6. Initialization Manager¶

6.2.1. 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.

6.2.2. 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.

6.2.3. 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.

6.2.4. 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 Chapter 21 Section 3.1 - Announcing a Fatal Error.

6.3.1. 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 Chapter 36 - 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 Chapter 24 - 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.

6.3.2. Shutting Down RTEMS¶

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

6.4.1. INITIALIZE_EXECUTIVE - Initialize RTEMS¶

CALLING SEQUENCE:

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.

6.4.2. SHUTDOWN_EXECUTIVE - Shutdown RTEMS¶

CALLING SEQUENCE:

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.