An Operating Systems Vade Mecum

Glossary 345Readers-writers problem. A situation in which any writer conflicts with all otherreaders and writers.Read/write mark. A logical position in a file that indicates where the next data transferwill occur.Realizable. An allocation state is realizable if it makes sense. For example, the numberof resources currently in use cannot exceed the total number of resources.Record. (1) A chunk of data stored on magnetic tape, delimited by inter-record gaps.(2) A chunk of data stored in a file and structured by fields.Reduction. A technique to manipulate a waits-for graph to detect deadlock.Redundancy. Using more bits than necessary to store or transmit data in order to providereliability.Reflecting. A virtualizing kernel receives traps when its clients perform privilegedinstructions and interrupts when devices need service. If the kernel causes one of its virtualmachines to see this trap or interrupt, we say the trap or interrupt has been reflected.Region. A set of instructions that must be executed excluding some other activity. (SeeMutual exclusion.)Register. A hardware-supplied fast region of store. Many computers have addressingmodes that use registers. Not to be confused with Device register.Relative. With respect to a standard reference point. For example, a relative file nameindicates where a file lies with respect to the current working directory. (See Absolute.)Remote job entry. Submitting a job from one computer to another, with the resultstransmitted back to the first computer.Remote logon. Logging on to a computer over a network. (See Logon.)Remote mounting. Placing the contents of a directory as a virtual subdirectory onanother machine. (See Mounting.)Resident monitor. A simple operating system that starts the next process when the previousone finishes.Resource class. A kind of resource (like a tape drive) for which the individual resourcesare indistinguishable, or at least the differences are immaterial to the requesting process.Resource graph. A graph of processes and resources with arcs from processes toresources they are waiting for and from resources to processes that are using them.Resource Principle. An operating system is a collection of algorithms that allocateresources to processes.Response ratio. The ratio of required time to response time for a given process,represented by R. R is always less than or equal to 1. (See Penalty ratio.)Response time. The elapsed time between the instant a process arrives in the readyqueue and the time it leaves that queue. Also called turnaround time, especially in abatch environment.