source: rtems-docs/c-user/initialization.rst @ 72a62ad

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

.. COMMENT: COPYRIGHT (c) 1988-2008.
.. COMMENT: On-Line Applications Research Corporation (OAR).
.. COMMENT: All rights reserved.

Initialization Manager
######################

Introduction
============

The Initialization Manager is responsible for initiating and shutting down
RTEMS.  Initiating RTEMS involves creating and starting all configured
initialization tasks, and for invoking the initialization routine for each
user-supplied device driver.  In a multiprocessor configuration, this manager
also initializes the interprocessor communications layer.  The directives
provided by the Initialization Manager are:

- rtems_initialize_executive_ - Initialize RTEMS

- rtems_shutdown_executive_ - Shutdown RTEMS

Background
==========

Initialization Tasks
--------------------
.. index:: initialization tasks

Initialization task(s) are the mechanism by which RTEMS transfers initial
control to the user's application.  Initialization tasks differ from other
application tasks in that they are defined in the User Initialization Tasks
Table and automatically created and started by RTEMS as part of its
initialization sequence.  Since the initialization tasks are scheduled using
the same algorithm as all other RTEMS tasks, they must be configured at a
priority and mode which will ensure that they will complete execution before
other application tasks execute.  Although there is no upper limit on the
number of initialization tasks, an application is required to define at least
one.

A typical initialization task will create and start the static set of
application tasks.  It may also create any other objects used by the
application.  Initialization tasks which only perform initialization should
delete themselves upon completion to free resources for other tasks.
Initialization tasks may transform themselves into a "normal" application task.
This transformation typically involves changing priority and execution mode.
RTEMS does not automatically delete the initialization tasks.

System Initialization
---------------------

System Initialization begins with board reset and continues through RTEMS
initialization, initialization of all device drivers, and eventually a context
switch to the first user task.  Remember, that interrupts are disabled during
initialization and the *initialization context* is not a task in any sense and
the user should be very careful during initialization.

The BSP must ensure that the there is enough stack space reserved for the
initialization context to successfully execute the initialization routines for
all device drivers and, in multiprocessor configurations, the Multiprocessor
Communications Interface Layer initialization routine.

The Idle Task
-------------

The Idle Task is the lowest priority task in a system and executes only when no
other task is ready to execute.  This default implementation of this task
consists of an infinite loop. RTEMS allows the Idle Task body to be replaced by
a CPU specific implementation, a BSP specific implementation or an application
specific implementation.

The Idle Task is preemptible and *WILL* be preempted when any other task is
made ready to execute.  This characteristic is critical to the overall behavior
of any application.

Initialization Manager Failure
------------------------------

The ``rtems_fatal_error_occurred`` directive will be invoked from
``rtems_initialize_executive`` for any of the following reasons:

- If either the Configuration Table or the CPU Dependent Information Table is
  not provided.

- If the starting address of the RTEMS RAM Workspace, supplied by the
  application in the Configuration Table, is NULL or is not aligned on a
  four-byte boundary.

- If the size of the RTEMS RAM Workspace is not large enough to initialize and
  configure the system.

- If the interrupt stack size specified is too small.

- If multiprocessing is configured and the node entry in the Multiprocessor
  Configuration Table is not between one and the maximum_nodes entry.

- If a multiprocessor system is being configured and no Multiprocessor
  Communications Interface is specified.

- If no user initialization tasks are configured.  At least one initialization
  task must be configured to allow RTEMS to pass control to the application at
  the end of the executive initialization sequence.

- If any of the user initialization tasks cannot be created or started
  successfully.

A discussion of RTEMS actions when a fatal error occurs may be found
:ref:`Announcing a Fatal Error`.

Operations
==========

Initializing RTEMS
------------------

The Initialization Manager ``rtems_initialize_executive`` directives is called
by the ``boot_card`` routine.  The ``boot_card`` routine is invoked by the
Board Support Package once a basic C run-time environment is set up.  This
consists of

- a valid and accessible text section, read-only data, read-write data and
  zero-initialized data,

- an initialization stack large enough to initialize the rest of the Board
  Support Package, RTEMS and the device drivers,

- all registers and components mandated by Application Binary Interface, and

- disabled interrupts.

