1. A method of identifying a cause of an interrupt related to an interrupt indication received from an interrupt indicating circuit in a processor, the method comprising: having the interrupt indicating circuit provided with a plurality of lowest-layer registers having a plurality of bits, each of the plurality of bits corresponding to the cause of the interrupt, and one or more upper-layer registers for aggregating the plurality of lowest-layer registers; forming a hierarchical structure with the one or more upper-layer registers and the plurality of lowest-layer register; and identifying the cause of the interrupt by having the processor read the upper-layer registers and the lowest-layer registers in order following the hierarchical structure.