INT 21,28 - Random Block Write Using FCB

	AH = 28h
	CX = number of records to write
	DS:DX = pointer to an opened FCB

	on return:
	AL = 00 if write successful
	   = 01 if diskette full or read only
	   = 02 if DTA is too small
	CX = number of records written

	- allows random access and sequential writing of a group
	  of records from a file opened with an FCB into the DTA
	- FCB must be setup with random record number and record size
	- updates random record number, current block and current
	  record fields