Achieving full functional coverage for the forwarding unit of pipelined processors

Generic instruction based testing methods do not always give good fault coverage for the complex units like Forwarding unit. Hence it becomes important to carefully craft the test which are best for different parts of these units. In this paper we have identified two different test generation methods for different sections of the forwarding unit, namely muxes and comparators which offers better coverage independently: template based test instruction generation and test instruction generation from constrained test vectors. A wrapper designed using the functional constraints of the forwarding unit, is used for test generation. Both methods combined delivers fault coverage of 97.33%. A detailed analysis of the non-detected faults revealed the fact that they cannot be tested using the instruction set of the processor (MIPS 5-stage pipelined processor). This proves that the proposed method achieved 100% functional coverage for stuck-at faults in the forwarding unit.