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Many commercially available embedded processors are capable of extending their base instruction set for a specific domain of applications. While steady progress has been made in the tools and methodologies of automatic instruction set extension for configurable processors, recent study has shown that the limited data bandwidth available in the core processor (e.g., the number of simultaneous accesses to the register file) becomes a serious performance bottleneck. In this paper, we propose a new low-cost architectural extension and associated compilation techniques to address the data bandwidth problem. Specifically, we embed a single control bit in the instruction op-codes to selectively copy the execution results to a set of hash-mapped shadow registers in the write-back stage. This can efficiently reduce the communication overhead due to data transfers between the core processor and the custom logic. We also present a novel simultaneous global shadow register binding with a hash function generation algorithm to take full advantage of the extension. The application of our approach leads to a nearly-optimal performance speedup (within 2% of the ideal speedup).