An improved testing apparatus and method for testing the kernel of a microprocessor based unit under test (UUT) where connection to the UUT is made at both the memory connection socket and at the microprocessor with the microprocessor being in place and active in the UUT. The apparatus and method permits subtantially full diagnostics of the kernel to be carried out in a systematic and automated manner in which the requirement of manual probing of the UUT is minimized. Connections at the microprocessor permit the development of high resolution sync signals for verification and evaluation of test results and permit testing without latching of signals on the data bus. The testing protocol implemented in the methodincludes the use of testing primitives which permit the development of a signature for each address and data bus line for the identification of the type as well as the location of any faults discovered by the apparatus. The method of testing exploits bootstrapping techniques including three primitives for bus test, data stimulus and address stimulus to optimize simultaneous testing and circuit fault diagnosis.