42. Glossary¶

ABI

This term is an acronym for Application Binary Interface.

active

A term used to describe an object which has been created by an application.

APA

This term is an acronym for Arbitrary Processor Affinity. APA schedulers allow a thread to have an arbitrary affinity to a processor set, rather than a restricted mapping to only one processor of the set or the ability to run on all processors of the set.

It has two variants, Weak APA and Strong APA.

A task which must execute only at irregular intervals and has only a soft deadline.

API

This term is an acronym for Application Programming Interface.

application

In this document, software which makes use of RTEMS.

ASR

This term is an acronym for Asynchronous Signal Routine.

assembler language

The assembler language is a programming language which can be translated very easily into machine code and data. For this project assembler languages are restricted to languages accepted by the GNU assembler program for the target architectures.

asynchronous

Not related in order or timing to other occurrences in the system.

Asynchronous Signal Routine

Similar to a hardware interrupt except that it is associated with a task and is run in the context of a task. The directives provided by the signal manager are used to service signals.

atomic operations

Atomic operations are defined in terms of C11.

awakened

A term used to describe a task that has been unblocked and may be scheduled to the CPU.

BCB

This term is an acronym for Barrier Control Block.

big endian

A data representation scheme in which the bytes composing a numeric value are arranged such that the most significant byte is at the lowest address.

bit-mapped

A data encoding scheme in which each bit in a variable is used to represent something different. This makes for compact data representation.

block

A physically contiguous area of memory.

The task state entered by a task which has been previously started and cannot continue execution until the reason for waiting has been satisfied. Blocked tasks are not an element of the set of ready tasks of a scheduler instance.

Board Support Package

A collection of device initialization and control routines specific to a particular type of board or collection of boards.

To simultaneously send a message to a logical set of destinations.

BSP

This term is an acronym for Board Support Package.

buffer

A fixed length block of memory allocated from a partition.

C language

The C language for this project is defined in terms of C11.

C++11

The standard ISO/IEC 14882:2011.

C++14

The standard ISO/IEC 14882:2014.

C++17

The standard ISO/IEC 14882:2017.

C++20

The standard ISO/IEC 14882:2020.

C11

The standard ISO/IEC 9899:2011.

C17

The standard ISO/IEC 9899:2018.

calling convention

The processor and compiler dependent rules which define the mechanism used to invoke subroutines in a high-level language. These rules define the passing of arguments, the call and return mechanism, and the register set which must be preserved.

CCB

This term is an acronym for Change Control Board.

Central Processing Unit

This term is equivalent to the terms processor and microprocessor.

chain

A data structure which allows for efficient dynamic addition and removal of elements. It differs from an array in that it is not limited to a predefined size.

Clock Driver

The Clock Driver is a driver which provides the clock tick and a time counter. The time counter is used to drive the CLOCK_REALTIME and CLOCK_MONOTONIC. The Clock Driver can be initialized by the application with the CONFIGURE_APPLICATION_NEEDS_CLOCK_DRIVER and CONFIGURE_MICROSECONDS_PER_TICK application configuration options.

clock tick

The clock tick is a coarse time measure provided by RTEMS. The Clock Driver emits clock ticks at rate specified by the CONFIGURE_MICROSECONDS_PER_TICK application configuration option. In contrast to CLOCK_REALTIME and CLOCK_MONOTONIC, the clock tick rate is not affected by incremental adjustments.

CLOCK_MONOTONIC

The CLOCK_MONOTONIC is a clock provided by RTEMS which measures the time since an unspecified starting point. In contrast to CLOCK_REALTIME, this clock cannot be set. It may be affected by incremental adjustments for example carried out by the NTP or the use of a PPS signal. See also CLOCK_REALTIME, clock tick, and Clock Driver.

