On the Functional Test of the Register Forwarding and Pipeline Interlocking Unit in Pipelined Processors

When the result of a previous instruction is needed in the pipeline before it is available, a “data hazard” occurs. Register Forwarding and Pipeline Interlock (RF&PI) are mechanisms suitable to avoid data corruption and to limit the performance penalty caused by data hazards in pipelined microprocessors. Data hazards handling is part of the microprocessor control logic; its test can hardly be achieved with a functional approach, unless a specific test algorithm is adopted. In this paper we analyze the causes for the low functional testability of the RF&PI logic and propose some techniques able to effectively perform its test. In particular, we describe a strategy to perform Software-Based Self-Test (SBST) on the RF&PI unit. The general structure of the unit is analyzed, a suitable test algorithm is proposed and the strategy to observe the test responses is explained. The method can be exploited for test both at the end of manufacturing and in the operational phase. Feasibility and effectiveness of the proposed approach are demonstrated on both an academic MIPS-like processor and an industrial System-on-Chip based on the Power Architecture™.