An Operating Systems Vade Mecum

338 GlossaryHint. Usually but not necessary true and up to date. For example, the name of themachine where a process resides may be cached as a hint on other machines but notupdated if the process moves. (See Absolute.)History. (1) A log of past commands recorded so that they can be modified and resubmitted.(2) A log of past events recorded for debugging or accounting purposes.Hit ratio. The percentage of time that accesses to a cache result in a cache hit.Hysteresis Principle. Resist change. In particular, when data are brought into a cache,leave them there for a while before archiving them again.Idle time. Time during which no process is ready to run.Icon. A graphical representation of an object or abstraction on a bit-mapped display.Implicit. Implicit process information is gathered by the operating system while it is servicingthe process.Indirect file. A file that contains the full name of another file. Opening the indirect filehas the effect of opening the file it names. Also called a symbolic link.Inheriting. When a process splits into two or creates a new process, we say that the newprocesses inherit from the old one if they start with the same contents of virtual space,open files, or other resources.Initialization file. A file that contains customized settings for a program. The programreads this file during initialization.Input port. A communication port with one reader and many writers.Intentions list. A list of operations that must be executed to finish a transaction. Intentionslists are usually written in stable storage.Inter-file gap. An empty region on a magnetic tape that separates one file from another.Inter-record gap. An empty region on a magnetic tape that separates one record fromanother.Interactive. A style for using a computer in which users type in commands and seeresponses on a display. New commands can therefore be based on the results of previouscommands. Interactive computing is often found in conjunction with monoprogrammingon single-user microcomputers and multiprogramming on larger machines.Internal waste. See Waste.Interrupt. A hardware event that signals some external condition, such as completion oftransput. Interrupts cause a context switch to the kernel.Intrinsic. Intrinsic properties of a process are characteristics that distinguish one processfrom another, such as service-time requirements, storage needs, and amount of transputrequired.Invariant expression. A syntax for specifying synchronization information.Invocation. Turning a program that is stored in a load image into a running process.