CLOCK_REALTIME

The CLOCK_REALTIME is a clock provided by RTEMS which measures the real time (also known as wall-clock time). It is defined by POSIX. In particular, every day is treated as if it contains exactly 86400 seconds and leap seconds are ignored. This clock can be set by the application which may result in time jumps. It may be affected by incremental adjustments for example carried out by the NTP or the use of a PPS signal. RTEMS can represent time points of this clock in nanoseconds ranging from 1988-01-01T00:00:00.000000000Z to 2514-05-31T01:53:03.999999999Z. See also CLOCK_MONOTONIC, clock tick, and Clock Driver.

cluster

We have clustered scheduling in case the set of processors of a system is partitioned into non-empty pairwise disjoint subsets. These subsets are called clusters. Clusters with a cardinality of one are partitions. Each cluster is owned by exactly one scheduler instance.

coalesce

The process of merging adjacent holes into a single larger hole. Sometimes this process is referred to as garbage collection.

Configuration Table

A table which contains information used to tailor RTEMS for a particular application.

context

All of the processor registers and operating system data structures associated with a task.

context switch

Alternate term for task switch. Taking control of the processor from one task and transferring it to another task.

control block

A data structure used by the executive to define and control an object.

core

When used in this manual, this term refers to the internal executive utility functions. In the interest of application portability, the core of the executive should not be used directly by applications.

CPU

This term is an acronym for Central Processing Unit.

critical section

A section of code which must be executed indivisibly.

CRT

This term is an acronym for Cathode Ray Tube. Normally used in reference to the man-machine interface.

current priority

The current priority of a task is the task priority with respect to the home scheduler of the task. It is an aggregation of the real priority and temporary priority adjustments due to locking protocols, the rate-monotonic period objects on some schedulers such as EDF, and the POSIX sporadic server. The current priority is an eligible priority.

A fixed time limit by which a task must have completed a set of actions. Beyond this point, the results are of reduced value and may even be considered useless or harmful.

device

A peripheral used by the application that requires special operation software. See also device driver.

device driver

Control software for special peripheral devices used by the application.

Device Driver Table

A table which contains the entry points for each of the configured device drivers.

directives

RTEMS’ provided routines that provide support mechanisms for real-time applications.

dispatch

Doorstop

Doorstop is a requirements management tool.

dormant

The state entered by a task after it is created and before it has been started.

DPCB

This term is an acronym for Dual-Ported Memory Control Block.

dual-ported

A term used to describe memory which can be accessed at two different addresses.

dynamic extension sets

The dynamic extension sets are a list of user extensions. The list is defined by the system services used by the application and directive calls such as rtems_extension_create(). See also initial extension sets.

EARS

This term is an acronym for Easy Approach to Requirements Syntax.

EDF

This term is an acronym for Earliest Deadline First.

ELF

This term is an acronym for Executable and Linkable Format.

eligible priority

An eligible priority of a task is the task priority with respect to the corresponding eligible scheduler of the task. An eligible priority is either the current priority or a helping priority of a task.

eligible scheduler

An eligible scheduler of a task is a scheduler which can be used by the task to allocate a processor for the task.

embedded

An application that is delivered as a hidden part of a larger system. For example, the software in a fuel-injection control system is an embedded application found in many late-model automobiles.

entry point

The address at which a function or task begins to execute. In C, the entry point of a function is the function’s name.

envelope

A buffer provided by the MPCI layer to RTEMS which is used to pass messages between nodes in a multiprocessor system. It typically contains routing information needed by the MPCI. The contents of an envelope are referred to as a packet.

error code

This term has the same meaning as status code.

ESCB

This term is an acronym for Extension Set Control Block.

events

A method for task communication and synchronization. The directives provided by the event manager are used to service events.

exception

A synonym for interrupt.

