A microarchitecture is a particular hardware organization implementing an instruction set architecture. It specifies mechanisms such as instruction decoding, execution units, pipelines, caches and branch prediction. Two processors can expose the same instruction-set contract while making different choices about those mechanisms.
For example, both processors may correctly execute a load followed by an integer addition. One can overlap independent instructions and obtain the load from a nearby cache, while the other waits for a longer memory access. The architectural result agrees, but cycle counts differ. Speculative work must be discarded or committed in a way that preserves the required visible behavior. This distinction explains why compatibility with one instruction set does not imply equal performance, power consumption or physical implementation.[ref]
