and various types of line printers (the impact mechanism must be activated at the right time as the print mechanism comes into alignment with the desired output). A failure in any of these would cause either missing output or misaligned output. In the context of multitasking systems the scheduling policy is normally priority driven (pre-emptive schedulers). In some situations, these can guarantee hard real-time performance (for instance if the set of tasks and their priorities is known in advance). There are other hard real-time schedulers such as rate-monotonic which is not common in general-purpose systems, as it requires additional information in order to schedule a task: namely a bound or worst-case estimate for how long the task must execute. Specific algorithms for scheduling such hard real-time tasks exist, such as earliest deadline first, which, ignoring the overhead of context switching, is sufficient for system loads of less than 100%. New overlay scheduling systems, such as an adaptive partition scheduler assist in managing large systems with a mixture of hard real-time and non real-time applications. Firm real-time systems are more nebulously defined, and some classifications do not include them, distinguishing only hard and soft real-time systems. Some examples of firm real-time systems: The assembly line machine described earlier as hard real-time could instead be considered firm real-time. A missed deadline still causes an error which needs to be dealt with: there might be machinery to mark a part as bad or eject it from the assembly line, or the assembly line could be stopped so an operator can correct the problem. However, as long as these errors are infrequent, they may be tolerated. Soft real-time systems are typically used to solve issues of concurrent access and the need to keep a number of connected systems up-to-date through changing situations. Some examples of soft real-time systems: Software that maintains and updates the flight plans for commercial airliners. The flight plans must be kept reasonably current, but they can operate with the latency of a few seconds. Live audio-video systems are also usually soft real-time. A frame of audio that's played late may cause a brief audio glitch (and may cause all subsequent audio to be delayed correspondingly, causing a perception that the audio is being played slower than normal), but this may be better than the alternatives of continuing to play silence, static, a previous audio frame, or estimated data. A frame