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.. SPDX-License-Identifier: CC-BY-SA-4.0
.. Copyright (C) 2020 Richi Dubey (richidubey@gmail.com)
.. Copyright (C) 2015, 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.
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 :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.
current priority
The current priority of a :term:`task` is the :term:`task priority` with
respect to the :term:`home scheduler` of the task. It is an aggregation of
the :term:`real priority` and temporary priority adjustments due to locking
protocols, the rate-monotonic period objects on some schedulers such as EDF,
and the :term:`POSIX` sporadic server. The current priority is an
:term:`eligible priority`.
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 :ref:`InterfaceRtemsExtensionCreate`. See also
:term:`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 <https://en.wikipedia.org/wiki/Executable_and_Linkable_Format>`_.
eligible priority
An eligible priority of a :term:`task` is the :term:`task priority` with
respect to the corresponding :term:`eligible scheduler` of the task. An
eligible priority is either the :term:`current priority` or a
:term:`helping priority` of a task.
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. See also
:term:`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 <https://man7.org/linux/man-pages/man2/futex.2.html>`_.
The futex support in RTEMS is provided for the barriers of the
:term:`OpenMP` library provided by :term:`GCC`. It could be used to
implement high performance :term:`SMP` synchronization primitives which
offer random-fairness.
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 priority
A helping priority of a :term:`task` is the :term:`task priority` with
respect to the corresponding :term:`helping scheduler` of the task. A
helping priority is an :term:`eligible priority`.
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.
higher priority
A :term:`task` ``H`` has a higher :term:`priority` than a task ``L``, if
task ``H`` is more important than task ``L``.
home scheduler
The home scheduler of a :term:`task` is a :term:`scheduler` which is an
:term:`eligible scheduler` and which is assigned to the task during its
initialization or explicitly via a directive call such as
:ref:`InterfaceRtemsTaskSetScheduler`.
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.
Last In First Out
A discipline for manipulating entries in a data structure. See also
:term:`First In First Out`.
LIFO
This term is an acronym for :term:`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.
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.
lower priority
A :term:`task` ``L`` has a lower :term:`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
:term:`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.
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.
OMIP
This term is an acronym for O(m) Independence-Preserving Protocol. OMIP is a
generalization of the :term:`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 <https://www.openmp.org/>`_.
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
The priority is a mechanism used to represent the relative importance of an
element in a set of items.
For example, :term:`RTEMS` uses :term:`task priorities <task priority>` to determine which
:term:`task` should execute on a processor. In RTEMS, priorities are
represented by non-negative integers.
For the Classic :term:`API`, if a numerical priority value ``A`` is greater
than a numerical priority value ``B``, then ``A`` expresses a
:term:`lower priority` than ``B``. If a numerical priority value ``C`` is
less than a numerical priority value ``D``, then ``C`` expresses a
:term:`higher priority` than ``D``.
For the :term:`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
:term:`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 :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 priority
Each :term:`task` has exactly one real priority. The real priority is
always with respect to the :term:`home scheduler` of a task. It is defined
during task initialization. It may be changed by directives such as
:ref:`InterfaceRtemsTaskSetPriority` and
:ref:`InterfaceRtemsTaskSetScheduler`. The real priority is the foundation
of the :term:`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 <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.
target architecture
The target architecture is the instruction set architecture (ISA) of the
:term:`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.
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 priority
A task :term:`priority` of a :term:`task` determines its importance
relative to other tasks.
The scheduler use task priorities to determine which :term:`ready task` gets
a processor allocated, see :term:`scheduled task`. The
:term:`eligible priorities <eligible priority>` of a task define the position of the task in a
:term:`wait queue` which uses the priority discipline. Each task has at
least the :term:`real priority`.
Task priorities are used in :term:`wait queues <wait queue>` which use the priority
discipline to determine the dequeueing order of tasks.
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.
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