Nonvolatile memory apparatus

In technology for enabling the replacement of part of an operating program of a controller by a modified program on a nonvolatile memory, the present invention prevents tampering and leak of storage information within the nonvolatile memory and the controller. At power-on reset, an encrypted alternative program, if present, is transferred from a nonvolatile memory to a volatile memory, and decrypted when actually executed. A long wait is not required until data processing by a data processor is enabled after the exit from the reset processing. Since the alternative program once decrypted is held in the volatile memory so as to be reusable, it does not need to be decrypted each time it is executed. Since the alternative program is encrypted, even if the nonvolatile memory is physically separated from the controller to illegally dump the alternative program, it is difficult to analyze the data.