byte is device
		  ID)
 18h	WORD	number of first cluster in file
 1Ah	WORD	current absolute cluster number on disk
 1Ch	WORD	current relative cluster number within file
		(0 = first cluster of file, 1 = second cluster, etc.)
 1Eh	BYTE	dirty flag (00h = not dirty)
 1Fh	BYTE	unused