A real-time system is one that must process information and produce a response within a specified time.
For Example Air Traffic Control Systems, Networked Multimedia Systems, Command Control Systems etc
Commercial RealTime System
- Windows CE
Hard RealTime System
Hard real time means timeliness of a computation in the system. If computation is not completed within the specified time then there is no value to a computation and the effects of a late computation may be catastrophic to the system.
Example In a anti missile defense system a system has to process the information (identification of threat and firing a anti missile) within a specified time else it would be of no use and can have vary hard consequences.
Soft Real-time System
Soft real time is a property of the timeliness of a computation where the value diminishes according to its tardiness. A soft realtime system can tolerate some late answers to soft realtime computations, as long as the value hasn't diminished to zero. A soft realtime system will often carry meta requirements such as a stochastic model of acceptable frequency of late computations.
Example: A practical example would be your Mobile phone when you try to find out a phone number and you expect your phone to respond in a timely manner. However, if it takes much time to respond rather than the few milliseconds nothing catastrophic will occur. Perhaps you may shut down some apps to make it fasters.
Many a time Soft real time is applied to an OS which does not guarantee timely computations and these OS are called as quasi-realtime or pseudo-realtime in that they execute realtime activities in preference to others whenever necessary, but don't adequately account for non-schedulable activities in the system.