76 lines
2.3 KiB
ReStructuredText
76 lines
2.3 KiB
ReStructuredText
.. _nanokernel_tasks:
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Task Services
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#############
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Concepts
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********
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A task is a preemptible thread of execution that implements a portion of
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an application's processing. It is is normally used to perform processing that
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is too lengthy or complex to be performed by a fiber or an ISR.
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A nanokernel application can define only a single application task, known
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as the *background task*, which executes when there is no fiber or ISR
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that needs to execute. The entry point function for the background task
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is :c:func:`main()`, and must be supplied by the application.
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.. note::
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The background task is very different from the tasks used by a microkernel
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application; for more information see
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:ref:`Microkernel Task Services <microkernel_tasks>`.
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Task Lifecycle
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==============
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The kernel automatically starts the background task during system
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initialization.
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Once the background task is started it executes forever. If the task attempts
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to terminate by returning from :c:func:`main()`, the kernel puts the task into
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a permanant idling state since the background task must always be available
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to execute.
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Task Scheduling
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===============
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The nanokernel's scheduler executes the background task only when there is no
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fiber or ISR that needs to execute, since fiber and ISR execution takes
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precedence.
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The kernel automatically takes care of saving the background task's CPU register
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values when it performs a context switch to a fiber or an ISR. It restores
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these values when the background task later resumes execution.
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Usage
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*****
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Defining the Background Task
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============================
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The application must supply a function of the following form:
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.. code-block:: c
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void main(void)
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{
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/* background task processing */
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...
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/* (optional) enter permanent idling state */
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return;
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}
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This function is used as the background task's entry point function. If a
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nanokernel application does not need to perform any task-level processing
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:c:func:`main()` can simply do an immediate return.
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The :option:`MAIN_STACK_SIZE` configuration option specifies the size,
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in bytes, of the memory region used for the background task's stack
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and for other execution context information.
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APIs
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****
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The nanokernel does not provide any APIs for manipulating the background task.
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