Improving efficiency of extensible processors by using approximate custom instructions

In this paper, we propose to move the conventional extensible processor design flow to the approximate computing domain to gain more speedup. In this domain, the instruction set architecture (ISA) design flow selects both exact and approximate custom instructions (CIs). The proposed approach could be used for the applications where imprecise results may be tolerated. In the CI identification phase of the flow, the CIs which do not satisfy the maximum propagation delay but can provide approximate results also may be included in the CI candidate set. Next, in the selection phase, we propose a merit function which selects CIs with higher cycle savings and small error rates. The efficacy of the proposed approximate design flow is investigated using the case studies of the discrete cosine transform (DCT) and inverse DCT (iDCT) of the MPEG2 application. Also, the impact of the process variation on the impreciseness of the results is investigated.