The task state entered by a task after it has been given control of the processor. In SMP configurations, a task may be registered as executing on more than one processor for short time frames during task migration. Blocked tasks can be executing until they issue a thread dispatch.

executive

In this document, this term is used to referred to RTEMS. Commonly, an executive is a small real-time operating system used in embedded systems.

exported

An object known by all nodes in a multiprocessor system. An object created with the GLOBAL attribute will be exported.

extension forward order

The user extensions may be invoked in extension forward order. In forward order, all user extensions of the initial extension sets are invoked before all user extensions of the dynamic extension sets. In the initial extension sets the order is defined by the table index. The user extension with the lowest table index is invoked first. In the dynamic extension sets the order is defined by the registration order. The first registered user extension is invoked first. See also extension reverse order.

extension reverse order

The user extensions may be invoked in extension reverse order. In reverse order, all user extensions of the dynamic extension sets are invoked before all user extensions of the initial extension sets. In the dynamic extension sets the order is defined by the registration order. The last registered user extension is invoked first. In the initial extension sets the order is defined by the table index. The user extension with the highest table index is invoked first. See also extension forward order.

The address used to access dual-ported memory by all the nodes in a system which do not own the memory.

FIFO

This term is an acronym for First In First Out.

First In First Out

A discipline for manipulating entries in a data structure. See also Last In First Out.

floating point coprocessor

A component used in computer systems to enhance performance in mathematically intensive situations. It is typically viewed as a logical extension of the primary processor.

freed

A resource that has been released by the application to RTEMS.

Futex

This term is an abbreviation for Fast User-Space Locking. The futex support in RTEMS is provided for the barriers of the OpenMP library provided by GCC. It could be used to implement high performance SMP synchronization primitives which offer random-fairness.

GCC

This term is an acronym for GNU Compiler Collection.

global

An object that has been created with the GLOBAL attribute and exported to all nodes in a multiprocessor system.

GNAT

GNAT is the GNU compiler for Ada, integrated into the GCC.

GNU

This term is an acronym for GNU’s Not Unix.

handler

The equivalent of a manager, except that it is internal to RTEMS and forms part of the core. A handler is a collection of routines which provide a related set of functions. For example, there is a handler used by RTEMS to manage all objects.

hard real-time system

A real-time system in which a missed deadline causes the worked performed to have no value or to result in a catastrophic effect on the integrity of the system.

heap

A data structure used to dynamically allocate and deallocate variable sized blocks of memory.

A task is an heir if it is registered as an heir in a processor of the system. A task can be the heir on at most one processor in the system. In case the executing and heir tasks differ on a processor and a thread dispatch is marked as necessary, then the next thread dispatch will make the heir task the executing task.

helping priority

A helping priority of a task is the task priority with respect to the corresponding helping scheduler of the task. A helping priority is an eligible priority.

helping scheduler

A helping scheduler of a task is a scheduler which is a eligible scheduler and which is not the home scheduler of the task.

heterogeneous

A multiprocessor computer system composed of dissimilar processors.

higher priority

A task H has a higher priority than a task L, if task H is more important than task L.

home scheduler

The home scheduler of a task is a scheduler which is an eligible scheduler and which is assigned to the task during its initialization or explicitly via a directive call such as rtems_task_set_scheduler().

homogeneous

A multiprocessor computer system composed of a single type of processor.

I/O

This term is an acronym for Input/Output.

ID

An RTEMS assigned identification tag used to access an active object.

A special low priority task which assumes control of the CPU when no other task is able to execute.

ineligible scheduler

An ineligible scheduler of a task is a scheduler which is not an eligible scheduler.

initial extension sets

The initial extension sets are a table of user extensions. The table is defined by the application configuration for example through the CONFIGURE_INITIAL_EXTENSIONS application configuration option. The initial extension sets cannot be altered during runtime through directive calls. See also dynamic extension sets.

interface

A specification of the methodology used to connect multiple independent subsystems.

The address used to access dual-ported memory by the node which owns the memory.

interrupt

A hardware facility that causes the CPU to suspend execution, save its status, and transfer control to a specific location.

interrupt level

A mask used to by the CPU to determine which pending interrupts should be serviced. If a pending interrupt is below the current interrupt level, then the CPU does not recognize that interrupt.

interrupt service

An interrupt service consists of an Interrupt Service Routine which is called with a user provided argument upon reception of an interrupt service request. The routine is invoked in interrupt context. Interrupt service requests may have a priority and an affinity to a set of processors. An interrupt service is a software component.

