This paper presents 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