부산대학교 도서관

Looking forward to the next generation of mobile streaming computing, the demanded energy efficiency of end-user terminals will become ever stringent. The Xetal-Pro processor, which is the latest member of the Xetal low-power single-instruction multiple data (SIMD) processor family from Philips, is presented in this paper. The predecessor of Xetal-Pro, known as Xetal-II, already ranks as one of the most computational-efficient [in terms of giga operations per second (GOPS)/Watt] processors available today, however, it cannot yet achieve the demanded energy efficiency (less than 1 pJ per operation). Unlike Xetal-II, Xetal-Pro supports ultrawide supply voltage $(V_{dd})$ scaling from the nominal supply to the subthreshold region. Although aggressive $V_{dd}$ scaling causes severe throughput degradation, this can be partly compensated for by the massive parallelism in the Xetal family. Xetal-II includes a large on-chip frame memory (FM), which cannot be scaled well to an ultralow $V_{dd}$ hence creating a big obstacle to increase energy efficiency. Therefore, we investigate both different FM realizations and memory organization alternatives. A hybrid memory system (HMS), which reduces the non-local memory traffic and enables further $V_{dd}$ scaling, is proposed. For design space exploration of the right number of the scratchpad memory (SM) entries, the corresponding data locality analysis is provided, too. Moreover, some unique circuit implementation issues of Xetal-Pro such as the customized level-shifter are also discussed. Compared to Xetal-II operating at the nominal voltage, Xetal-Pro provides up to two times energy efficiency improvement even without $V_{dd}$ scaling (essentially a consequence of data localization in the SM) when delivering the same amount of ultrahigh throughput. With $V_{dd}$ scaling into the sub/near threshold region, Xetal-Pro could gain more than ten times energy reduction while still delivering a high throughput of 0.69 GOPS (counting multiply and add operations only). The new insight of Xetal-Pro sheds light on the direction of future ultralow-energy SIMD processors.