The ``rtems_initialize_executive`` directive uses a system initialization
linker set to initialize only those parts of the overall RTEMS feature set that
is necessary for a particular application.  See :ref:`Linker Sets`.  Each RTEMS
feature used the application may optionally register an initialization handler.
The system initialization API is available via``#included <rtems/sysinit.h>``.

A list of all initialization steps follows.  Some steps are optional depending
on the requested feature set of the application.  The initialization steps are
execute in the order presented here.

`RTEMS_SYSINIT_BSP_WORK_AREAS`
    The work areas consisting of C Program Heap and the RTEMS Workspace are
    initialized by the Board Support Package.  This step is mandatory.

`RTEMS_SYSINIT_BSP_START`
    Basic initialization step provided by the Board Support Package.  This step
    is mandatory.

`RTEMS_SYSINIT_DATA_STRUCTURES`
    This directive is called when the Board Support Package has completed its
    basic initialization and allows RTEMS to initialize the application
    environment based upon the information in the Configuration Table, User
    Initialization Tasks Table, Device Driver Table, User Extension Table,
    Multiprocessor Configuration Table, and the Multiprocessor Communications
    Interface (MPCI) Table.

`RTEMS_SYSINIT_BSP_LIBC`
    Depending on the application configuration the IO library and root
    filesystem is initialized.  This step is mandatory.

`RTEMS_SYSINIT_BEFORE_DRIVERS`
    This directive performs initialization that must occur between basis RTEMS
    data structure initialization and device driver initialization.  In
    particular, in a multiprocessor configuration, this directive will create
    the MPCI Server Task.

`RTEMS_SYSINIT_BSP_PRE_DRIVERS`
    Initialization step performed right before device drivers are initialized
    provided by the Board Support Package.  This step is mandatory.

`RTEMS_SYSINIT_DEVICE_DRIVERS`
    This step initializes all statically configured device drivers and performs
    all RTEMS initialization which requires device drivers to be initialized.
    This step is mandatory.  In a multiprocessor configuration, this service
    will initialize the Multiprocessor Communications Interface (MPCI) and
    synchronize with the other nodes in the system.

`RTEMS_SYSINIT_BSP_POST_DRIVERS`
    Initialization step performed right after device drivers are initialized
    provided by the Board Support Package.  This step is mandatory.

The final action of the ``rtems_initialize_executive`` directive is to start
multitasking.  RTEMS does not return to the initialization context and the
initialization stack may be re-used for interrupt processing.

Many of RTEMS actions during initialization are based upon the contents of the
Configuration Table.  For more information regarding the format and contents of
this table, please refer to the chapter :ref:`Configuring a System`.

The final action in the initialization sequence is the initiation of
multitasking.  When the scheduler and dispatcher are enabled, the highest
priority, ready task will be dispatched to run.  Control will not be returned
to the Board Support Package after multitasking is enabled.  The initialization
stack may be re-used for interrupt processing.

Shutting Down RTEMS
-------------------

The ``rtems_shutdown_executive`` directive is invoked by the application to end
multitasking and terminate the system.

Directives
==========

This section details the Initialization Manager's directives.  A subsection is
dedicated to each of this manager's directives and describes the calling
sequence, related constants, usage, and status codes.

.. _rtems_initialize_executive:

INITIALIZE_EXECUTIVE - Initialize RTEMS
---------------------------------------
.. index:: initialize RTEMS
.. index:: start multitasking

**CALLING SEQUENCE:**

.. index:: rtems_initialize_executive

.. code-block:: c

    void rtems_initialize_executive(void);

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

Iterates through the system initialization linker set and invokes the
registered handlers.  The final step is to start multitasking.

**NOTES:**

This directive should be called by ``boot_card`` only.

This directive *does not return* to the caller.  Errors in the initialization
sequence are usually fatal and lead to a system termination.

.. _rtems_shutdown_executive:

SHUTDOWN_EXECUTIVE - Shutdown RTEMS
-----------------------------------
.. index:: shutdown RTEMS

**CALLING SEQUENCE:**

.. index:: rtems_shutdown_executive

.. code-block:: c

    void rtems_shutdown_executive(
        uint32_t result
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive is called when the application wishes to shutdown RTEMS.  The
system is terminated with a fatal source of ``RTEMS_FATAL_SOURCE_EXIT`` and the
specified ``result`` code.

**NOTES:**

This directive *must* be the last RTEMS directive invoked by an application and
it *does not return* to the caller.

This directive may be called any time.