A method and apparatus is disclosed for initiating execution of selected application processors in a clustered multiprocessor system without disrupting the addressing mode of the processors. In one embodiment, this is accomplished by initializing the processors in a conventional way, setting the addressing mode, and then leaving each of the processors in an active mode spinning on a designated Startup Address Location. To start an application processor, the OS simply write a target processor ID and a desired start-up address into the Startup Address Location. The application processor with the matching processor ID reads the Startup Address Location, jumps to the address specified in the Start-Up Address code section, and begins execution at that location. In another embodiment, a processor is provided that does not switch addressing modes when switched from a halt state.