Opal threads execute within protection domains in a single shared virtual address space.
Sharing is simplified, because addresses are context independent. We have designed and implemented an Opal prototype using the Mach 3.0 microkernel as a base. This article justifies the Opal model and its goals for sharing and protection, presents the system and its abstractions, describes the prototype implementation, and reports experience with integrated applications. The Digital Library is published by the Association for Computing Machinery.