Operating Systems

Relocatable Dynamic Partitions

a memory allocation scheme in which the system relocates programs in memory to gather together all of the empty blocks and compact them to make one block of memory that's large enough to accommodate some or all of the jobs waiting for memory.

Demand Paging

a memory allocation scheme that loads into memory a program's page at the time it is needed for processing.

Server

a node that provides to clients various network services, such as file retrieval, printing, or database access services.

Address

a number that designates a particular memory location.

PMT

a page map table is a table in main memory with the vital information for each page including the page number and its corresponding page frame memory address.

Storage

a place where data is stored in the computer system. Primary storage is main memory and secondary storage is non-volatile media, such as disks and flash memory.

Aging

a policy used to ensure that jobs that have been in the system for a long time in the lower-level queues will eventually complete their execution.

Thread

a portion of a program that can run independently of other portions. Multithreaded application programs can have several threads running at one time with the same or different priorities.

Indefinite Postponement

signifies that a job's execution is delayed indefinitely because it is repeatedly preempted so other jobs can be processed.

Firmware

software instructions or data that are stored in a fixed or firm way, usually implemented on read only memory. It is built into the computer to make its operation simpler for the user to understand.

First Generation

the 1940 to 1955 era of the first computers, characterized by their use of vacuum tubes and their very large physical size.

Second Generation

the 1955 to 1965 era of technological development of computers, when the transistor replaced the vacuum tube. Computers were smaller and faster and had larger storage capacity than first-generation computers and were developed to meet the needs of the business market.

MMT

the Memory Map Table is a table in main memory that contains as many entries as there are page frames and lists the location and free/busy status for each one.

Context Switching

the acts of saving a job's processing information in its PCB so the job can be swapped out of memory, and of loading the processing information from PCB of another job into the appropriate registers so the CPU can process it. Context switching occurs in all preemptive policies.

Job Status

the condition of a job as it moves through the system from the beginning to the end of its execution: HOLD, READY, RUNNING, WAITING, or FINISHED.

Virtualization

the creation of a virtual version of hardware or software. Operating system virtualization allows a single CPU to run multiple operating system images at the same time.

Clock Cycle

the elapsed time between two ticks of the computer's system clock.

Supercomputers

the fastest, most sophisticated computers made, used for complex calculations at the fastest speed permitted by current technology.

Mainframe

the historical name given to a large computer system characterized by its large size, high cost, and high performance.

Third Generation

the mid-1960's era of computer development, roughly around 1965 to 1975 that introduced multiprogramming, integrated circuits and miniaturization of components to replace transistors, reduced costs, work faster, and increase reliability.

Associative Memory

the name given to several registers, allocated to each active process, whose contents associate several of the process segments and page numbers with their main memory addresses.

OS

the operating system is the software that manages all the resources of a computer system.

Page Fault Handler

the part of the Memory Manager that determines if there are empty page frames in memory so that the requested page can be immediately copied from secondary storage, or determines which page must be swapped out if all page frames are busy. Also known as a page interrupt handler.

Hardware

the physical machine and its components, including main memory, I/O devices, I/O channels, direct access storage devices, and the central processing unit.

Kernel

the primary part of the operating system that remains in random access memory and is charged with performing the system's most essential tasks, such as managing main memory and disk access.

Address Resolution

the process of changing the address of an instruction or data item to the address in main memory at which it is to be loaded or relocated.

Compaction

the process of collecting fragments of available memory space into contiguous blocks by moving programs and data into a computer's memory or disk, secondary storage. Also called garbage collection.

Deallocation

the process of freeing an allocated resource, whether memory space, a device, a file, or a CPU.

Page Swap

the process of moving a page out of main memory and into secondary storage so another page can be moved into memory in its place.

Relocation

the process of moving a program from one area of memory to another. It is also the process of adjusting address references in a program, by either software or hardware means, to allow the program to execute correctly when loaded in different sections of memory.

Interrupt Handler

the program that controls what action should be taken by the operating system when a sequence of events is interrupted.

Network Manager

the section of the operating system responsible for controlling access to and the use of networked resources.

Device Manager

the section of the operating system responsible for controlling the use of devices. It monitors every device, channel, and control unit and chooses the most efficient way to allocate all of the system's devices.

File Manager

the section of the operating system responsible for controlling the use of files. It tracks every file in the system including data files, assemblers, compilers, and application programs. By using predetermined access policies, it enforces access restrictions on each file.

Memory Manager

the section of the operating system responsible for controlling the use of memory. It checks the validity of each request for memory space and, if it is a legal request, allocates the amount of memory needed to execute the job.

