Energy-efficient data prefetch buffering for low-end embedded processors

An energy-efficient architecture should jointly optimize energy consumption and throughput, as captured by the Energy-Delay-Square Product (ED2P) metric. This paper introduces a prefetch data buffer micro-architecture, which achieves that goal with the aid of software-inserted control words to govern the prefetch process. The proposed architecture is aimed at low-end embedded processors, which, so as to reduce energy consumption, lack a cache-based memory hierarchy. By identifying after compilation which data should be prefetched and modifying the object code, the rate of prefetch misses is reduced. And by pre-computing memory addresses using auxiliary software after compilation and modifying the object code, address computation by hardware at run time is avoided, reducing pipeline stalls and, thus, improving throughput. Additionally in the case of branches, by prefetching two data items at any one time, alternative instruction outcomes are anticipated. The paper contains results from running a range of well-known and representative benchmarks on the proposed architecture. There was an improvement of 620% compared to an unbuffered architecture in execution times when tested over those seven benchmarks. Furthermore, the average ED2P for the buffered architecture when normalized against the same architecture without buffering was found to vary between 54% and 90% according to benchmarking, though there is a cost in code size increase. That is to say, for the benchmarks tested there was a net energy efficiency improvement of between 10% and 46% in comparison with the equivalent unbuffered architecture with a lower area overhead.