Method of ensuring operation of calculator

A method of ensuring operation of a calculator executed by a CPU of a microcomputer including a memory, the calculator, and the CPU, the method comprising:

• in a state where the memory stores a function for normal operation that immediately returns to a caller, and a function for abnormal operation that performs a process for abnormal operation without using the calculator, where a first address of the memory acquired by adding a predetermined value to a starting address of an area of a predetermined size having a predetermined address as the starting address stores an address of the function for normal operation, and where all addresses of the memory other than the first address store an address of the function for abnormal operation,
• causing, by the CPU, the calculator to perform a predetermined operation verification calculation of which a correct value that is a resultant value in normal operation is set to the predetermined value;
• acquiring, by the CPU, the resultant value of the operation verification calculation performed by the calculator;
• referencing, by the CPU using a relative address referencing capability of the CPU, an address that is stored in an address acquired by adding the acquired resultant value to the starting address of the memory;
• and
• executing, by the CPU, a function stored at the referenced address.