PURPOSE: To easily know a varied address, in the case an arbitrary bit of a control register is varied.

CONSTITUTION: In the firmware diagnostic device, when it is desired to know an address at the time the contents of an X-th bit of a control register 20 are varied, a firmware is run by setting X to a first detecting circuit 30 and a second detecting circuit 40. When it is detected that X is varied from '0' to '1', a first detecting circuit 30 writes the contents of a program counter 10 to a memory circuit 50 by a write signal (b). In the same way, when it is detected that X is varied from '1' to '0', to a second detecting circuit writes the contents of the program counter 10 to the memory circuit 50 by a write signal (c). Thereafter, by looking at the contents of the memory circuit 50, a running address of the firmware at the time of a change in a bit of X can be known.