path: root/c_user/initialization.rst

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 `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 `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.

:dfn:`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.

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

:dfn:`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.

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

:dfn:`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.

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

:dfn:`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.

:dfn:`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 `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.

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

**CALLING SEQUENCE:**

.. index:: rtems_initialize_executive

.. code:: 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.

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

**CALLING SEQUENCE:**

.. index:: rtems_shutdown_executive

.. code:: 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.

.. COMMENT: COPYRIGHT (c) 1988-2014.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.