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@c  idlethread.t,v 1.5 2002/01/17 21:47:45 joel Exp

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@chapter IDLE Thread

@section 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:

@example

#define CPU_IDLE_TASK_IS_FP      FALSE

@end example

@section CPU Dependent Idle Thread Body

@subsection CPU_PROVIDES_IDLE_THREAD_BODY Macro Setting

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:

@enumerate

@item BSP provided

@item CPU dependent (if provided)

@item generic (if no BSP and no CPU dependent)

@end enumerate

The following illustrates setting the CPU_PROVIDES_IDLE_THREAD_BODY macro:

@example

#define CPU_PROVIDES_IDLE_THREAD_BODY    TRUE

@end example

Implementation details of a CPU model specific IDLE thread body are in the

next section.

@subsection 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:

@example

void _CPU_Thread_Idle_body( void );

@end example

As mentioned above, RTEMS does not require that a CPU dependent IDLE

thread body be provided as part of the port.  If

CPU_PROVIDES_IDLE_THREAD_BODY is defined to FALSE, then the CPU

independent algorithm is used.  This algorithm consists of a "branch to

self" which is implemented in the XXX routine as follows.

@example

XXX check name and exact implementation

void _Thread_Idle_body( void )

@{

  while( 1 ) ;

@}

@end example

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.

@example

void _CPU_Thread_Idle_body( void )

@{

  for( ; ; )

    /* insert your "halt" instruction here */ ;

@}

@end example

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.