This paper describes the methodological results of a research project involving the security characteristics of a large-scale computer operating system. The research involved an in-depth analysis of a complex subsystem, and the preparation of diagrammatic, as well as textual, results. To achieve diagrammatic representation, the authors developed and implemented the Focused Flow Diagram (FFD), defined as a terse, unambiguous scheme for easily representing selected aspects of processing in a generally familiar format. Some observations of the FFD methodology are: (1) FFDs work better than flowcharts in an analytical situation because they have fewer symbols to remember and explicitly provide for the inclusion of textual amplification to the diagram. (2) The FFD representation is easy to read and has a relatively high data rate, easing final documentation of results. (3) Similarity to flowcharts means only a small investment in new learning. 15 pp.
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