A design framework for hybrid-access caches

High-speed microprocessors need fast on-chip caches in order to keep busy. Direct-mapped caches have better access times than set-associative caches, but poorer miss rates. This has led to several hybrid on-chip caches combining the speed of direct-mapped caches with the hit rates of associative caches. In this paper, we unify these hybrids within a single framework which we call the hybrid access cache (HAC) model. Existing hybrid caches lie near the edges of the HAC design space, leaving the middle untouched. We study a group of caches in this middle region, a group we call half-and-half caches, which are half direct-mapped and half set-associative. Simulations confirm the predictive valve of the HAC model, and demonstrate that, for medium to large caches, this middle region yields more efficient cache designs.<>