Interrupt Service Routine

An ISR is invoked by the CPU to process a pending interrupt.

ISR

This term is an acronym for Interrupt Service Routine.

ISVV

This term is an acronym for Independent Software Verification and Validation.

kernel

In this document, this term is used as a synonym for executive.

Last In First Out

A discipline for manipulating entries in a data structure. See also First In First Out.

LIFO

This term is an acronym for Last In First Out.

list

A data structure which allows for dynamic addition and removal of entries. It is not statically limited to a particular size.

little endian

A data representation scheme in which the bytes composing a numeric value are arranged such that the least significant byte is at the lowest address.

local

An object which was created with the LOCAL attribute and is accessible only on the node it was created and resides upon. In a single processor configuration, all objects are local.

local operation

The manipulation of an object which resides on the same node as the calling task.

An address used by an application. In a system without memory management, logical addresses will equal physical addresses.

loosely-coupled

A multiprocessor configuration where shared memory is not used for communication.

lower priority

A task L has a lower priority than a task H, if task L is less important than task H.

major number

The index of a device driver in the Device Driver Table.

manager

A group of related RTEMS’ directives which provide access and control over resources.

MCS

This term is an acronym for Mellor-Crummey Scott.

memory pool

Used interchangeably with heap.

message

A sixteen byte entity used to communicate between tasks. Messages are sent to message queues and stored in message buffers.

message buffer

A block of memory used to store messages.

message queue

An RTEMS object used to synchronize and communicate between tasks by transporting messages between sending and receiving tasks.

Message Queue Control Block

A data structure associated with each message queue used by RTEMS to manage that message queue.

minor number

A numeric value passed to a device driver, the exact usage of which is driver dependent.

mode

An entry in a task’s control block that is used to determine if the task allows preemption, timeslicing, processing of signals, and the interrupt disable level used by the task.

MPCI

This term is an acronym for Multiprocessor Communications Interface Layer.

MrsP

This term is an acronym for Multiprocessor Resource-Sharing Protocol.

multiprocessing

The simultaneous execution of two or more processes by a multiple processor computer system.

multiprocessor

A computer with multiple CPUs available for executing applications.

Multiprocessor Communications Interface Layer

A set of user-provided routines which enable the nodes in a multiprocessor system to communicate with one another.

Multiprocessor Configuration Table

The data structure defining the characteristics of the multiprocessor target system with which RTEMS will communicate.

The alternation of execution amongst a group of processes on a single CPU. A scheduling algorithm is used to determine which process executes at which time.

mutual exclusion

A term used to describe the act of preventing other tasks from accessing a resource simultaneously.

nested

A term used to describe an ASR that occurs during another ASR or an ISR that occurs during another ISR.

node

A term used to reference a processor running RTEMS in a multiprocessor system.

non-existent

The state occupied by an uncreated or deleted task.

NTP

This term is an acronym for Network Time Protocol.

NUMA

This term is an acronym for Non-Uniform Memory Access.

numeric coprocessor

A component used in computer systems to enhance performance in mathematically intensive situations. It is typically viewed as a logical extension of the primary processor.

object

In this document, this term is used to refer collectively to tasks, timers, message queues, partitions, regions, semaphores, ports, and rate monotonic periods. All RTEMS objects have IDs and user-assigned names.

object-oriented

A term used to describe systems with common mechanisms for utilizing a variety of entities. Object-oriented systems shield the application from implementation details.

OMIP

This term is an acronym for O(m) Independence-Preserving Protocol. OMIP is a generalization of the priority inheritance locking protocol to clustered scheduling. The m denotes the number of processors in the system.

OpenMP

This term is an acronym for Open Multi-Processing.

operating system

The software which controls all the computer’s resources and provides the base upon which application programs can be written.

