MEGA-MAC: A Merged Accumulation based Approximate MAC Unit for Error Resilient Applications

Abstract

This paper proposes a novel merged-accumulation-based approximate MAC (multiply-accumulate) unit, MEGA-MAC, for accelerating error-resilient applications. MEGA-MAC utilizes a novel rearrangement and compression strategy in the multiplication stage and a novel approximate "carry predicting adder" (CPA) in the accumulation stage. Addition and multiplication operations are merged, which reduces the delay. MEGA-MAC provides knobs to exercise a tradeoff between accuracy and resource overhead. Compared to the accurate MAC unit, MEGA-MAC(8,6) (i.e., a MEGA-MAC unit with a chunk size of 6 bits, operating on 8-bit input operands) reduces the power-delay-product (PDP) by 49.4%, while incurring a mean error percentage of only 4.2%. Compared to state-of-art approximate MAC units, MEGA-MAC achieves a better balance between resource-saving and accuracy-loss. The source code is available at https://sites.google.com/view/mega-mac-approximate-mac-unit/.