Access to embedded processor cores for application of test has greatly complicated the testability of large systems on silicon. Scan based testing methods cannot be applied to processor cores which cannot be modified to meet the design requirements for scan insertion. Instruction Randomization Self Test (IRST) achieves stuck-at-fault coverage for an embedded processor core without the need for scan insertion or mux isolation for application of test patterns. This is a new built-in self test method which combines the execution of microprocessor instructions with a small amount of on-chip test hardware which is used to randomize those instructions. IRST is well suited for meeting the challenges of testing ASIC systems which contain embedded processor cores.