The portion of the CPUs processing power consumed by the operating system.

packet

A buffer which contains the messages passed between nodes in a multiprocessor system. A packet is the contents of an envelope.

partition

This term has two definitions:

1. A partition is an RTEMS object which is used to allocate and deallocate fixed size blocks of memory from an dynamically specified area of memory.

2. A cluster with a cardinality of one is a partition.

Partition Control Block

A data structure associated with each partition used by RTEMS to manage that partition.

PCB

This term is an acronym for Period Control Block.

pending

A term used to describe a task blocked waiting for an event, message, semaphore, or signal.

A task which must execute at regular intervals and comply with a hard deadline.

The actual hardware address of a resource.

poll

A mechanism used to determine if an event has occurred by periodically checking for a particular status. Typical events include arrival of data, completion of an action, and errors.

pool

A collection from which resources are allocated.

portability

A term used to describe the ease with which software can be rehosted on another computer.

POSIX

This term is an acronym for Portable Operating System Interface.

posting

The act of sending an event, message, semaphore, or signal to a task.

PPS

This term is an acronym for Pulse-Per-Second.

preempt

The act of forcing a task to relinquish the processor and dispatching to another task.

priority

The priority is a mechanism used to represent the relative importance of an element in a set of items.

For example, RTEMS uses task priorities to determine which task should execute on a processor. In RTEMS, priorities are represented by non-negative integers.

For the Classic API, if a numerical priority value A is greater than a numerical priority value B, then A expresses a lower priority than B. If a numerical priority value C is less than a numerical priority value D, then C expresses a higher priority than D.

For the POSIX API, if a numerical priority value R is less than a numerical priority value S, then R expresses a lower priority than S. If a numerical priority value T is greater than a numerical priority value U, then T expresses a higher priority than U.

priority boosting

A simple approach to extend the priority inheritance protocol for clustered scheduling is priority boosting. In case a mutex is owned by a task of another cluster, then the priority of the owner task is raised to an artificially high priority. This approach is not used in RTEMS, see also OMIP.

priority inheritance

An algorithm that calls for the lower priority task holding a resource to have its priority increased to that of the highest priority task blocked waiting for that resource. This avoids the problem of priority inversion.

priority inversion

A form of indefinite postponement which occurs when a high priority tasks requests access to shared resource currently allocated to low priority task. The high priority task must block until the low priority task releases the resource.

processor utilization

The percentage of processor time used by a task or a set of tasks.

proxy

An RTEMS control structure used to represent, on a remote node, a task which must block as part of a remote operation.

Proxy Control Block

A data structure associated with each proxy used by RTEMS to manage that proxy.

PTCB

This term is an acronym for Partition Control Block.

PXCB

This term is an acronym for Proxy Control Block.

QCB

This term is an acronym for Message Queue Control Block.

quantum

The application defined unit of time in which the processor is allocated.

queue

Alternate term for message queue.

A task occupies this state when it is available to be given control of a processor. A ready task has no processor assigned. The scheduler decided that other tasks are currently more important. A task that is ready to execute and has a processor assigned is called scheduled.

real priority

Each task has exactly one real priority. The real priority is always with respect to the home scheduler of a task. It is defined during task initialization. It may be changed by directives such as rtems_task_set_priority() and rtems_task_set_scheduler(). The real priority is the foundation of the current priority.

real-time

A term used to describe systems which are characterized by requiring deterministic response times to external stimuli. The external stimuli require that the response occur at a precise time or the response is incorrect.

reentrant

A term used to describe routines which do not modify themselves or global variables.

region

An RTEMS object which is used to allocate and deallocate variable size blocks of memory from a dynamically specified area of memory.

Region Control Block

A data structure associated with each region used by RTEMS to manage that region.

registers

Registers are locations physically located within a component, typically used for device control or general purpose storage.

remote

Any object that does not reside on the local node.

remote operation

The manipulation of an object which does not reside on the same node as the calling task.

ReqIF

This term is an acronym for Requirements Interchange Format.

resource

A hardware or software entity to which access must be controlled.

resume

Removing a task from the suspend state. If the task’s state is ready following a call to the rtems_task_resume directive, then the task is available for scheduling.

return code

This term has the same meaning as status code.

return value

The value returned by a function. A return value may be a status code.

