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Computer system performance depends on high access rate and low miss rate in the instruction cache, which also affects energy consumed by fetching instructions. Simulation of a small computer typical for embedded systems shows that up to 20% of the overall processor energy is consumed in the instruction fetch path and as much as 23% of the execution time is spent on instruction fetch. Therefore it is of key importance to reduce the energy dissipated during instruction fetch. One way to increase the instruction memory bandwidth is to fetch more instructions in each access without increasing the bus width. We propose an extension to a RISC ISA, with variable length instructions, yielding higher information density without compromising programmability. Based on profiling of dynamic instruction usage and argument locality of a set of SPEC CPU2000 applications, we present a scheme using 816- and 24-bit instructions accompanied by lookup tables inside the processor Our scheme yields a 20-30% reduction in main memory usage, and experiments show that up to 60% of all executed instructions consist of short instructions. The overall energy savings are up to 15% for the entire data path and memory system, and up to 20% in the instruction fetch path.
Date of Conference: 17-20 Sept. 2003