INT 21,15 - Sequential Write Using FCB

	AH = 15h
	DS:DX = pointer to an opened FCB

	on return:
	AL = 00 if write was successful
	   = 01 if diskette is full or read only
	   = 02 if DTA is too small

	- writes a record from the DTA to the current record position
	  in file specified by the opened FCB
	- record size and output location are maintained in the FCB