Page 258 - Embedded Microprocessor Systems Real World Design

SEQUENTIAL SCHEDULING TIME SLICING
SAY THERE ARE THREE TASKS, EACH THE SAME THREE TASKS EACH
WITH THREE OPERATIONS TO PERFORM IS GIMNSPEC~F~C TIMESLCES
IN SEQUENTIAL OPERATION. EACH TASK EACH TASK RUNS ONLY UNTIL ITS
RUNS UNTIL FINISHED TIME SLICE IS UP
ONE TIME SLICE
-- 4 -
TASK 1, OPERATION 1
OPERATION 2 --f-

--f -
OPERATION 3
TASK 3 IS HIGHER PRIORITY
GETS TWO TIME SLICES
EACH TIME IT RUNS
NOTE: FOR SIMPLICIM, EACH OPERATION IS
ONE TIME SLICE IN LENGTH. IN AN ACTUAL
SYSTEM, THE OPERATIONS WOULD BE OF
VARYING LENGTHS AND WOULD BE HALTED
IN MID-OPERATION AT THE END OF A
TIME SLICE
Figure 9.2
Sequential Versus Time-Sliced Operation.

Most RTOSs can support time slicing or sequential scheduling. Sequential
scheduling also can check for and stop tasks that hog the CPU. In any scheduling
system, of course, only one task at a time actually has control of the CPU. Figure
9.2 illustrates the difference between time slicing and sequential operation.

Preemptive Scheduling

Preemptive scheduling is the most common method of scheduling tasks when using
an RTOS, and it is one of the primary advantages of using an RTOS. Under
preemptive scheduling, a task runs until it is finished or until a task of higher
priority preempts it. Before going into more detail about preemptive scheduling,
however, we should look at RTOS’s task handling in general.

Activation and Deactivation of Tasks
Tasks under RTOS can be ready or not ready. The RTOS keeps a list of tasks that
are ready and what their execution priority is. A ready task is added to the task list
and executed in sequence. When a task becomes not ready, it is removed from the
list. Going back to the protocol converter example, the output task might go ready
when there is data in the FIFO buffer and become not ready when the FIFO buffer
is empty.
A ready task may be inhibited from running because something blocks further
execution. For example, the protocol converter output task may be ready because

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