The basic idea of speculative way activation is to make a prediction of the way where the required data may be located. If the prediction is correct, the cache access latency and power consumption is similar to that of a direct-mapped cache of the same size. If the prediction is wrong, the cache is accessed again to retrieve the desired data. The cache is accessed as a direct-mapped cache twice. Because of high prediction accuracy, proposed designs have saved both time and power. Some designs have also been industrialized. Prior work can be categorized by the way the cache is probed.