Read data blocks from cassette (INT 15h, 02h - PC,Jr)

        Entry:
          AH = 02h
          CX = count of bytes
          ES:BX = pointer to data area
        Return:
          AH = status
               01h CRC error
               02h bad tape signals
               04h no data
               80h invalid command
               86h no cassette present
          CF set on error
          DX = count of bytes read
          ES:BX = pointer past last byte read