Книга: Distributed operating systems



Like Amoeba and Mach, Chorus is a microkernel-based operating system for use in distributed systems. It provides binary compatibility with System V UNIX, support for real-time applications, and object-oriented programming.

Chorus consists of three conceptual layers: the kernel layer, the subsystems, and the user processes. The kernel layer contains the microkernel proper, as well as some kernel processes that run in kernel mode and share the microkernel's address space. The middle layer contains the subsystems, which are used to build provide operating system support to user programs, which reside in the top layer.

The microkernel provides six key abstractions: processes, threads, regions, messages, ports, port groups, and unique identifiers. Processes provide a way to collect and manage resources. Threads are the active entities in the system, and are scheduled by the kernel using a priority-based scheduler. Regions are areas of virtual address space that can have segments mapped into them. Ports are buffers used to hold incoming messages not yet read. Unique identifiers are binary names used to identifying resources.

The microkernel and subsystems together provide three additional constructs: capabilities, protection identifiers, and segments. The first two are used to name and protect subsystem resources. The third is the basis of memory allocation, both within a running process and on disk.

Two subsystems were described in this chapter. The UNIX subsystem consists of the process, object, streams, and interprocess communication managers, which work together to provide binary compatible UNIX emulation. The COOL subsystem provides support for object-oriented programming.

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