Format of FCB reserved field for DOS 2.x:
Offset	Size	Description	)
 18h	BYTE	bit 7: set if logical device
		bit 6: set if open???
		bits 5-0: ???
 19h	WORD	starting cluster number on disk
 1Bh	WORD	???
 1Dh	BYTE	???
 1Eh	BYTE	???
 1Fh	BYTE	???