Main Memory

the unit that works directly with the CPU and in which the data and instructions must reside in order to be processed. Also called random access memory, primary storage, or internal memory.

Hard Real-time System

they risk total system failure if the predicted time deadline is missed.

Soft Real-time System

they suffer performance degradation, but not total system failure, as a consequence of a missed deadline.

Multithreading

this allows applications to manage a separate process with several threads of control.

Single-user Systems

this supports one user, on one computer running one job at a time. Sharing is not possible.

1970's

2 key technological developments that were introduced in the 1970's were: The first Cray supercomputer released in 1976 and the development of virtual memory.

CPU

Central Processing Unit is the component with the circuitry, the chips, to control the interpretation and execution of instructions. It controls the operation of the entire computer system. All storage references, data manipulations, and I/O operations are initiated or performed by the CPU.

FCFS

First-come First-served is the simplest scheduling algorithm for direct access storage devices that satisfies track requests in the order in which they are received. A non-preemptive process scheduling policy that handles jobs according to their arrival time; the first job in the READY queue will be processed first by the CPU.

FIFO Policy

First-in First-out Policy is a page replacement policy that removes from main memory the pages that were brought in first. It is based on the assumption that these pages are the least likely to be used again in the near future.

Segmented/Demand Paged Memory Allocation

a memory allocation scheme based on the concept of dividing a user's job into logical groupings of code and loading them into memory as needed to minimize fragmentation.

I/O-bound

Input Output-bound is a job that requires a large number of input/output operations, resulting in much free time for the CPU.

JT

Job Table is a table in main memory that contains two values/entries for each active job—the size of the job and the memory location where its page map table is stored. It is used for paged memory allocation schemes.

K

Kilobyte is a unit of memory or storage space equal to 1,024 bytes or 2 to the tenth bytes.

LRU Policy

Least-recently-used policy is a page-replacement policy that removes from main memory the pages that show the least amount of recent activity. It is based on the assumption that these pages are the least likely to be used again in the immediate future.

Segmented Memory Allocation

a memory allocation scheme based on the concept of dividing a user's job into logical groupings of code to allow for non-contiguous program storage during execution.

Core

The processing part of a CPU chip made up of the control unit and the arithmetic logic unit. The core does not include the cache.

Paged Memory Allocation

a memory allocation scheme based on the concept of dividing a user's job into sections of equal size to allow for non-contiguous program storage during execution. This was implemented to further increase the level of multi-programming.

Dynamic Partitions

a memory allocation scheme in which jobs are given as much memory as they request when they are loaded for processing, thus creating their own partitions in main memory.

Fixed or Static Partitions

a memory allocation scheme in which main memory is sectioned off, with portions assigned to each job/user.

FIFO Anomaly

an unusual circumstance through which adding more page frames causes an increase in page interrupts when using a FIFO page replacement policy.

Software

a collection of programs used to perform certain tasks. Software falls into three main categories: operating system programs, compilers and assemblers, and application programs.

Throughput

a composite measure of a system's efficiency that counts the number of jobs served in a given unit of time.

Processor Manager

a composite of two submanagers, the Job Scheduler and the Process Scheduler. It decides how to allocate the CPU, monitors whether it is executing a process or waiting, and controls job entry to ensure balanced use of resources.

Hybrid System

a computer system that supports both batch and interactive processes. It appears to be interactive because individual users can access the system via terminals and get fast responses, but it accepts and runs batch programs in the background when the interactive load is light.

Real-time System

a computing system used in time-critical environments that require guaranteed response times, such as navigation systems, rapid transit systems, and industrial control systems.

Embedded Computer System

a dedicated computer system, often small and fast, that resides in a larger physical system such as jet aircraft or ships. It must be small and fast and work with real-time constraints, fail-safe execution, and nonstandard I/O devices.

Workstation

a desktop computer attached to a local area network that serves as an access point to that network.

Sector

a division in a disk's track, sometimes called a "block." The tracks are divided into sectors during the formatting process.

Page

a fixed-size section of a user's job that corresponds in size to page frames in main memory.

GUI

a graphical user interface allows the user to activate operating system commands by clicking on desktop icons or menus using a pointing device such as a mouse or touch screen. GUIs evolved from command-driven user interfaces and are also called menu-driven interfaces.

Interrupt

a hardware signal that suspends execution of a program and activates the execution of a special program known as the interrupt handler. It breaks the normal flow of the program being executed.

