path: root/porting/idle_thread.rst
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authorAmar Takhar <>2016-01-16 19:08:48 -0500
committerAmar Takhar <>2016-05-02 20:51:23 -0400
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+IDLE Thread
+Does Idle Thread Have a Floating Point Context?
+The setting of the macro CPU_IDLE_TASK_IS_FP is based on the answer to the
+question: Should the IDLE task have a floating point context? If the
+answer to this question is TRUE, then the IDLE task has a floating point
+context associated. This is equivalent to creating a task in the Classic
+API (using rtems_task_create) as a RTEMS_FLOATING_POINT task. If
+CPU_IDLE_TASK_IS_FP is set to TRUE, then a floating point context switch
+occurs when the IDLE task is switched in and out. This adds to the
+execution overhead of the system but is necessary on some ports.
+If FALSE, then the IDLE task does not have a floating point context.
+NOTE: Setting CPU_IDLE_TASK_IS_FP to TRUE negatively impacts the time
+required to preempt the IDLE task from an interrupt because the floating
+point context must be saved as part of the preemption.
+The following illustrates how to set this macro:
+.. code:: c
+CPU Dependent Idle Thread Body
+The CPU_PROVIDES_IDLE_THREAD_BODY macro setting is based upon the answer
+to the question: Does this port provide a CPU dependent IDLE task
+implementation? If the answer to this question is yes, then the
+CPU_PROVIDES_IDLE_THREAD_BODY macro should be set to TRUE, and the routine
+_CPU_Thread_Idle_body must be provided. This routine overrides the
+default IDLE thread body of _Thread_Idle_body. If the
+CPU_PROVIDES_IDLE_THREAD_BODY macro is set to FALSE, then the generic
+_Thread_Idle_body is the default IDLE thread body for this port.
+Regardless of whether or not a CPU dependent IDLE thread implementation is
+provided, the BSP can still override it.
+This is intended to allow for supporting processors which have a low power
+or idle mode. When the IDLE thread is executed, then the CPU can be
+powered down when the processor is idle.
+The order of precedence for selecting the IDLE thread body is:
+# BSP provided
+# CPU dependent (if provided)
+# generic (if no BSP and no CPU dependent)
+The following illustrates setting the CPU_PROVIDES_IDLE_THREAD_BODY macro:
+.. code:: c
+Implementation details of a CPU model specific IDLE thread body are in the
+next section.
+Idle Thread Body
+The _CPU_Thread_Idle_body routine only needs to be provided if the porter
+wishes to include a CPU dependent IDLE thread body. If the port includes
+a CPU dependent implementation of the IDLE thread body, then the
+CPU_PROVIDES_IDLE_THREAD_BODY macro should be defined to TRUE. This
+routine is prototyped as follows:
+.. code:: c
+ void \*_CPU_Thread_Idle_body( uintptr_t );
+As mentioned above, RTEMS does not require that a CPU dependent IDLE
+thread body be provided as part of the port. If
+independent algorithm is used. This algorithm consists of a "branch to
+self" which is implemented in a routine as follows.
+.. code:: c
+ void \*_Thread_Idle_body( uintptr_t ignored )
+ {
+ while( 1 ) ;
+ }
+If the CPU dependent IDLE thread body is implementation centers upon using
+a "halt", "idle", or "shutdown" instruction, then don’t forget to put it
+in an infinite loop as the CPU will have to reexecute this instruction
+each time the IDLE thread is dispatched.
+.. code:: c
+ void \*_CPU_Thread_Idle_body( uintptr_t ignored )
+ {
+ for( ; ; )
+ /* insert your "halt" instruction here \*/ ;
+ }
+Be warned. Some processors with onboard DMA have been known to stop the
+DMA if the CPU were put in IDLE mode. This might also be a problem with
+other on-chip peripherals. So use this hook with caution.
+.. COMMENT: COPYRIGHT (c) 1988-2002.
+.. COMMENT: On-Line Applications Research Corporation (OAR).
+.. COMMENT: All rights reserved.