Computer designers and users have felt the need for monitoring the behavior of their systems since the earliest days of the computer era. However, only very few and recent systems have been adequately instrumented by their designers during the design stage (see for example reference 1). This situation has led to the development of hardware and software measurement tools having the common property of requiring relatively minor or no modifications of the system to be monitored. The applicability of these tools in certain measurement problems is somewhat limited, since on one hand the system was not specifically designed to be measurable, and on the other hand the tool was designed to be applicable to a class of systems, configurations, or versions, and its measurement power with respect to a particular system may have been sacrificed to its generality.