Function 02h    Read Blocks From Cassette		(PC, PCjr only) 
entry   AH      02h 
	CX      count of bytes to read 
	ES:BX   pointer to data buffer 
return  CF      set on error 
		AH      error code 
	DX      count of bytes actually read 
	ES:BX   pointer past last byte written 
note 1) NOP for systems where cassette not supported 
     2) Cassette operations normally read 256 byte blocks