Operating System Control Structures
If the operating system is to manage processes and resources, then it must have information about the current status of each process and resource. The universal approach to providing this information is: The operating system constructs and maintains tables of information about each entity that it is managing.
Operating system maintains four functions and it should be capable of supporting four functions they are memory, I/O, file and process. Although the details will differ from one operating system to another, all operating systems maintain information in these four categories.
Memory tables are used to keep the track of both main (real) and secondary (virtual) memory. Some of main memory is reserved for use by the operating system; the remainder is available for use by processes. Processes are maintained on secondary memory by using some sort of virtual memory or simple swapping mechanism. The memory tables must include the following information. The allocation of main memory to processes, the allocation of secondary memory to processes and any protection attributes of segments of main or virtual memory, such as which processes may access certain shared memory regions. Any information needed to manage virtual memory.
I/O tables are used by the operating system to manage the I/O devices and channels of the computer system. At any given time, an I/O device may be available or assigned to a particular process. If an I/O operation is in progress, the operating system needs to know the status of the I/O operation and the location in main memory being used as the source or destination of the I/O transfer.
The operating system may also maintain file tables, which provide information about the existence of files, their location on secondary memory, their current status, and other attributes. Much, if not all, of this information may be maintained and used by a file management system, in which case the operating systems has little or no knowledge of files. In other operating systems much of the detail of file management is managed by the operating system itself.
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Finally, the operating system must maintain process tables to manage process. The remainder of this section is devoted to an examination of the required process tables. Tables must be linked or cross referenced in some fashion. Memory, I/O, and files are managed on behalf of processes, so there must be some direct or indirect reference to these resources in the process tables.
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