Real Time Operating System

A real time operating system is used in environments where a large number of events, mostly external to the computer system, must be accepted and processed in a short time or with in certain deadline.

Such application include industrial control, telephone switching equipment, real time simulations and flight control. A real-time application requires timely response from the computer for the correctness of its functioning.

All the real-time applications will have a worst-case response time, which it can tolerate, i.e. the largest value of the response time for which it can function correctly.

Real-time systems are also used in military applications. A real-time operating system is one, which fulfills the worst-case response time requirements of an application. The primary objective of the real-time operating system is to meet the scheduling deadline. User convenience and resource utilization are of secondary

concern to real-time operating system. A real time operating system provides the following facilities to fulfill the worst-case response time requirements of the application:

·        Multitasking within an application

·        Deadline oriented or priority driven scheduling

·        Programmer defined interrupts

Most of real time system use priority based preemptive scheduling in which higher priority process preempts execution of lower priority process. Each process is assigned a certain level priority that corresponds to the relative importance of the event that it services. The processor is normally allocated to the highest priority process among those that are ready to execute. The ability to specify priorities for the tasks provides additional flexibility while structuring an application to meet its response time.

A real time operating system also permits the application program to define new kinds of interrupts relevant in application domain, assigns processing priorities for these interrupts, and supplies its own routines to perform interrupt processing.

There are two types of real time systems: hard real time systems and soft real time systems.  A hard real time system makes sure that a critical task complete on time, to achieve this goal all the delays in the system must be bounded. The soft real time system is less restrictive type of real time system in which the real time tasks gets priority over other tasks and retains that priority until it completes.

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