A hard real-time system (also known as an immediate real-time system) is hardware or software that must operate within the confines of a stringent deadline. The application may be considered to have failed if it does not complete its function within the allotted time span. Simply put hard real-time systems are systems that consider any missed time deadline as system failure. Hard real-time systems are not necessarily fast systems rather these systems focus on predictability.
Examples of hard real-time systems
Air France Flight 447 crashed into the ocean after a sensor malfunction caused a series of system errors. The pilots stalled the aircraft while responding to outdated instrument readings. All 12 crew and 216 passengers were killed.
Mars Pathfinder spacecraft was nearly lost when a priority inversion caused system restarts. A higher priority task was not completed on time due to being blocked by a lower priority task. The problem was corrected and the spacecraft landed successfully.
An Inkjet printer has a print head with control software for depositing the correct amount of ink onto a specific part of the paper. If a deadline is missed then the print job is ruined.
Why stringent time requirements for real-time systems?
Consider the example of a car crash where the sensor detects the collision and passes on the information to the ECU responsible to deploy the airbag to protect the occupants of the vehicle. The whole system should complete detection and associated calculation to the deployment of airbags within a specified time which is approx 55 mili seconds. If the airbag system fails to deploy within the set deadline then it will result in serious injury or possible death of passenger and/or driver. For example, consider a car traveling at 100 kmph and a sudden crash occurs and it took 500ms to deploy the airbags. In this case, it does not matter whether the intended task (airbag deployment) was accurately completed or not, if timelines are not met it won't be useful. Hence hard real-time systems, not only depends on accuracy result or action but also whether such action or results are completed within the set deadline or not.
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