Nonpreemptive Scheduling Policy

a job scheduling strategy that functions without external interrupts so that once a job captures the processor and begins execution, it remains in the RUNNING state uninterrupted until it issues an I/O request or it is finished.

CPU-bound

a job that will perform a great deal of nonstop processing before issuing an interrupt. This can tie up the CPU for long periods of time.

Best-fit Memory Allocation

a main memory allocation scheme that considers all free blocks and selects for allocation the one that will result in the least amount of wasted space.

First-fit Memory Allocation

a main memory allocation scheme that searches from the beginning of the free block list and selects for allocation the first block of memory large enough to fulfill the request.

Multiple-level Queues

a process-scheduling scheme, used with other scheduling algorithms, that groups jobs according to a common characteristic. The processor is then allocated to serve the jobs in these queues in a predetermined manner.

Object-oriented

a programming philosophy whereby programs consist of self-contained, reusable modules called objects, each of which supports a specific function, but which are categorized into classes of objects that share the same function.

Relocation Register

a register that contains the value that must be added to each address referenced in the program so that it will be able to access the correct memory addresses after relocation. If the program hasn't been relocated, the value stored in the program's relocation register is 0.

Bounds Register

a register used to store the highest location in memory legally accessible by each program.

Middle-level Scheduler

a scheduler used by the Processor Manager when the system to remove active processes from memory becomes overloaded. It swaps these processes back into memory when the system overload has cleared. It is also a scheduler used by the Processor Manager to manage processes that have been interrupted because they have exceeded their allocated CPU time slice. It is used in some highly interactive environments.

SMT

a segment map table is a table in main memory with the vital information for each segment including the segment number and its corresponding memory address.

Internal Fragmentation

a situation in which a fixed partition is only partially used by the program. The remaining space within the partition is unavailable to any other job and is therefore wasted.

External Fragmentation

a situation in which the dynamic allocation of memory creates unusable fragments of free memory between blocks of busy, or allocated, memory.

Microcomputer

a small computer equipped with all the hardware and software necessary to perform one or more tasks.

Midrange Computer aka Minicomputer

a small to medium-sized computer system developed to meet the needs of smaller institutions. It was originally developed for sites with only a few dozen users.

Cache Memory

a small, fast memory used to hold selected data and to provide faster access than would otherwise be possible.

Network

a system of interconnected computer systems and peripheral devices that exchange information with one another.

Interactive System

a system that allows each user to interact directly with the operating system via commands entered from a keyboard. Also called time-sharing system.

Multiprogramming

a technique that allows a single processor to process several programs residing simultaneously in main memory and interleaving their execution by overlapping I/O requests with CPU requests. Also called concurrent programming or multitasking.

Page Fault

a type of hardware interrupt caused by a reference to a page not residing in memory. The effect is to move a page out of main memory and into secondary storage so another page can be moved into memory.

Batch System

a type of system developed for the earliest computers that used punched cards or tape for input. Each job was entered by assembling the cards together into a deck and several jobs were grouped, or batched, together before being sent through the card reader.

Segment

a variable-size section of a user's job that contains a logical grouping of code.

Clock Page Replacement Policy

a variation of the LRU policy that removes from main memory the pages that show the least amount of activity during recent clock cycles.

Page Frame

an individual section of main memory of uniform size into which a single page may be loaded without causing external fragmentation.

Subroutine

also called a "subprogram," a segment of a program that can perform a specific function. Subroutines can reduce programming time when a specific function is required at more than one point in a program.

Page Replacement Policy

an algorithm used by virtual memory systems to decide which page or segment to remove from main memory when a page frame is needed and memory is full.

Null Entry

an empty entry in a list. It assumes different meanings based on the list's application.

Locality of Reference

behavior observed in many executing programs in which memory locations recently referenced, and those near them, are likely to be referenced in the near future.

Reentrant Code

code that can be used by two or more processes at the same time; each shares the same copy of the executable code but has separate data areas.

Natural Wait

common term used to identify an I/O request from a program in a multiprogramming environment that would cause a process to wait "naturally" before resuming execution.

1980's

firmware and multiprocessing were developed.

Displacement aka Offset

in a paged or segmented memory allocation environment, it's the difference between a page's relative address and the actual machine language address. It is used to locate an instruction or data value within its page frame.

2000's

virtualization and multi-core processors were developed.

Multiprocessing

when two or more CPUs share the same main memory, most I/O devices, and the same control program routines. They service the same job stream and execute distinct processing programs concurrently.

1990's

world wide web was conceived by Tim Berners-Lee and there was a proliferation of multimedia applications.