Sommerville-Software-Engineering-10ed

21.4 ■ Real-time operating systems 631
only runs when processor time is available. In general, it is simpler to design a system
so that there are a small number of process frequencies. The execution times
represent the required worst-case execution times of the processes.
The final step in the design process is to design a scheduling system that will
ensure that a process will always be scheduled to meet its deadlines. You can only do
this if you know the scheduling approaches that are supported by the real-time operating
system (OS) used (Burns and Wellings 2009). The scheduler in the real-time
OS allocates a process to a processor for a given amount of time. The time can be
fixed, or it may vary depending on the priority of the process.
In allocating process priorities, you have to consider the deadlines of each process so
that processes with short deadlines receive processor time to meet these deadlines. For
example, the voltage monitor process in the burglar alarm needs to be scheduled so that
voltage drops can be detected and a switch made to backup power before the system
fails. This should therefore have a higher priority than the processes that check sensor
values, as these have fairly relaxed deadlines compared to their expected execution time.
21.4 Real-time operating systems
The execution platform for most application systems is an operating system that
manages shared resources and provides features such as a file system and runtime
process management. However, the extensive functionality in a conventional operating
system takes up a great deal of space and slows down the operation of programs.
Furthermore, the process management features in the system may not be designed to
allow fine-grain control over the scheduling of processes.
For these reasons, standard operating systems, such as Linux and Windows, are not
normally used as the execution platform for real-time systems. Very simple embedded
systems may be implemented as “bare metal” systems. The systems provide their own
execution support and so include system startup and shutdown, process and resource
management, and process scheduling. More commonly, however, embedded applications
are built on top of a real-time operating system (RTOS), which is an efficient
operating system that offers the features needed by real-time systems. Examples of
RTOS are Windows Embedded Compact, VxWorks, and RTLinux.
A real-time operating system manages processes and resource allocation for a
real-time system. It starts and stops processes so that stimuli can be handled, and it
allocates memory and processor resources. The components of an RTOS (Figure
21.18) depend on the size and complexity of the real-time system being developed.
For all except the simplest systems, they usually include:
1. A real-time clock, which provides the information required to schedule processes
periodically.
2. If interrupts are supported, an interrupt handler, which manages aperiodic
requests for service.