White box testing or Black box testing

When performing verification, the target device can be viewed as a white box or a black box. During whitebox testing, detailed knowledge is available describing the internal workings of the device to be tested. This knowledge can be used to direct the verification effort. Forexample, an engineer verifying a digital circuit may have schematics, block diagrams, RTL code that may or may not be suitably annotated, and textual descriptions including timing diagrams and state transition graphs. All or a subset of these can be used to advantage when developing test programs. The logic designer responsible for the correctness of the design, armed with knowledge of the internal workings of the design, writes stimuli based on this knowledge; hence he or she is performing white-box testing.

During black-box testing, it is assumed that there is no visibility into the internal workings of the device being tested. A functional description exists which outlines, in more or less detail, how the device must respond to various externally applied stimuli. This description, or specification, may or may not describe behavior of the device in the presence of all possible combinations of inputs. For example, a microprocessor may have op-code combinations that are left unused and unspecified. From one release to the next, these unused op-codes may respond very differently if invoked. PCB designers, concerned with obtaining ICs that work correctly with other ICs plugged into the same PCB or backplane, are most likely to perform black-box testing, unless they are able to persuade their vendor to provide them with more detailed information.