A circuit by which an SWI instruction held in a memory circuit is switchedly input in response to a signal supplied externally is provided in a processor, whereby the substitution of a program word can be realized using such an internal circuit, so that a high-speed and reliable break in program excution can be effected. Moreover, an interrupt function separate from normal interrupts is established by setting a specified operation mode, whereby, when a data processor is used as an emulator, the break of a user program including the ordinary interrupt processes can be easily effected by utilizing the special interrupt function. Also, by additionally providing the function of delivering out a signal indicative of the break status, an external circuit can be simplified.