Difference Between Soft, Hard and Firm Real-Time System

A real-time system can be classified into either hard, soft, or firm real-time system based on the consequences of the task missing the deadline. It is not necessary that all tasks of a real-time application belong to the same category but it is possible that different tasks of a real-time system can belong to different categories.

Soft Real-Time System

A soft real-time system is one in which performance is degraded but not destroyed by failure to meet response-time constraints.

If several tasks miss deadline, then the performance of the system is said to have degraded.

Soft Real Time System

An example of a soft real-time task is web browsing. Normally, after an URL (Uniform Resource Locater) is clicked, the corresponding web page is fetched and displayed within a couple of seconds on the average. However, when it takes several minutes to display a requested page, we still do not consider the system to have failed, but merely express that the performance of the system has degraded.


Hard Real-Time System

A hard real-time system is one in which failure to meet even a single deadline may lead to complete or catastrophic system failure.

The task deadlines are in the order of micro or milliseconds.

Many hard real-time systems are safety critical

Examples: Industrial Control Applications, On-board computers, Robots

Hard Real Time System

Firm Real-Time System

A firm real-time system is one in which a few missed deadlines will not lead to total failure, but missing more than a few may lead to complete or catastrophic system failure.

Firm Real Time System

Every firm real-time task is associated with some predefined deadline before which it is required to produce its results. However, unlike a hard real-time task, even a firm real-time task does not complete within its deadline, the system doesn’t fail but the late results are merely discarded. In other words, the utility of the results computed by a firm real-time task becomes zero after the deadline. 

Examples: A video conferencing application, Satellite-based surveillance applications

A Sampling of Hard, Firm, and Soft Real-Time Systems


Real-Time Classification


Avionic weapons delivery 

system in which pressing a 

button launches air-to-air 



Missing the deadline to launch 

the missile within a specified 

time after pressing the button 

may cause the target to be 

missed, which will result in 


Navigation controller for an 

autonomous weed-killer robot


Missing a few navigation 

deadlines causes the robot to 

veer out from a planned path 

and damage some crops.

Console hockey game


Missing even a several 

deadlines will only degrade 


The Library of Congress print-

manuscript database system


The database system is being 

used for research purpose and 

it limits its urgency. It seems it 

would be acceptable for 

missed deadlines to degrade 

the system.

A police database that 

provides information about 

stolen automobile


If the system miss the 

deadline  to give the result, it 

will not 

result in system failure and the 

late result would only degrade 

the performance.

An Automatic Teller Machine


As long as the ATM is 

executing timely even though 


miss deadline frequently it’s 

output will still have value

A coin operated video game


Missing even a several 

deadlines will only degrade 


A university grade-processing 



Although the system may 


response times of days or 


weeks, it must respond within 

certain time or there could be 

an academic or financial 


Video Conferencing


When a certain frame is being 

played, if some preceding 

frame arrives at the receiver, 

then this frame is of no use 

and is discarded.

Satellite based tracking of 

enemy movements


When the ground computer is 

being overloaded, an new 

image may be received even 

before an older image is taken 

up for processing. In this case, 

the older image is of not much 


Difference Between Soft, Hard and Firm Real-Time System Difference Between Soft, Hard and Firm Real-Time System Reviewed by Nischal Lal Shrestha on February 14, 2021 Rating: 5

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