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Exokernel is an operating system kernel developed by the MIT Parallel and Distributed Operating Systems group, and also a class of similar operating systems. Operating System Kernels generally present the hardware-resources to applications through high level abstractions such as the (virtual) file-system. The idea behind exokernels is to force as few abstractions as possible on developers, enabling them to make as many decisions as possible about hardware abstractions. Exokernels are tiny, since functionality is limited to ensuring protection and multiplexing of resources, which are vastly simpler than conventional microkernels' implementation of message passing and monolithic kernels' implementation of abstractions. Implemented applications are called library operating systems; they may request specific memory addresses, disk blocks, etc. The kernel only ensures that the requested resource is free, and the application is allowed to access it. This low-level hardware access allows the programmer to implement custom abstractions, and omit unnecessary ones, most commonly to improve a program's performance. It also allows programmers to choose what level of abstraction they want, high, or low. Exokernels can be seen as an application of the end-to-end principle to operating systems, in that they do not force an application program to layer its abstractions on top of other abstractions that were designed with different requirements in mind. For example, in the MIT Exokernel project, the Cheetah web server stores preformatted Internet Protocol packets on the disk, the kernel provides safe access to the disk by preventing unauthorized reading and writing, but how the disk is abstracted is up to the application or the libraries the application uses. ==Motivation== Traditionally kernel designers have sought to make individual hardware resources invisible to application programs by requiring the programs to interact with the hardware via some abstraction model. These models include file systems for disk storage, virtual address spaces for memory, schedulers for task management, and sockets for network communication. These abstractions of the hardware make it easier to write programs in general, but limit performance and stifle experimentation in new abstractions. A security-oriented application might need a file system that does not leave old data on the disk, while a reliability-oriented application might need a file system that keeps such data for failure recovery. One option is to remove the kernel completely and program directly to the hardware, but then the entire machine would be dedicated to the application being written (and, conversely, the entire application codebase would be dedicated to that machine). The exokernel concept is a compromise: let the kernel allocate the basic physical resources of the machine (e.g. disk blocks, memory pages, and processor time) to multiple application programs, and let each program decide what to do with these resources. The program can then link to a support library that implements the abstractions it needs (or it can implement its own). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「exokernel」の詳細全文を読む スポンサード リンク
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