Replacing functional units of an extensible processor with reconfigurable fnctional units enhances performance and flexibility ofprocessors to execute custom instructions. That is due to the ability ofreconfigurable fnctional units to perform computations in hardware to increase performance, while retaining much of the flexibility of a software solution. In this paper, we develop a heterogeneous architecture for the reconfigurable fnctional unit of an extensible processor. To verify the efficiency of our architecture, we applied it to 8 applications of Mibench. Our experiments show that compared to the similar architectures, ours supports a wide range of custom instructions. In addition, use of the new architecture improves execution time of custom instructions by 20% to 30% on average. Moreover, compared with the previous architecture, area is reduced by 15%.