An Operating Systems Vade Mecum

344 GlossaryProcess. The execution of a program. The kernel causes the process to run by switchingcontext to it. The process submits service calls to request resources from the kernel.Also called task or job.Process interface. The interface between a process and the kernel, consisting of servicecalls and their responses.Processor. A device able to execute computer instructions. Most computers have justone processor; those with more are called multiprocessors. Also called central processingunit.Processor sharing. A theoretical scheduling policy in which every process gets anidentical fraction of the computing resources. Equivalently, each process has its own virtualprocessor that runs at a varying fraction of the speed of the physical processor.Processor state. A hardware state that determines which instructions are currently legaland which address-translation tables are currently in use. Some machines have only twostates, privileged and non-privileged; others have more.Processor-synchronous. A class of mutual-exclusion and synchronization methods thatwork by making a processor uninterruptible. (Opposite of Store-synchronous.)Process switch. The actions taken by the kernel to cause a different process to run thenext time the context switches from kernel to process. (See Context switch.)Producer-consumer. A situation in which one set of activities creates data and anotherset of activities uses those data. (See Bounded buffer.)Program. A set of instructions packaged into a form that can be run by a process.Program counter. A hardware register that indicates the address of the currently executinginstruction.Programmable clock. A device that can be set to generate an interrupt after an arbitraryinterval.Progress diagram. An n -dimensional picture where dimension i refers to virtual timein process i . A path through that diagram starting at the origin shows how thoseprocesses are scheduled. Some regions of the diagram may be forbidden because ofresource conflicts between processes.Protection. Preventing a subject from modifying objects it should not modify. Forexample, preventing a process from modifying the virtual store of another process orpreventing a user from writing into another user’s file. (See Security.)Protocol. A set of conventions that governs the co-operation between two activities. Itincludes a specification of the kinds of data that are transmitted between them and whatthose data mean.Queueing network. A mathematical description of a set of queues, servers, andprocesses that can be used to derive quantities such as expected penalty ratio.Read-ahead. An eager-evaluation policy that prefetches data blocks from serial devicesinto a main-store cache before any process requests them. Also called prefetching andanticipatory buffering.