Programming Embedded Systems, 2nd Edition

Tasks

Different types of tasks can run in an operating system. For example, a task can be periodic, where it exits after its work is complete. The task can then be restarted when there is more work to be done. Typically, however, a task runs forever, similar to the infinite loop discussed in Chapter 3. Each task also has its own stack that is typically allocated statically.

Task States

Remember how we said that only one task could actually use the processor at a given time? That task is said to be the running task, and no other task can be in that same state at the same time. Tasks that are ready to run—but are not currently using the processor—are in the ready state, and tasks that are waiting for some event external to themselves to occur before going on are in the waiting state. Figure 10-4 shows the relationships between these three states.

Figure 10-4. Possible states of a task

A transition between the ready and running states occurs whenever the operating system selects a new task to run during a scheduling point. The task that was previously running leaves the running state, and the new task (selected from the queue of tasks in the ready state) is promoted to running. Once it is running, a task will leave that state only if it terminates, if a higher-priority task becomes ready, or if it needs to wait for some event external to itself to occur before continuing. In the latter ...