INT 21,F - Open a File Using FCB

	AH = 0F
	DS:DX = pointer to unopened FCB

	on return:
	AL = 00 if file opened
	   = FF if unable to open

	- opens an existing file using a previously setup FCB
	- the FCB fields drive identifier, filename and extension
	  must be filled in before call
	- sets default FCB fields; current block number is set to 0;
	  record size is set to 80h; file size, date and time are set
	  to the values from the directory
	- does not create file, see  INT 21,16
	- DOS 2.x allows opening of subdirectories, DOS 3.x does not