Power Savings in Embedded Processors through Decode Filter Cache

In embedded processors, instruction fetch and decode can consume more than 40% of processor power. An instruction filter cache can be placed between the CPU core and the instruction cache to service the instruction stream. Power savings in instruction fetch result from accesses to a small cache. In this paper, we introduce decode filter cache to provide decoded instruction stream. On a hit in the decode filter cache, fetching from the instruction cache and the subsequent decoding is eliminated, which results in power savings in both instruction fetch and instruction decode. We propose to classify instructions into cacheable or uncacheable depending on the decoded width. Then sectored cache design is used in the decode filter cache so that cacheable and uncacheable instructions can coexist in a decode filter cache sector. Finally, a prediction mechanism is presented to reduce the decode filter cache miss penalty. Experimental results show average 34% processor power reduction and less than 1% performance degradation.