INT 15,2 - Read Blocks from Cassette (PC,PCjr only)

	AH = 02
	CX = count of bytes to read
	ES:BX = pointer to data buffer

	on return:

	AH = error code if CF=1
	   = 1	if CRC error
	   = 2	data transition lost, (bit signal scrambled)
	   = 3	no data found on tape

	DX = count of bytes actually read
	ES:BX = pointer to byte following last byte read

	- data is transferred in 256 byte blocks, though only the
	  number of bytes requested are actually stored