Failure to do so could cause the aircraft to go away

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Unformatted text preview: only the minimal facilities necessary for implementing additional operating system services. The only services provided by the kernel in this model are low-level device management, a limited amount of low-level process management, and some memory management. All other operating system services, such as file management, additional process and memory management activities, and security are implemented as user-level server process. Each server process has its lown address space and can be programmed separately. Resident and Non-Resident Operating System Modules With all the functionalities of an operating system implemented, it becomes a large software. Obviously, all the functionalities of an operating system are not needed all the time. As the main memory capacity of a system is limited, it is customary to always keep in the system's memory only a very small part of the operating system and to keep its remaining part on an on-line storage device such as hard disk. Those modules of an operating system that are always kept in the system's main memory are called resident modules and those that are kept onjlard disk are called non-resident modules. The non-resident modules are loaded into the memory on demand, that is, as and when they are needed for execution. The system kernel should not be confused with the resident modules of the operating system. The two are not necessarily the same. In fact, for most operating systems they are different. The following two criteria normally determine whether a particular operating system module should be resident: 1. Its frequency of use, and 2. Whether the system can operate at all without it. For example, file directories can be maintained on disk and loaded into the memory when required. Status information for inactive processes can similarly be swapped out on disk. In fact, the resident part of an operating system is a subset of its kernel. OTHER RELATED CONCEPTS Few other concepts related to operating systems are briefly described below. Real-time Operating Systems For many applications, successful processing...
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