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Chapter 5
Note that the OSTaskSuspend() function unconditionally blocks a task and this
task will not actually wait for an event to occur but in fact, waits until another
task calls OSTaskResume() to make the task ready to run.
F5-6(5)
Assuming that CPU interrupts are enabled, an interrupting device will suspend
execution of a task and execute an Interrupt Service Routine (ISR). ISRs are
typically events that tasks wait for. Generally speaking, an ISR should simply
notify a task that an event occurred and let the task process the event. ISRs
should be as short as possible and most of the work of handling the
interrupting devices should be done at the task level where it can be managed
by μC/OS-III. ISRs are only allowed to make “Post” calls (i.e., OSFlagPost() ,
OSQPost() , OSSemPost() , OSTaskQPost() and OSTaskSemPost() ). The only
post call not allowed to be made from an ISR is OSMutexPost() since mutexes,
as will be addressed later, are assumed to be services that are only accessible at
the task level.
As the state diagram indicates, an interrupt can interrupt another interrupt. This
is called interrupt nesting and most processors allow this. However,
interrupt nesting easily leads to stack overflow if not managed properly.
Internally, μC/OS-III keeps track of task states using the state machine shown in Figure 5-7.
The task state is actually maintained in a variable that is part of a data structure associated
with each task, the task’s TCB. The task state diagram was referenced throughout the design
of μC/OS-III when implementing most of μC/OS-III’s services. The number in parentheses is
the state number of the task and thus, a task can be in any one of eight (8) states (see OS.H,
OS_TASK_STATE_??? ).
Note that the diagram does not keep track of a dormant task, as a dormant task is not
known to μC/OS-III. Also, interrupts and interrupt nesting is tracked differently as will be
explained further in the text.
This state diagram should be quite useful to understand how to use several functions and
their impact on the state of tasks. In fact, I’d highly recommend that the reader bookmark
the page of the diagram.
94
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