On inherent untestability of unaugmented microprogrammed control

Effective and efficient testing of the control part of a processor has remained a difficult problem. While several approaches have been proposed in the literature for handling unaugmented control parts, they involve questionable assumptions, and the results have not been encouraging. Here it is shown that unless some DFT (Design for Testability) approaches are taken, microprogrammed control is inherently a poorly testable structure. The considerations include lack of an elegant fault model, presence of components with low random testability, the length of a checking sequence and information-theoretic considerations. The design approaches must therefore include DFT augmentations and/or removal of sub-functional logic.