In this paper, the authors present a Reconfigurable Functional Unit (RFU) for an adaptive dynamic extensible processor. The processor can tune its extended instructions to the target applications, after chip-fabrication. The Custom Instructions (CIs) are generated deploying the hot basic blocks during the training mode. In the normal mode, CIs are executed on the RFU. A quantitative approach was used for designing the RFU. The RFU is a matrix of functional units with 8 inputs and 6 outputs. Performance is enhanced up to 1.25 using the proposed RFU for 22 applications of Mibench.