What are some examples of real time operating systems and why are they needed?

Real time
operating systems are generally special-purpose ones designed to run embedded or specialized
systems. These systems often operate in environments where there are significant constraints on
hardware design and often narrow tasks that need to be done with great accuracy and speed. Real
time systems are used in military, engineering, medical, and data communications systems (among
others).

A traditional, general-purpose operating system such as Mac OS or
Windows is designed for general consumers and imposes a massive overhead on the hardware,
demanding significant amounts of storage for a wide variety of features ranging from playing and
recording videos to editing photographs, playing music, monitoring internet security,
downloading updates, and running business applications. This means that a significant amount of
storage space and processing power end up devoted to things that simply aren't needed in a
missile guidance system or an intelligent thermostat. Real time operating systems often consist
of very small executive programs that simply monitor or control devices, have basic user
interfaces, and are capable of running a very small number of applications, sometimes just one
piece of software that accomplishes its main function and some sort of diagnostic
software.

Typical examples of real time operating systems are those custom
written for the Galileo and Voyager space probes. Because of extreme power and weight
constraints, they needed to be coded extremely efficiently to use as little memory and to
accomplish their tasks in as few operations as possible. The operating system needed to be able
to interface with a limited group of devices in short windows, such as planet fly-bys or course
changes, and then go into a dormant mode to conserve power.

Process control
and embedded hardware and software are often paired with real time operating systems. The
software embedded in intelligent missiles and military drones also needs to operate in real
time. It needs to be able to quickly process and prioritize interrupts from software and
hardware including commands from external sources and information from various sensors including
navigational devices and internal monitoring devices that might give warnings of system
malfunctions. The reason that a real time operating system is required for these is that
off-the-shelf systems are often too inefficient and resource hungry for such
applications.