Term Definition -------------------------------------------------- 16777152,#FALSE#,0,#FALSE#,"Arial",20,#FALSE# 12648384,#FALSE#,0,#FALSE#,"Arial",20,#FALSE# -------------------------------------------------- "merlin" "",0,0 "",0,0 -------------------------------------------------- SEQ=-1,PRS=-1,TTS=-1,MRK=-1,NUM=192,STR=1,ADV=-1,IN1=1,IN2=1,MSG=N,BOB,1-,2-,3-,4-,5-,6-,7-,8-,9-,10-,11-,12-,13-,14-,15-,16-,17-,18-,19-,20-,21-,22-,23-,24-,25-,26-,27-,28-,29-,30-,31-,32-,33-,34-,35-,36-,37-,38-,39-,40-,41-,42-,43-,44-,45-,46-,47-,48-,49-,50-,51-,52-,53-,54-,55-,56-,57-,58-,59-,60-,61-,62-,63-,64-,65-,66-,67-,68-,69-,70-,71-,72-,73-,74-,75-,76-,77-,78-,79-,80-,81-,82-,83-,84-,85-,86-,87-,88-,89-,90-,91-,92-,93-,94-,95-,96-,97-,98-,99-,100-,101-,102-,103-,104-,105-,106-,107-,108-,109-,110-,111-,112-,113-,114-,115-,116-,117-,118-,119-,120-,121-,122-,123-,124-,125-,126-,127-,128-,129-,130-,131-,132-,133-,134-,135-,136-,137-,138-,139-,140-,141-,142-,143-,144-,145-,146-,147-,148-,149-,150-,151-,152-,153-,154-,155-,156-,157-,158-,159-,160-,161-,162-,163-,164-,165-,166-,167-,168-,169-,170-,171-,172-,173-,174-,175-,176-,177-,178-,179-,180-,181-,182-,183-,184-,185-,186-,187-,188-,189-,190-,191-,192-,EOB -------------------------------------------------- ASCII. A 7 bit code for representing American English characters and device controls. Belady's Anomaly. A counterintuitive effect in which increasing the number of page frames may increase the number of page faults. CPU bound. A property of a process in which processor use is high, and I/O requests are low. Circular LOOK disk scheduling. A disk scheduling strategy which satisfies the pending disk request which is physically closest to the current position of the access arm in the direction toward the inside of the disk, reversing direction and seeking to the outside when it reaches the cylinder of the innermost request, and repeating. Circular SCAN disk scheduling. A disk scheduling strategy which satisfies the pending disk request which is physically closest to the current position of the access arm in the direction toward the inside of the disk, reversing direction and seeking to the outside when it reaches the innermost cylinder, and repeating. DRAM. Semiconductor read-write memory in which individual bit cells must be peridically refreshed. Direct Memory Access. The transfer of data between memory and a peripheral directly, without passing through the CPU. EBCDIC. An 8 bit code for representing characters and device controls. FIFO Page Replacement. A strategy which replaces the page which has been in memory for the longest time. FIFO scheduling. A scheduling policy which schedules tasks in the same order in which they enter the ready state. I/O bound. A property of a process in which processor use is low, and I/O requests are high. LFU Page Replacement. A strategy which replaces the page which has been used least often. LOOK disk scheduling. A disk scheduling strategy which satisfies the pending disk request which is physically closest to the current position of the access arm in the current direction of motion, reversing direction when there are no more requests in the current direction. LRU Page Replacement. A strategy which replaces the page which has not been used for the longest time. NUR Page Replacement. A strategy which replaces a page which has not been used recently. Round Robin scheduling. A scheduling policy which scheduleses tasks in a fixed circular order. SCAN disk scheduling. A disk scheduling strategy which satisfies the pending disk request which is physically closest to the current position of the access arm in the current direction of motion, reversing direction when the first and last cylinders are reached. SRAM. Semiconductor read-write memory in which individual bit cells stable as long as power is maintained. Unicode. A 16 bit code for representing characters. Absolute path. A path beginning at the root directory. Access time. The time from a data transfer request to the completion of the transfer of the data. Anticipatory fetch strategy. A strategy which fetches items which may be requested soon. Anticipatory paging. A paging policy which reads a page into real memory when its future use is predicted. Assembly language. A symbolic language closely related to a machine language. Assembly language. A symbolic language closely related to a machine language. Asynchronous signal. Signal generated for reasons unrelated to the current instruction of the running thread. Atomic operation. An operation which cannot be interrupted. Backup. The creation of redundant copies of data from files. Base register. A register which contains a value which is added to virtual memory addresses to produce a memory address. Batch process. A process which executes without user interaction. Best fit. A placement strategy which selects the smallest space from the free list which is large enough. Binary semaphore. A semaphore that can only have the values 0 and 1. Blocked list. An operating system data structure which contains all of the blocked processes in the system. Blocked state. A process state entered by a task which is waiting for some event or action