Apparatus and methods to execute an instruction of an application of a first bit size ported to a second bit size environment, including methods and apparatus to confine the application to a first bit size address space subset. An embodiment in accordance with the present invention includes a method to confine an application to an address space subset, the method including determining that the application is confined to a first bit size address subset, the application including an instruction; generating an address reference of a second bit size as part of execution of the instruction; truncating the generated address reference from the second bit size to the first bit size; and extending the truncated, generated address reference from the first bit size to the second bit size based at least in part on an address format control flag.