On the functional test of Branch Prediction Units based on Branch History Table

Branch Prediction Units (BPUs) are highly efficient modules that can significantly decrease the negative impact of branches in superscalar and RISC processors. Traditional test solutions, mainly based on scan test, are often inadequate to tackle the complexity of these architectures, especially when dealing with delay faults that require at-speed stimuli application. Moreover, scan test does not represent a viable solution when Incoming Inspection or on-line test are considered. In this paper a functional approach targeting BPU test is proposed, allowing to generate a suitable test program whose effectiveness is independent on the specific implementation of the BPU. The effectiveness of the approach is validated on a Branch History Table (BHT) resorting to an open-source computer architecture simulator and to an ad hoc developed HDL testbench. Experimental results show that the proposed method is able to thoroughly test the BHT, reaching complete static fault coverage.