Device Request Header Format

     Offset Size		Description

	00   byte	length in bytes of the request header
	01   byte	unit code;  the sub-unit the operation is for (block
			devices);  meaningless for character devices
	02   word	command code, (see DEVICE COMMANDS)
	04  8bytes	reserved for DOS
	0C  nbytes	request data  (variable length)


	- see  INT 21,44  or	DEVICE ATTRIBUTES  or  DEVICE CODES
	- see also  IOCTL,n where "n" is an IOCTL function