Implementing a cache for a high-performance GaAs microprocessor

Abstract

In the near future, microprocessor systems with very high clock rates will use multichip module (MCM) pack- aging technology to reduce chip-crossing delays. In this paper we present the results of a study for the design of a 250 MHz Gallium Arsenide (GaAs) microprocessor t,lrat employs h4CM technology to improve performance. The design study for the resulting two-level split cache st.arts with a baseline cache architecture and then ex- amines the following aspects: 1) primary cache size and degree of associativity; 2) primary data-cache write pol- icy; 3) secondary cache size and organization; 4) pri- mary cache fetch size; 5) concurrency between instruc- tion and data accesses. A trace-driven simulator is used to analyze each design's performance. The results show that memory access time and page-size constraints ef- Cectively limit the size of the primary data and instruc- tion caches to 4I<W (16KB). For such cache sizes, a write-through policy is better than a write-back policy. Three cache mechanisms that contribute to improved performance are introduced. The first is a variant of the write-through policy called write-only. This write policy provides most of the performance benefits of sub- Ilod placernenl without extra valid bits. The second, is the use of a split secondary cache. Finally, the third mechanism allows loads to pass stores without associa- tive matching. Keywords-two-level caches, high performance pro- cessors, gallium arsenide, multichip modules, trace- driven cache simulation.