When writing of data into nonvolatile memories 8H and 8L is started, data
D7 and D15 corresponding to the 128th word of a data input section 8B are
inverted and outputted. When accurate writing is subsequently performed,
the data D7 and D15 are outputted as they are. By monitoring a change of
the data D7 and D15 from the nonvolatile memories 8H and 8L, it is
possible to detect whether writing is still continuing or has already
completed. Thus, by using nonvolatile memories of 8 bit data width or the
like, a 16-bit microcomputer can be easily realized.