Format of DoubleSpace DSPACKET structure:
Offset	Size	Description	)
 00h	WORD	signature 444Dh ("DM")
 02h	BYTE	command code
		46h ('F') flush internal caches
		49h ('I') flush and invalidate internal caches
 03h	WORD	result code
		(ret) 4F4Bh ("OK") if successful, else unchanged
 05h  5 BYTEs	padding