Format of FCB reserved field for DOS 1.10-1.25:
Offset	Size	Description	)
 18h	BYTE	bit 7: set if logical device
		bit 6: not dirty
		bits 5-0: disk number or logical device ID
 19h	WORD	starting cluster number on disk
 1Bh	WORD	current absolute cluster number on disk
 1Dh	WORD	current relative cluster number within file
 1Fh	BYTE	unused