@c
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@c
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@c COPYRIGHT (c) 1988-2002.
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@c COPYRIGHT (c) 1988-2002.
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@c On-Line Applications Research Corporation (OAR).
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@c On-Line Applications Research Corporation (OAR).
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@c All rights reserved.
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@c All rights reserved.
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@c
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@c
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@c idlethread.t,v 1.5 2002/01/17 21:47:45 joel Exp
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@c idlethread.t,v 1.5 2002/01/17 21:47:45 joel Exp
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@c
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@c
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@chapter IDLE Thread
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@chapter IDLE Thread
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@section Does Idle Thread Have a Floating Point Context?
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@section Does Idle Thread Have a Floating Point Context?
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The setting of the macro CPU_IDLE_TASK_IS_FP is based on the answer to the
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The setting of the macro CPU_IDLE_TASK_IS_FP is based on the answer to the
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question: Should the IDLE task have a floating point context? If the
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question: Should the IDLE task have a floating point context? If the
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answer to this question is TRUE, then the IDLE task has a floating point
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answer to this question is TRUE, then the IDLE task has a floating point
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context associated. This is equivalent to creating a task in the Classic
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context associated. This is equivalent to creating a task in the Classic
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API (using rtems_task_create) as a RTEMS_FLOATING_POINT task. If
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API (using rtems_task_create) as a RTEMS_FLOATING_POINT task. If
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CPU_IDLE_TASK_IS_FP is set to TRUE, then a floating point context switch
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CPU_IDLE_TASK_IS_FP is set to TRUE, then a floating point context switch
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occurs when the IDLE task is switched in and out. This adds to the
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occurs when the IDLE task is switched in and out. This adds to the
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execution overhead of the system but is necessary on some ports.
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execution overhead of the system but is necessary on some ports.
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If FALSE, then the IDLE task does not have a floating point context.
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If FALSE, then the IDLE task does not have a floating point context.
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NOTE: Setting CPU_IDLE_TASK_IS_FP to TRUE negatively impacts the time
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NOTE: Setting CPU_IDLE_TASK_IS_FP to TRUE negatively impacts the time
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required to preempt the IDLE task from an interrupt because the floating
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required to preempt the IDLE task from an interrupt because the floating
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point context must be saved as part of the preemption.
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point context must be saved as part of the preemption.
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The following illustrates how to set this macro:
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The following illustrates how to set this macro:
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@example
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@example
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#define CPU_IDLE_TASK_IS_FP FALSE
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#define CPU_IDLE_TASK_IS_FP FALSE
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@end example
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@end example
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@section CPU Dependent Idle Thread Body
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@section CPU Dependent Idle Thread Body
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@subsection CPU_PROVIDES_IDLE_THREAD_BODY Macro Setting
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@subsection CPU_PROVIDES_IDLE_THREAD_BODY Macro Setting
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The CPU_PROVIDES_IDLE_THREAD_BODY macro setting is based upon the answer
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The CPU_PROVIDES_IDLE_THREAD_BODY macro setting is based upon the answer
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to the question: Does this port provide a CPU dependent IDLE task
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to the question: Does this port provide a CPU dependent IDLE task
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implementation? If the answer to this question is yes, then the
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implementation? If the answer to this question is yes, then the
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CPU_PROVIDES_IDLE_THREAD_BODY macro should be set to TRUE, and the routine
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CPU_PROVIDES_IDLE_THREAD_BODY macro should be set to TRUE, and the routine
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_CPU_Thread_Idle_body must be provided. This routine overrides the
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_CPU_Thread_Idle_body must be provided. This routine overrides the
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default IDLE thread body of _Thread_Idle_body. If the
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default IDLE thread body of _Thread_Idle_body. If the
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CPU_PROVIDES_IDLE_THREAD_BODY macro is set to FALSE, then the generic
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CPU_PROVIDES_IDLE_THREAD_BODY macro is set to FALSE, then the generic
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_Thread_Idle_body is the default IDLE thread body for this port.
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_Thread_Idle_body is the default IDLE thread body for this port.
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Regardless of whether or not a CPU dependent IDLE thread implementation is
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Regardless of whether or not a CPU dependent IDLE thread implementation is
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provided, the BSP can still override it.
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provided, the BSP can still override it.
