Partial Address Directory for Cache Access

Abstract

In most high performance computers the speeds of cache accessing are critical in determining the cycle times. A classical method for designing set-associative caches is to late-select array data based on the results of cache directory lookups. The impact on the critical path timing due to late-select will become more significant in future microprocessors with very high clock frequencies. In this paper we propose a new approach to the optimization of array access timing for set-associative caches. The basic idea is to utilize a relatively small partial address directory (PAD) for fast and accurate approximations of cache access coordinates. The PAD can speed up most cache array access by accurately predicting cache locations without having to wait for results from conventional cache directory lookups. Occasionally when the PAD guesses wrong, a memory access can be re-issued with only 1-cycle delays. The PAD may be closely integrated with the array design. The effectiveness of the PAD method is analyzed through combinatorial and simulation studies. © 1994 IEEE