Information flow metrics for the evaluation of operating systems' structure

ion. Lower levels are not aware of the resources used at higher levels» Levels of abstraction are used in the resource management to provide users with virtual devices (Liskov, 1972b). In addition to the work described above in implementing specific hierarchies, research has been active in exploring various concepts used in the design of hierarchically-structured systems. These concepts in­ clude the notions of a module, a program family, kernels, system struc­ turing, specification techniques, and language features. Each are dis­ cussed, in turn, below. The term module has been used extensively in the literature with various definitions. Even without a specific definition for module, the concept of modularization has been generally accepted as a necessary ingredient of good structure. Modularization implies a system which: 1) allows internal module design decisions to be made independently of other modules; 2) increases comprehensibility since a module may be under­ stood in terms of its effects and not its implementation; the system may be understood one module at a time; 3) creates system stability since each module can be tested and debugged independently of other modules ; and 4) increases system flexibility because the implementation of one module can be drastically changed without affecting other modules. These benefits imply greater ease in detailed design, and an increase in speed of implementation, more adequate testing and debugging, and less costly maintenance. These all, in turn, are used to enhance reliability. Pa ma s (1972a) pursues three issues with regard to module design structures. First, the criteria used in decomposing systems modules