A Class Hierarchy for Building Stream - Oriented

This paper describes the object-oriented design and implementation of a family of stream-oriented le systems under UNIX and under an object-oriented operating system called Choices. A class hierarchy provides an object-oriented taxonomy of the algorithms and data structures used in the design of this family. The family includes the System V le system, the 4.2 BSD le system, and the MS-DOS le system. The class hierarchy has been developed by a series of experiments that are designed to lead to a framework for object-oriented le systems. The class hierarchy for stream-oriented le systems is the product of the second experiment in this series in which we revised a class hierarchy for UNIX-like le systems[MLRC88] to include the MS-DOS le system. We describe the hierarchy, how it evolved from the rst experiment to the second, and review the lessons that we have learned from the two experiments. 1 INTRODUCTION The Choices operating system architecture [CJR87, CRJ87, RJC88] is motivated by the diculties of building portable and extensible operating systems for high-performance multiprocessor and uniprocessor computers. The solution we adopt to the organizational problems inherent in such systems is to design Choices as an object-oriented system. In addition, Choices provides application programs with an object-oriented system interface. 2 Associated with the operating system is an extensive class hierarchy that denes the interfaces and components of the system [RJC88, RC89]. Choices presents an object-oriented environment to applications. Object method invo-cation is used to invoke both the operating system services and the services provided by \server objects" running as applications on the system. Choices provides secure method invocation on server objects by using virtual memory protection mechanisms 2 Choices is implemented in the C++[Str86] programming language. C++ provides a suciently ecient implementation of classes and inheritance to support operating system construction.