RNCB

This term is an acronym for Region Control Block.

round-robin

RS-232

A standard for serial communications.

RTEMS

This term is an acronym for Real-Time Executive for Multiprocessor Systems.

RTEMS epoch

The RTEMS epoch is a point in time. It is 1988-01-01T00:00:00Z in ISO 8601 time format.

running

The state of a rate monotonic timer while it is being used to delineate a period. The timer exits this state by either expiring or being canceled.

schedulable

A set of tasks which can be guaranteed to meet their deadlines based upon a specific scheduling algorithm.

schedule

The process of choosing which task should next enter the executing state.

A task is scheduled if it is allowed to execute and has a processor assigned. Such a task executes currently on a processor or is about to start execution. A task about to start execution it is an heir task on exactly one processor in the system.

scheduler

A scheduler or scheduling algorithm allocates processors to a subset of its set of ready tasks. So it manages access to the processor resource. Various algorithms exist to choose the tasks allowed to use a processor out of the set of ready tasks. One method is to assign each task a priority number and assign the tasks with the lowest priority number to one processor of the set of processors owned by a scheduler instance.

A scheduler is either an eligible scheduler or a ineligible scheduler for a task. An eligible scheduler is either the home scheduler or a helping scheduler for a task.

scheduler instance

A scheduler instance is a scheduling algorithm with a corresponding context to store its internal state. Each processor in the system is owned by at most one scheduler instance. The processor to scheduler instance assignment is determined at application configuration time. See Configuring a System.

segments

Variable sized memory blocks allocated from a region.

semaphore

Semaphore Control Block

A data structure associated with each semaphore used by RTEMS to manage that semaphore.

shared memory

Memory which is accessible by multiple nodes in a multiprocessor system.

signal

An RTEMS provided mechanism to communicate asynchronously with a task. Upon reception of a signal, the ASR of the receiving task will be invoked.

signal set

A thirty-two bit entity which is used to represent a task’s collection of pending signals and the signals sent to a task.

SMCB

This term is an acronym for Semaphore Control Block.

SMP

This term is an acronym for Symmetric Multiprocessing.

SMP barriers

The SMP barriers ensure that a defined set of independent threads of execution on a set of processors reaches a common synchronization point in time. They are implemented using atomic operations. Currently a sense barrier is used in RTEMS.

SMP locks

The SMP locks ensure mutual exclusion on the lowest level and are a replacement for the sections of disabled interrupts. Interrupts are usually disabled while holding an SMP lock. They are implemented using atomic operations. Currently a ticket lock is used in RTEMS.

soft real-time system

A real-time system in which a missed deadline does not compromise the integrity of the system.

software component

This term is defined by ECSS-E-ST-40C 3.2.28 as a “part of a software system”. For this project a software component shall be any of the following items and nothing else:

Please note that explicitly defined ELF symbols and assembler language data are considered a software component only if they are defined in a source code file. For example, this rules out symbols and data generated as side-effects by the toolchain (compiler, assembler, linker) such as jump tables, linker trampolines, exception frame information, etc.

software item

This term has the same meaning as software product.

software product

The software product is the RTEMS real-time operating system.

software unit

This term is defined by ECSS-E-ST-40C 3.2.24 as a “separately compilable piece of source code”. For this project a software unit shall be any of the following items and nothing else:

A software unit is a software component.

source code

This project uses the source code definition of the Linux Information Project: “Source code (also referred to as source or code) is the version of software as it is originally written (i.e., typed into a computer) by a human in plain text (i.e., human readable alphanumeric characters).”

A task which executes at irregular intervals and must comply with a hard deadline. A minimum period of time between successive iterations of the task can be guaranteed.

stack

A data structure that is managed using a Last In First Out (LIFO) discipline. Each task has a stack associated with it which is used to store return information and local variables.

status code

A status code indicates the completion status of an operation. For example most RTEMS directives return a status code through the return value to indicate a successful operation or error conditions.