to occur. Born state. The tread state in which a new thread begins life. Bus. A communication pathway that connects two or more of the major components of a computer system, including CPU, memory, and device controllers. Busy wait. A situation in which a process, while not blocked, continuously checks for a condition which will allow it to proceed. Byte. The smallest addressable part of memory. Cache. Solid state memory used to buffer and store data temporarily. Circular buffer. A fixed capacity buffer which wraps around from one end to the other, simulating a circle. Coalescence. The combination of two (or more) physically adjacent unused spaces into a single unused space. Compiler. A translator from a high level language to a low level language. Compiler language. A high level language which is to be processed by a compiler. Consumer. A process or thread that removes data from a shared object and processes it. Context switch. Action performed by the operating system to remove a process from the run state and replace it with another. Contiguous memory allocation. Allocation in which the address space is continuously connected without breaks. Counting semaphore. A semaphore that maintains a count between zero and some maximum value. Critical section. A section of code which performs operations on a shared resources. Cylinder. The set of tracks of a disk drive which can be accessed without changing the position of the access arm. Data compression. The encoding of data in such a way as to reduce its size. Dead state. The state of a thread after it has been terminated. Deadline scheduling. A scheduling policy which schedules tasks based on the timing needs of the tasks. Deadlock. A situation in which two or more processes are prevented from continuing while each waits for resources to be freed by the continuation of the other. Deadlock. A situation in which two or more processes are prevented from continuing while each waits for resources to be freed by the continuation of the other. Demand fetch strategy. A strategy which fetches items only when requested. Demand paging. A paging policy which reads in a page only when it is referenced. Device independence. An operating system characteristic in which devices are handled as virtual files by processes. Directory. A data structure in a file system which maps to names to file system objects such as files or other directories. Dispatcher. The operating system component which transitions a process to the running state. Dispatcher. The operating system component which transitions a process to the running state. Dynamic address translation. The real time conversion of virtual addresses to real addresses during execution. Dynamic priority. A priority which is adjusted automatically by the system according to task behavior and system loading. Earliest deadline first scheduling. A deadline sheduling policy which dispatches the task with the earliest completion deadline. Extent. A section of a file occupying logically contiguous clusters. Fair share scheduling. A scheduling policy which assigns tasks to groups, and allocates a percentage of CPU time to each group. Fetch strategy. The method used to determine which block is to be obtained next. Field. A single attribute of the entity which the record describes. File. A data structure on secondary storage which acts as a non-volatile container for data. File allocation table. A table on a disk volume containing chained lists of the physical locations of all files on the volume. Firmware. Software stored in nonvolatile memory such as ROM. First fit. A placement strategy which selects the first space on the free list which is large enough. First-come-first-served disk scheduling. A disk scheduling strategy which satisfies disk requests in the same order in which they are received. Flat directory structure. A directory structure having a single directory. Fork. A system call which creates a second (child) process identical to the calling (parent) process Hard link. A directory entry which is the location of a file. Hard real time scheduling. A scheduling policy which ensures that deadlines are met. Hierarchical file system. A file system in which a directory can logically contain other directories. High level language. A language that conceptually closer the problems to be solved than to the hardware on which it runs. Indefinite postponement. A situation in which a process or thread is waiting for an event which may never occur. Information hiding. A program style which which masks implementation details of a module from the rest of the code. Interactive process. A process which requires user interaction while executing. Interpreter. A program expressed in one language which executes programs expressed in another language. Interrupt. An asynchronous service request from hardware or software to the CPU. Interrupt handler. A software routine which processes interrupts. Join. Thread operation in which the calling thread is blocked until the thread it joins terminates. Kernel-level thread. A thread created by an operating system. Laxity. The difference between the time until a tasks completion deadline and its