It will prefetch the instructions and put into the pipeline the branch it thinks is mostly likely. It may do ahead-of-time speculative execution on certain instructions but not always. If it missed the correct branch it will flush the pipeline and start the pipeline over again from the correct branch. Afaik it doesn't execute or prefetch both branches. The other guy is saying it does but that doesn't really gel with my own recollection or the Wikipedia article he cited. You can see some further discussion that suggests only one branch gets prefetched here here and here. Reasons cited for only predicting one branch are: 1) Two pipelines with all the associated circuitry to look ahead, decode, and speculatively execute is incredibly expensive in terms of both processing requirements and die real estate. 2) Caching both would thrash your caches with new data constantly. 3) Modern branch prediction is already so accurate, there's really no need for two pipelines anyways.