Strong APA

suspend

A term used to describe a task that is not competing for the CPU because it has had a rtems_task_suspend directive.

synchronous

Related in order or timing to other occurrences in the system.

system call

In this document, this is used as an alternate term for directive.

target

The system on which the application will ultimately execute.

target architecture

The target architecture is the instruction set architecture (ISA) of the target. Some RTEMS features depend on the target architecture. For the details consult the RTEMS CPU Architecture Supplement.

TAS

This term is an acronym for Test-And-Set.

This project uses the thread definition of Wikipedia: “a thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically a part of the operating system.”

It consists normally of a set of registers and a stack. The scheduler assigns processors to a subset of the ready tasks. The terms task and thread are synonym in RTEMS. The term task is used throughout the Classic API, however, internally in the operating system implementation and the POSIX API the term thread is used.

A task is a software component.

A data structure associated with each task used by RTEMS to manage that task.

Task migration happens in case a task stops execution on one processor and resumes execution on another processor.

The scheduler use task priorities to determine which ready task gets a processor allocated, see scheduled task. The eligible priorities of a task define the position of the task in a wait queue which uses the priority discipline. Each task has at least the real priority.

Task priorities are used in wait queues which use the priority discipline to determine the dequeueing order of tasks.

The set of processors on which a task is allowed to execute.

Alternate terminology for context switch. Taking control of the processor from one task and given to another.

TCB

This term is an acronym for Task Control Block.

This term has the same meaning as task.

The thread dispatch transfers control of the processor from the currently executing thread to the heir thread of the processor.

tick

The basic unit of time used by RTEMS. It is a user-configurable number of microseconds. The current tick expires when a clock tick directive is invoked.

tightly-coupled

A multiprocessor configuration system which communicates via shared memory.

timeout

An argument provided to a number of directives which determines the maximum length of time an application task is willing to wait to acquire the resource if it is not immediately available.

timer

An RTEMS object used to invoke subprograms at a later time.

Timer Control Block

A data structure associated with each timer used by RTEMS to manage that timer.

timeslice

The application defined unit of time in which the processor is allocated.

timeslicing

A task scheduling discipline in which tasks of equal priority are executed for a specific period of time before being preempted by another task.

TLS

This term is an acronym for Thread-Local Storage [Dre13]. TLS is available in C11 and C++11. The support for TLS depends on the CPU port [RTE].

TMCB

This term is an acronym for Timer Control Block.

A temporary rise in system activity which may cause deadlines to be missed. Rate Monotonic Scheduling can be used to determine if all deadlines will be met under transient overload.

TTAS

This term is an acronym for Test and Test-And-Set.

Unix epoch

The Unix epoch is a point in time. It is 1970-01-01T00:00:00Z in ISO 8601 time format.

User Extension Table

A table which contains the entry points for each user extensions.

user extensions

User extensions are software routines provided by the application to enhance the functionality of RTEMS. An active user extension is either in the initial extension sets or the dynamic extension sets. User extensions are invoked in extension forward order or extension reverse order.

A table which contains the information needed to create and start each of the user initialization tasks.

user-provided

These terms are used to designate any software routines which must be written by the application designer.

user-supplied

This term has the same meaning as user-provided.

vector

Memory pointers used by the processor to fetch the address of routines which will handle various exceptions and interrupts.

wait queue

The list of tasks blocked pending the release of a particular resource. Message queues, regions, and semaphores have a wait queue associated with them.

Weak APA

Weak APA is a specialization of APA. This refers to Linux’s push and pull implementation of APA model. When a thread becomes ready for execution, it is allocated a processor if there is an idle processor, or a processor executing a lower priority thread in its affinity set. Unlike Strong APA, no thread is migrated from its processor to find a thread to processor mapping. See also [CGB14].

YAML

This term is an acronym for YAML Ain’t Markup Language.

yield

When a task voluntarily releases control of the processor.