40. Glossary — RTEMS Classic API Guide 5.4-rc2 (21st August 2026) documentation

# 40. 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.

aperiodic task

  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.

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.

blocked task

  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.

broadcast

  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.

C11

  The standard ISO/IEC 9899:2011.

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.

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.

deadline

  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

  The act of loading a task’s context onto the CPU and transferring control of the CPU to that task.

Doorstop

  Doorstop is a requirements management tool.

dormant

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

dual-ported

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

EARS

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

ELF

  This term is an acronym for Executable and Linkable Format.

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.

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.

executing task

  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.

external address

  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.

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.

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.

heir task

  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.

heterogeneous

  A multiprocessor computer system composed of dissimilar processors.

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.

IDLE task

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

interface

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

internal address

  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.

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.

logical address

  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.

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.

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.

multitasking

  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.

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.

operating system

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

overhead

  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.

pending

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

periodic task

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

physical address

  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.

posting

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

preempt

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

priority

  A mechanism used to represent the relative importance of an element in a set of items. RTEMS uses priority to determine which task should execute.

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, the pseudo-interrupt priority.

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.

ready task

  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-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

  A task scheduling discipline in which tasks of equal priority are executed in the order in which they are made ready.

RS-232

  A standard for serial communications.

RTEMS

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

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.

scheduled task

  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.

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

  An RTEMS object which is used to synchronize tasks and provide mutually exclusive access to resources.

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:

  - software unit

  - explicitly defined ELF symbol in a source code file

  - assembler language data in a source code file

  - C language object with static storage duration

  - C language object with thread-local storage duration

  - thread

  - interrupt service

  - collection of software components (this is a software architecture element)

  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:

  - assembler language function in a source code file

  - C language function (external and internal linkage)

  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).”

sporadic task

  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.

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.

TAS

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

task

  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.

Task Control Block

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

task migration

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

task processor affinity

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

task switch

  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.

thread

  This term has the same meaning as task.

thread dispatch

  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.

transient overload

  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.

User Extension Table

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

user extensions

  Software routines provided by the application to enhance the functionality of RTEMS.

User Initialization Tasks Table

  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.

YAML

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

yield

  When a task voluntarily releases control of the processor.