Real-time Operating System

Real-time operating systems are 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 within certain deadlines. Such applications are industrial control , telephone switching equipment, flight control, and real-time simulation .

The real-time operating systems can be of two types -

1. Hard Real-time Operating System- These operating systems guarantee that critical tasks be completed within a certain range of time. For example, suppose in take an concept of rocket propulsion.... Time is very much important factor for rocket propulsion... Certain deadline is set , and after that deadline rocket is been crashed without even flying.
2. Soft Real-time Operating System- This operating system provides some relaxation in time limit. For example- Multimedia systems , digital audio system etc..

Explicit, programmer-defined and controlled processes are encountered in real-time systems. A separate process is charged with handling a single external event. The process is activated upon occurrence of the related event signaled by an interrupt.

Multitasking operation is accomplished by scheduling processes for execution independently for each other. Each process is assigned a certain level of priority that corresponds to the relative importance of the event that it services. The processor is allocated to the highest-priority process. Higher-priority processes preempt execution of the lower-priority processes. This type of scheduling , called , priority-based preemptive scheduling is used by real-time systems.

Memory management in real-time systems is comparatively less demanding than in other types of multiprogramming systems. The primary reason for this is that many processes permanently reside in memory in order to provide quick response time.

One of the main characteristics of real-time systems is time-critical device management. In addition to providing sophisticated forms of interrupt management and I/O buffering, real-time operating systems provide system calls to allow user processes to connect themselves to interrupt vectors and to service events directly.

File management is found only in larger installations of real-time systems. The file-management of real-time systems includes protection and access control. The primary objective of file-management in real-time systems is speed of access , rather then efficient utilization of secondary storage .