An Operating Systems Vade Mecum

342 GlossaryOpen shop. A style for using a computer in which users sign up for a block of time, typically15 or 30 minutes. During that time, the user has complete control of the machine.Optional utility. A program that is distributed as part of the operating system softwarebut is not needed by every user. (See Essential utility and Kernel.)Output port. A communication port with one writer and many readers.Overflow file descriptor. A block on the disk that holds disk-block pointers that did notfit in the file descriptor.Overhead space. See Waste.Overhead time. Time needed to perform the computations needed for a policy.Overlay. A technique whereby the same region of virtual store is used for different purposesat different times.Overloading. Use of one object for two purposes. The object could be a field of arecord, the name of a subroutine, an error message, or anything else. Which purpose isintended depends on the context in which the object is used.Overrun. An error that occurs in direct-memory-access transfers if main store cannotsupply the next byte to write to the device when it is needed, or if it cannot store the nextbyte from the device before that byte has been discarded to make room for more data.Page. A region of virtual store. The page size is determined by the hardware (and occasionallyby the operating system) and is a constant over all pages.Page fault. A trap generated by the hardware when a page absent from main store isaccessed by a program.Page frame. A region of physical store exactly long enough to hold one page.Page reference string. A list of the pages referenced by a process during the course ofexecution.Page-replacement policy. A policy that decides which page to swap out to make roomfor a page that must be swapped in.Page table. The name of the address translation table when paging is used.Parameter. (1) An adjustable quantity that tunes a distribution to fit the data or todescribe the situation of interest. (2) An argument to a procedure. (3) An argument to acommand in command language. (See Flag.)Parent. A direct ancestor in a tree, whether the tree is composed of files, processes, orother entities. The parent of a process is the one that submitted a service call leading tothe creation of that process. (Opposite of Child.)Parity. The count of some event, modulo 2. A parity bit is an extra bit on a word that isset or cleared to make the number of one bits in the word 0 or 1 mod 2 (that is, even orodd).Partial deletion. A file is partially deleted if it is no longer directly accessible but can berecovered without much effort. (See Delete.)