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This is intended to allow for supporting processors which have a low power
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This is intended to allow for supporting processors which have a low power
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or idle mode. When the IDLE thread is executed, then the CPU can be
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or idle mode. When the IDLE thread is executed, then the CPU can be
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powered down when the processor is idle.
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powered down when the processor is idle.
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The order of precedence for selecting the IDLE thread body is:
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The order of precedence for selecting the IDLE thread body is:
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@enumerate
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@enumerate
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@item BSP provided
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@item BSP provided
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@item CPU dependent (if provided)
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@item CPU dependent (if provided)
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@item generic (if no BSP and no CPU dependent)
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@item generic (if no BSP and no CPU dependent)
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@end enumerate
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@end enumerate
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The following illustrates setting the CPU_PROVIDES_IDLE_THREAD_BODY macro:
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The following illustrates setting the CPU_PROVIDES_IDLE_THREAD_BODY macro:
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@example
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@example
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#define CPU_PROVIDES_IDLE_THREAD_BODY TRUE
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#define CPU_PROVIDES_IDLE_THREAD_BODY TRUE
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@end example
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@end example
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Implementation details of a CPU model specific IDLE thread body are in the
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Implementation details of a CPU model specific IDLE thread body are in the
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next section.
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next section.
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@subsection Idle Thread Body
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@subsection Idle Thread Body
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The _CPU_Thread_Idle_body routine only needs to be provided if the porter
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The _CPU_Thread_Idle_body routine only needs to be provided if the porter
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wishes to include a CPU dependent IDLE thread body. If the port includes
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wishes to include a CPU dependent IDLE thread body. If the port includes
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a CPU dependent implementation of the IDLE thread body, then the
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a CPU dependent implementation of the IDLE thread body, then the
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CPU_PROVIDES_IDLE_THREAD_BODY macro should be defined to TRUE. This
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CPU_PROVIDES_IDLE_THREAD_BODY macro should be defined to TRUE. This
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routine is prototyped as follows:
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routine is prototyped as follows:
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@example
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@example
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void _CPU_Thread_Idle_body( void );
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void _CPU_Thread_Idle_body( void );
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@end example
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@end example
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As mentioned above, RTEMS does not require that a CPU dependent IDLE
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As mentioned above, RTEMS does not require that a CPU dependent IDLE
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thread body be provided as part of the port. If
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thread body be provided as part of the port. If
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CPU_PROVIDES_IDLE_THREAD_BODY is defined to FALSE, then the CPU
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CPU_PROVIDES_IDLE_THREAD_BODY is defined to FALSE, then the CPU
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independent algorithm is used. This algorithm consists of a "branch to
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independent algorithm is used. This algorithm consists of a "branch to
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self" which is implemented in the XXX routine as follows.
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self" which is implemented in the XXX routine as follows.
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@example
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@example
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XXX check name and exact implementation
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XXX check name and exact implementation
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void _Thread_Idle_body( void )
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void _Thread_Idle_body( void )
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@{
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@{
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while( 1 ) ;
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while( 1 ) ;
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@}
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@}
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@end example
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@end example
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If the CPU dependent IDLE thread body is implementation centers upon using
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If the CPU dependent IDLE thread body is implementation centers upon using
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a "halt", "idle", or "shutdown" instruction, then don't forget to put it
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a "halt", "idle", or "shutdown" instruction, then don't forget to put it
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in an infinite loop as the CPU will have to reexecute this instruction
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in an infinite loop as the CPU will have to reexecute this instruction
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each time the IDLE thread is dispatched.
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each time the IDLE thread is dispatched.
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@example
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@example
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void _CPU_Thread_Idle_body( void )
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void _CPU_Thread_Idle_body( void )
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@{
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@{
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for( ; ; )
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for( ; ; )
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/* insert your "halt" instruction here */ ;
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/* insert your "halt" instruction here */ ;
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@}
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@}
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@end example
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@end example
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Be warned. Some processors with onboard DMA have been known to stop the
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Be warned. Some processors with onboard DMA have been known to stop the
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DMA if the CPU were put in IDLE mode. This might also be a problem with
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DMA if the CPU were put in IDLE mode. This might also be a problem with
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other on-chip peripherals. So use this hook with caution.
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other on-chip peripherals. So use this hook with caution.
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