Many modern processors now contain multiple processing cores, and multi-threaded programming is usually the most efficient way to use more computational power in a single program. Some modern processors are highly optimized with techniques including out-of-order execution, but with complex logic to detect when this can be done without violating the von Neumann semantics of the logical programming model. Separate instruction and data caches are widely used, making the hardware a Modified Harvard architecture, but again with logic to detect cases where the optimization fails, to be able to execute self-modifying code.