An Operating Systems Vade Mecum

Glossary 331Bound port. A communication port with one writer and one reader.Bounds. A hardware register indicating the length of a segment.Broadcast. A situation in which many recipients can see the same message.Buffer. A region in main store used to hold data that are about to be output to a deviceor that have recently come in from a device.Buffer Principle. The purpose of a buffer pool is to smooth out short-term variations inspeeds between consumers and producers. This smoothing prevents needless blocking.Bulletin board. A program that accepts notices and distributes them to interested users.Busy waiting. Polling in a tight loop.Byte stuffing. Use of an escape convention to avoid transmitting a header byte byaccident.Cache. (1) A place where data can be accessed quickly. (2) To transfer data from anarchive to a cache. (See Archive.)Cache hit. The situation in which a cache is accessed and is found to contain the desiredinformation.Cache miss. The situation in which a cache is accessed but does not contain the desiredinformation.Cache Principle. The more frequently data are accessed, the faster the access should be.Capability. An entity owned by a process that gives the process certain rights to accessa particular object.Capability list. A list of all the capabilities owned by a process.Central processing unit (cpu). The part of the hardware of a computer that executesmachine instructions. It communicates with main store and with devices.Chaining. See Automatic chaining and Command chaining.Channel. A subsidiary computer whose programs, called channel programs, are kept inmain store. Channels govern transput for a set of devices.Characteristic number. The area under a curve in a fault-rate graph.Character-oriented. A protocol that sends messages 1 character (typically 8 bits) at atime. (Opposite of Bit-oriented.)Cheap-process philosophy. Using a new process for each stage in a computation.Checkpoint. A copy made of a process so that it can be restarted later from that samepoint if there is a hardware failure that prevents it from completing normally.Checksum. A bit pattern derived from a chunk of data, usually by exclusive-or operations,appended to that chunk so that errors in storing or transmitting that chunk of datacan be detected.Child. A subnode in a tree, whether the tree is composed of files, processes, or otherentities. (Opposite of Parent.)