Chapter
5
Task Management
The design process of a real-time application generally involves splitting the work to be
completed into tasks, each responsible for a portion of the problem. μC/OS-III makes it
easy for an application programmer to adopt this paradigm. A task (also called a thread ) is
a simple program that thinks it has the Central Processing Unit (CPU) all to itself. On a
single CPU, only one task can execute at any given time.
μC/OS-III supports multitasking and allows the application to have any number of tasks. The
maximum number of task is actually only limited by the amount of memory (both code and
data space) available to the processor. Multitasking is the process of scheduling and switching
the CPU between several tasks (this will be expanded upon later). The CPU switches its
attention between several sequential tasks. Multitasking provides the illusion of having multiple
CPUs and, actually maximizes the use of the CPU. Multitasking also helps in the creation of
modular applications. One of the most important aspects of multitasking is that it allows the
application programmer to manage the complexity inherent in real-time applications.
Application programs are typically easier to design and maintain when multitasking is used.
Tasks are used for such chores as monitoring inputs, updating outputs, performing
computations, control loops, update one or more displays, reading buttons and keyboards,
communicating with other systems, and more. One application may contain a handful of
tasks while another application may require hundreds. The number of tasks does not
establish how good or effective a design may be, it really depends on what the application
(or product) needs to do. The amount of work a task performs also depends on the
application. One task may have a few microseconds worth of work to perform while
another task may require tens of milliseconds.
Tasks look like just any other C function except for a few small differences. There are two
types of tasks: run-to-completion (Listing 5-1) and infinite loop (Listing 5-2). In most
embedded systems, tasks typically take the form of an infinite loop. Also, no task is allowed
to return as other C functions can. Given that a task is a regular C function, it can declare
local variables.
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