2864	 Characteristics of Program Localities	 The term locality has been used to denote that subset of a program s segments which are referenced during a particular phase of its execution. A program s behavior can be characterized in terms of its residence in localities of various sizes and lifetimes and the transitions between these localities. In this paper the concept of a locality is made more explicit through a formal definition of what constitutes a phase of localized reference behavior and by a corresponding mechanism for the detection of localities in actual reference strings. This definition provides for the existence of a hierarchy of localities at any given time and the reasonableness of the definition is supported by examples taken from actual programs. Empirical data from a sample of production Algol programs is used to display distributions of locality sizes and lifetimes and these results are discussed in terms of their implications for the modeling of program behavior and memory management in virtual memory systems. program behavior memory management locality
