Abstract: Each of microcodes 1 has an interrupt prohibit bit 10 that specifies acceptance or nonacceptance of an interrupt request. Upon occurrence of an interrupt request, a processor refers to a value set in the interrupt prohibit bit 10 of the microcode 1 being currently executed. When "0" is set to the interrupt prohibit bit 10, the processor accepts the interrupt request, and when "1" is set to the interrupt prohibit bit 10, the processor rejects the interrupt request and starts executing a succeeding microcode. This allows the processor to safely execute certain instructions without interruption, as indicated by the interrupt prohibit bit 10 contained in the microcode 1 being currently executed.