path: root/c-user/glossary.rst

.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 2020 Richi Dubey (richidubey@gmail.com)
.. Copyright (C) 2017, 2021 embedded brains GmbH (http://www.embedded-brains.de)
.. Copyright (C) 1988, 1998 On-Line Applications Research Corporation (OAR)

Glossary
********

.. glossary::
    :sorted:

    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, :term:`Weak APA` and :term:`Strong APA`.

    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 :term:`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 :term:`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 :term:`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.

    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 :term:`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
        :term:`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.

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

    clock tick
        The clock tick is a coarse time measure provided by RTEMS.  The
        :term:`Clock Driver` emits clock ticks at rate specified by the
        :ref:`CONFIGURE_MICROSECONDS_PER_TICK` application configuration option.  In
        contrast to :term:`CLOCK_REALTIME` and :term:`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 :term:`CLOCK_REALTIME`,
        this clock cannot be set.  It may be affected by incremental adjustments for
        example carried out by the :term:`NTP` or the use of a :term:`PPS` signal.
        See also :term:`CLOCK_REALTIME`, :term:`clock tick`, and
        :term:`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 :term:`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 :term:`NTP` or the use of a :term:`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
        :term:`CLOCK_MONOTONIC`, :term:`clock tick`, and :term:`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 :term:`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 <https://github.com/doorstop-dev/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 :term:`user extensions`.  The list
        is defined by the system services used by the application and directive calls
        such as :c:func:`rtems_extension_create`.  See also
        :term:`initial extension sets`.

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

    ELF
        This term is an acronym for
        `Executable and Linkable Format <https://en.wikipedia.org/wiki/Executable_and_Linkable_Format>`_.

    eligible scheduler
        An eligible scheduler of a :term:`task` is a :term:`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 :term:`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.

    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.

    extension forward order
        The :term:`user extensions` may be invoked in extension forward order.  In
        forward order, all user extensions of the :term:`initial extension sets` are
        invoked before all user extensions of the :term:`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
        :term:`extension reverse order`.

    extension reverse order
        The :term:`user extensions` may be invoked in extension reverse order.  In
        reverse order, all user extensions of the :term:`dynamic extension sets` are
        invoked before all user extensions of the :term:`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
        :term:`extension forward order`.

    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 :term:`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 <https://gcc.gnu.org/>`_.

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

    GNAT
        *GNAT* is the :term:`GNU` compiler for Ada, integrated into the
        :term:`GCC`.

    GNU
        This term is an acronym for `GNU's Not Unix <https://www.gnu.org/>`_.

    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.

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

    heterogeneous
        A multiprocessor computer system composed of dissimilar processors.

    home scheduler
        The home scheduler of a :term:`task` is a :term:`scheduler` which is a
        :term:`eligible scheduler` and which is assigned to the task during the
        initialization or explicitly via a directive call such as
        :c:func:`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.

    IDLE task
        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 :term:`task` is a :term:`scheduler` which is
        not an :term:`eligible scheduler`.

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

    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
        :term:`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 :term:`software component`.

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

    ISR
        This term is an acronym for :term:`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
        :term:`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.

    NTP
        This term is an acronym for
        `Network Time Protocol <https://en.wikipedia.org/wiki/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.

    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 :term:`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.

    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.

    POSIX
        This term is an acronym for
        `Portable Operating System Interface <https://en.wikipedia.org/wiki/POSIX>`_.

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

    PPS
        This term is an acronym for
        `Pulse-Per-Second <https://en.wikipedia.org/wiki/Pulse-per-second_signal>`_.

    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 :term:`Partition Control Block`.

    PXCB
        This term is an acronym for :term:`Proxy Control Block`.

    QCB
        This term is an acronym for :term:`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 <https://www.omg.org/spec/ReqIF/About-ReqIF/>`_.

    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 :term:`status code`.

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

    RNCB
        This term is an acronym for :term:`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.

    RTEMS epoch
        The RTEMS epoch is a point in time.  It is 1988-01-01T00:00:00Z in
        `ISO 8601 <https://en.wikipedia.org/wiki/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.

    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.

        A scheduler is either an :term:`eligible scheduler` or a
        :term:`ineligible scheduler` for a task.  An :term:`eligible scheduler` is
        either the :term:`home scheduler` or a :term:`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
        :ref:`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 :term:`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:

        * :term:`software unit`

        * explicitly defined :term:`ELF` symbol in a
          :term:`source code` file

        * :term:`assembler language` data in a source code file

        * :term:`C language` object with static storage duration

        * C language object with thread-local storage duration

        * :term:`thread`

        * :term:`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
        :term:`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 :term:`software product`.

    software product
        The *software product* is the :term:`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:

        * :term:`assembler language` function in a
          :term:`source code` file

        * :term:`C language` function (external and internal linkage)

        A *software unit* is a :term:`software component`.

    source code
        This project uses the *source code* definition of the
        `Linux Information Project <http://www.linfo.org/source_code.html>`_:
        "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
        :term:`return value` to indicate a successful operation or error
        conditions.

    Strong APA
        Strong APA is a specialization of :term:`APA`.  Schedulers which implement
        strong APA recursively searches for a processor in the :term:`thread`'s
        affinity set, whenever a thread becomes ready for execution, followed by the
        processors in the affinity set of threads that are assigned the processor
        present in the ready thread's affinity set. This is done to find a thread to
        processor mapping that does not violate the priority ordering and to provide
        a thread to processor mapping with a higher total priority of the threads
        allocated a processor.  Similar analysis is done when a thread blocks.  See
        also :cite:`Cerqueira:2014:LPA`.

    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 <https://en.wikipedia.org/wiki/Thread_(computing)>`_:
        "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
        :term:`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 :term:`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 :term:`Task Control Block`.

    thread
        This term has the same meaning as :term:`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 :cite:`Drepper:2013:TLS`.
        TLS is available in :term:`C11` and :term:`C++11`.  The support for TLS
        depends on the CPU port :cite:`RTEMS:CPU`.

    TMCB
        This term is an acronym for :term:`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.

    Unix epoch
        The Unix epoch is a point in time.  It is 1970-01-01T00:00:00Z in
        `ISO 8601 <https://en.wikipedia.org/wiki/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
        :term:`initial extension sets` or the :term:`dynamic extension sets`.  User
        extensions are invoked in :term:`extension forward order` or
        :term:`extension reverse order`.

    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 :term:`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 :term:`APA`.  This refers to Linux's push
        and pull implementation of APA model. When a :term:`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
        :term:`Strong APA`, no thread is migrated from its processor to find a thread
        to processor mapping.  See also :cite:`Cerqueira:2014:LPA`.

    YAML
        This term is an acronym for `YAML Ain't Markup Language <https://yaml.org/>`_.

    yield
        When a task voluntarily releases control of the processor.