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c-user: Rework glossary

Define exactly one term per definition. Use references for alternative
terms. Add :term: text roles to acronym definitions of glossary defined
terms.

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Glossary

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.. glossary::
   :sorted:
   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
      An acronym for Application Programming Interface.
   application
      In this document, software which makes use of RTEMS.
   ASR
      see Asynchronous Signal Routine.
   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.
   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.
   broadcast
      To simultaneously send a message to a logical set of destinations.
   Board Support Package
      A collection of device initialization and control routines specific to a
      particular type of board or collection of boards.
   buffer
      A fixed length block of memory allocated from a partition.
   BSP
      An acronym for :term:`Board Support Package`.
   C11
      The standard ISO/IEC 9899:2011.
   C++11
      The standard ISO/IEC 14882:2011.
   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.
   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
      An acronym for :term:`Central Processing Unit`.
   critical section
      A section of code which must be executed indivisibly.
   CRT
      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.
   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.
   dormant
      The state entered by a task after it is created and before it has been
      started.
   Device Driver Table
      A table which contains the entry points for each of the configured device
      drivers.
   dual-ported
      A term used to describe memory which can be accessed at two different
      addresses.
   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.
   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.
   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.
   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
      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.
   global
      An object that has been created with the GLOBAL attribute and exported to
      all nodes in a multiprocessor system.
   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.
   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 Routine
      An ISR is invoked by the CPU to process a pending interrupt.
   I/O
      An acronym for Input/Output.
   ISR
      An acronym for :term:`Interrupt Service Routine`.
   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
      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
      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.
   NUMA
      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
      An RTEMS object which is used to allocate and deallocate fixed size
      blocks of memory from an dynamically specified area of memory.
   partition
      Clusters with a cardinality of one are partitions.
   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
      An acronym for :term:`Partition Control Block`.
   PXCB
      An acronym for :term:`Proxy Control Block`.
   quantum
      The application defined unit of time in which the processor is allocated.
   queue
      Alternate term for message queue.
   QCB
      An acronym for :term:`Message Queue Control Block`.
   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.
   return code
      Also known as error code or return value.
   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
      A value returned by RTEMS directives to indicate the completion status of
      the directive.
   RNCB
      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.
   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
      :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
      An acronym for :term:`Semaphore Control Block`.
   SMP
      An acronym for Symmetric Multiprocessing.
   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.
   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.
   soft real-time system
      A real-time system in which a missed deadline does not compromise the
      integrity of the system.
   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
      Also known as error code or return value.
   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
      An acronym for Test-And-Set.
   task
      A logically complete thread of execution.  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.
   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
      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.
   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.
   timeslice
      The application defined unit of time in which the processor is allocated.
   TLS
      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
      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
      An acronym for Test and Test-And-Set.
   user extensions
      Software routines provided by the application to enhance the
      functionality of RTEMS.
   User Extension Table
      A table which contains the entry points for each user extensions.
   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.
   yield
      When a task voluntarily releases control of the processor.
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