A real time system is defined as follows - A real-time system is one in which the correctness of the computations not only depends upon the logical correctness of the computation but also upon the time at which the result is produced. If the timing constraints of the system are not met, system failure is said to be occurred.


Two types Hard real time operating system Strict time constraints Secondary storage limited or absent Conflicts with the time sharing systems Not supported by general purpose OS Soft real time operating system Reduced Time Constraints Limited utility in industrial control or robotics Useful in applications (multimedia, virtual reality) requiring advanced operating-system features. In the robot example, it would be hard real time if the robot arriving late causes completely incorrect operation. It would be soft real time if the robot arriving late meant a loss of throughput. Much of what is done in real time programming is actually soft real time system. Good system design often implies a level of fe/correct behaviour even if the computer system never completes the computation. So if the computer is only a little late, the system effects may be somewhat mitigated.



Hat makes an os a rtos?
1. A RTOS (Real-Time Operating System) has to be multi-threaded and preemptible.
2. The notion of thread priority has to exist as there is for the moment no deadline driven OS.
3. The OS has to support predictable thread synchronisation mechanisms
4. A system of priority inheritance has to exist
5. For every system call, the maximum it takes. It should be predictable and independent from the number of objects in the system
6. the maximum time the OS and drivers mask the interrupts. The following points should also be known by the developer:
1. System Interrupt Levels.

2. Device driver IRQ Levels, maximum time they take, etc.