Abstract: A mask ROM having a defect repairing function stores address signals corresponding to a defective memory cell and then, selectively activates either a redundancy row decoder or a row decoder according to whether the address signals stored are identical to address signals supplied externally. The mask ROM includes first and second memory cell arrays formed by grouping in a word line direction a plurality of read only memory cells arranged in rows and columns; first and second row decoders for combining row address signals supplied externally so as to selectively drive the word lines of the first and second memory cell arrays; and a row decoder selector for storing therein address signals according to a row block including a defective memory cell, of the first memory cell array so as to inactivate the first row decoder and activate the second row decoder when the external row address signals are equal to the address signals stored in the row decoder selector.

Abstract: A device for changing a frequency of an internal control clock for testing a chip, by incorporating a mode selection circuit (30) and a high voltage detection circuit (40) in a serial input/output memory. The mode selection circuit (30) is connected between two selected adjacent circuits C.sub.n-2, C.sub.n-1 among a plurality of frequency conversion circuits C.sub.1 . . . C.sub.n, for accessing selectively either a clock pulse CP.sub.n-2 from the frequency conversion circuit C.sub.n-2, arranged in front thereof or a system clock XSK, in dependence upon an internal voltage sense signal IV, IVB. The high voltage detection circuit (40) transmits the internal voltage sense signal to the mode selection circuit (30) by detecting a level of externally applied voltage XV. The internal control clock ICK provided by this device attains a period of T.sub.XSK .times.2.sup.n-M+1, wherein "M" is a number of the counter receiving the mode selection signals MS, MSB next to the mode selection circuit.

Abstract: A normal decoder and a redundant decoder having address program devices are used for the replacement of bad cells. The number of address program devices is one more than the number of input address bits for selecting a normal row or column. The input signals of the additional program device are complementary to the input signals of one of the other program devices. The program of the program devices have two steps to repair the faulty cells. To increase the reliability of redundancy, a nonvolatile memory element used in the program devices is a bridge connected four cell FLOTOX type nonvolatile memory device.