remaining processing time requirement. Limit register. A register indicating the highest accessible memory address. Linker. A program which combines two or more object modules into a single object module or into an executable file. Loader. A program which loads a program into main memory in executable form. Logical record. A set of data which is treated as a unit by a program. Long term schedule. The part of an operating system which places new tasks into the ready state. Low level language. A machine language or assembly language Machine language. A programming language which can be directly executed by a machine. Memory management unit. The hardware of a computer which performs dynamic address translation. Message. A unit of data sent by one task or thread that is guaranteed to be delivered atomically to another task or thread. Metadata. Data which describes the data in a file or table. Microcode. Computer code which implements the machine language of a computer. Middleware. Software that sits between two or more types of software and translates information between them. Minimum laxity first scheduling. A deadline scheduling policy which dispatches the task with the least laxity. Modified bit. A bit in a page table entry or a segment descriptor which is set when a memory address in the page or segment is written. Monitor. A synchronization mechanism which encapsulates the representation of a shared resource and provides operations that are the only way of manipulating it. Multithreading. The incorporation of multiple threads of execution within a process. Mutual exclusion. A situation in which at most one process can be engaged in a specified activity at a time. Mutual exclusion primitive. A fundamental operation which is needed to implement mutual exclusion. Noncontiguous memory allocation. Allocation in which the address space is separated into nonadjacent regions. Nonpreemptive. Not having the capability of preempting running tasks. Nonvolatile. Having the ability to retain information in the absence of power. Object module. A file containing machine language code and data in a form that the linker can use to create an executable program or shared library. Object-oriented programming. A style of programming that defines data as objects with attributes and methods that are applied to those objects, and which can be inherited by other objects. Operating system. Software that manages system resources to provide services that allow applications to execute properly. Optimal page replacement. A strategy which replaces the page which will not be used for the longest time. Page. A fixed size block of contiguous memory addresses in a virtual address space which is managed as a unit. Page fault. An interrupt generated when a program references a page that is not currently resident. Page fetch policy. The policy which determines when a memory page should be brought into real memory. Page frame. A fixed size block of contiguous memory addresses in a real address space which is managed as a unit. Page replacement policy. The policy which determines which memory page is to be removed from real memory. Paging. Virtual memory based on fixed size blocks. Parity. A property of a data word or character which is even if the number of 1s in the data is even, and odd if the number of 1s in the data is odd. Path. A series of directories and a filename which uniquely identify a file in a hierarchical file system. Pending signal. A signal which has not yet been delivered. Physical record. A set of data which can be read or written as a unit by a storage device. Placement strategy. The method used to determine where to put a new block. Polling. A technique which examines external device interfaces to determine if the device needs attention. Preemption. The operating system act of interrupting a running task, removing it from the run state, and placing it in the ready state. Preemptive. Having the capability of preempting running tasks. Priority. An ordinal number which represents the relative importance of a task. Priority aging. A process which dynamically raises the priority of a task according to the time it has been waiting in the ready queue. Priority scheduling. A scheduling policy which schedules tasks in descending order of priority. Process. The operating system concept of a (dynamic) instance of a program in execution. Process control block. An operating system data structure that characterizes a process. Process state. The status of a process as running, ready, blocked, etc. Process table. An operating system data structure which pointers to all of the processes in the system. Producer. A process or thread that creates data and places it into a shared object. Program. A (static) set of instructions which can be used as the basis for a process. Quantum. The maximum amount of time that a task is allowed to run before being preempted. Race condition. A situation where the (uncontrolled) ordering of events can affect the outcome of the situation. Ready list. An operating system data structure which contains all of the ready processes in the system. Ready state. A process state in which all resources except the