INT 21,21 - Random Read Using FCB

	AH = 21h
	DS:DX = pointer to an opened FCB

	on return:
	AL = 00 if read successful
	   = 01 if EOF (no data read)
	   = 02 if DTA is too small
	   = 03 if EOF (partial record read)

	- reads random records from a file opened with an FCB
	  to the DTA
	- FCB must be setup with drive id, filename, extension,
	  record position and record length before call
	- current record position field in FCB is not updated