US Patent 9720661 Selectively controlling use of extended mode features

An optimizer identifies application code to be modified and modifies that code, such that the application includes unmodified code and modified code. Based on generating the modified code, the optimizer sets one or more controls to selectively enable use of extended mode features by the application. The extended mode features include one or more features at an instruction set level different from an instruction set level available to the application. The controls may be set in a control register or entries of address translation structures, as examples.