Examples of Real-time Operating Systems

author is ramu@madras , some of the concepts form the manuscripts of class

Real-time operating systems(RTOS) are used in situations to handle real-life scenarios. A few examples of real-time operating systems include:
1. VxWorks: This OS is part of the Mars 2020 rover (2020 launch). Also, in the past, it was used in Phonix Mars lander, Boing 787, Honda Robot ASIMO etc.
2. QNX: QNX Neutrino TROS finds widespread use in embedded systems. Thus, it is compatible with platforms like ARM and x86. Industries using QNS are automotive, railway transportation and health-care.
3. eCos: eCos is an open-source real-time operating system. Example of eCos use is Chibis-M microsatellite‘s attitude and stabilization control system. 
4. RTLinux: RTLinux is a hard real-time operating system. It runs the Linux operating system as a full preemptive process. As a result, it is useful in controlling robots, data acquisition systems, manufacturing plants

What is a Real-Time Operating System(RTOS)?

A real-time operating system is a time crucial operating system. This means that the response to any event must come in a specified time interval only. Hence, a delay in response will result in disastrous effects.

For instance, an operating system like Windows10 which comes as the default OS in most laptops or PCs is not a real-time operating system. The reason is that even if your application say, VLC player starts with a delay it is not going to have any harmful impact. On the contrary, an OS used in an aircraft is a real-time operating system because if the landing gears do not come out in a specified interval after issuing the command, the aircraft will crash.

So, all situations like Red-light crossing, autonomous cars etc., are all real-life operating systems. An RTOS can be event-driven or time-sharing. In event-driven strategy, the OS shifts tasks only if a higher priority task requires servicing. In time-sharing, tasks are shifted in a round-robin fashion based on time quantum.

Types of Real-time operating systems

1. Soft Real Time OS

A Soft RTOS is a system in which the deadline for certain tasks can be delayed to some extent. For example, if the task deadline is 1:20:30PM, then the task can on occasions complete at let us say 1:20:35PM every. However, it can not delay for too long say 1:30PM.

2. Hard Real Time OS

A Hard RTOS is a system which meets the deadline for every process at all times. For example, if the task deadline is 1:20:30PM, then the task has to complete before 1:20:30PM every time.

Differences between Soft RTOS and Hard RTOS

Characteristic	Hard RTOS	Soft RTOS
Response Time	Strict deadline	soft deadline
Safety	Critical	Not critical
Data Integrity	Short term	Long Term
Error Detection	Automatic	User assisted

Scheduling Algorithms for RTOS

CPU scheduling algorithms used for RTOS are also different from normal CPU scheduling algorithms like FCFS, SJF etc. For example, a couple of popular algorithms are:
1. Rate Monotonic scheduling
2. Earliest Deadline First scheduling