Testing of digital systems has passed through many different approaches over time; among the most common self testing approaches are built-in self-test (BIST) targeting structural faults, and software-based self-testing (SBST) targeting functional faults.

BIST doesn't need an external tester, and proved good fault coverage, but it requires some design changes which consumes time, money, and power. Furthermore, it places the device under test (DUT) in a non-functional testing mode.

On the other hand, SBST tests a DUT online in the functional mode. But as the complexity of modern microprocessor increases, number of internal states is exploding, and testing them all is impossible.

A promising testing approach that has the advantages of the above two approaches is a software-based self-testing technique that targets structural faults. This approach is based on imposing some constraints on the DUT's interfaces, so that an ATPG tool can be utilized to generate test patterns that can be expressed as functional instructions.

The proposed method is described and applied on the floating-point units of two SUN's UltraSPARC processors which each of them is considered a true system-on-a-chip.