Page 257 - Embedded Microprocessor Systems Real World Design

operations, such as activating a task. Knowing this, you can predict the impact the
RTOS will have on system performance.
An RTOS comes in two basic flavors: kernels and full operating systems. A kernel
usually implements the basic task and memory management functions. A full
operating system may have drivers for disk drives, serial ports, and other common
resources. One common characteristic of RTOSs is that the system hardware must
generate a regular interrupt (called a timer tick or just a tick), say, at 20Hz (50ms).
This is used for timekeeping, task scheduling, and other functions. Not all RTOSs
require a system timer interrupt. Real-time operating systems typically support the
following functions:

Multitasking, which includes:
Activation and deactivation of tasks
Setting task priorities
Scheduling tasks

Communication between tasks
Memory management

Multitasking

This is the process of scheduling tasks or processes so that they all appear to operate
simultaneously. In the protocol converter example, the receive, XON, output, and
XOFF processes appear to a human user to run simultaneously because the sequen-
tial, one-at-a-time operation is so fast. All the functions of task activation/deactiva-
tion, scheduling, and ranking are part of the multitasking function. In a sequential
program, none of these operations, which are required for true multitasking, is
implemented.
Multitasking also can be implemented by time slicing. In this method, tasks are
switched every tick. Every time the interrupt occurs, a different task is given control,
so each task gets to execute for one tick time (50ms for the 20Hz tick example
given previously). The overall execution speed for a given task depends on the
number of tasks. Higher-priority tasks can be allowed to execute for more than one
tick time. A task that needs less than a full tick to execute can terminate early, giving
the remainder of its time to the next task.
Sequential scheduling and time slicing are essentially the same except that
sequentially scheduled tasks run until finished and time-sliced tasks run until their
time is up. Tasks operating under either scheduling scheme can voluntarily relin-
quish control before finishing. In that case, they can be restarted where they left
off instead of starting over.

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