processor are available. Real address. An address used by the hardware of a computer system. Real address space. The set of possible real addresses. Real time system. A system which must respond in real time to changing conditions. Record. A subdivision of a file, containing data related to a single entity. Recovery. The restoration of file data after its loss. Redundancy. The portion of the total representation of a message that can be eliminated without loss of information. Redundant array of inexpensive disks. An array of multiple disk drives which appears as a single drive to the system. Referenced bit. A bit in a page table entry or a segment descriptor which is set when a memory address in the page or segment is read or written. Register. A fast memory-like data storage element which is part of the CPU or other control unit. Relative path. A path beginning at the current directory. Replacement strategy. The method used to determine which resident block is to be displaced. Restore. The copying of data from redundant copies back to the files from which they were created. Rotational delay. The time taken for a disk to move the correct sector to the read/write heads. Scheduler. The part of an operating system which assigns resources to processes, tasks, or threads. Secondary storage. Online peripheral data storage. Seek. (files)To move to a specified location in a file. Seek time. The time taken for a disk drive to move the access arm to the correct cylinder. Segment. A variable size block of contiguous memory addresses in a virtual address space which is managed as a unit. Semaphore. A synchronization variable accessable only through wait and signal procedures. Serialize. To control access to a variable so that only one thread can access the variable at a time. Serially reusable shared resource. A resource which can be used by at most one thread or process at a time. Shortest process first scheduling. A scheduling policy which schedules tasks in ascending order of estimated processing time. Shortest remaining time scheduling. A scheduling policy which schedules tasks in ascending order of estimated remaining processing time. Shortest-seek-time-first disk scheduling. A disk scheduling strategy which satisfies the pending disk request which is physically closest to the current position of the access arm. Signal. A mechanism by which a process may be notified by the kernel of an event occurring in the system. Signal handler. Code that is executed in response to a particular signal type. Signal mask. A data structure which blocks specified signals from being delivered to a thread. Sleeping state. A blocked thread state which can be transitioned to ready when notified of a timeout event. Soft link. A directory entry which is a pathname to a file. Soft real time scheduling. A scheduling policy which attempts to meet deadlines. Source module. A file containing source code for a part of a program. Spatial locality. A condition in which references to addresses in a given period are close tegether Spatial locality. A condition in which references to addresses in a given period are close tegether Spindle. The axle on which a disk turns. Static priority. A priority which is not automatically adjusted by the system. Structured programming. A style programming in which a hierarchy of structures is used, each having a single entry and a single exit point. Swapping. The process of copying a memory image to secondary storage and making the memory space available for other usage. Synchronous signal. signal generated due to execution of the currently running thread's execution. Temporal locality. A condition in which reference to a single address are close together in time. Temporal locality. A condition in which reference to a single address are close together in time. Thrashing. A condition in which the system is spending almost all of its time writing out virtual memory pages and reading them back in. Thread. An independently executable stream of program instructions. Track. The (circular) area on a disk platter which can be accessed by a single head without moving the access arm of the drive. Transfer time. The time required to transfer the data from a sector, once the transfer has begun. User-level thread. A thread which has the same operating system execution context as its parent. Virtual address. The address seen by a process in a virtual memory system. Virtual address space. The set of possible virtual addresses. Virtual memory. Memory simulated in software by swapping with a disk file. Volatile. Depending on the maintenance of power for the retention of information. Waiting state. A blocked thread state which can be transitioned to ready when notified of an event. Word. A set of bits on which a computer can perform arithmetic operations with a single instruction. Working set. The set of pages, at any time, required in real memory for a program to make reasonable progress without thrashing. Worst fit. A placement strategy which selects the largest space from the free list (if it is large enough.) -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- --------------------------------------------------