??? Real-time systems are constrained
to execute within limited and defined time windows. A computer recording frequent readings from an instrument
such as a frequency counter or voltmeter could be an example. Recording the values and reacting to them, if
necessary, within a specified time is critical for real-time systems.
Real-time operating systems include special features that tailor them to critical real-world responsiveness.
For instance, process scheduling is usually based strictly on priority rather than on some round-robin timesharing
scheme. The highest-priority task is always the one that will be executing. For example, with a real-time system,
when the valve controlling cooling water to the nuclear reactor needs adjustment, that task will get the CPU,
and it will not have to share time with a word processor.
Some real-time systems also permit preallocation of disk space, either as part of the file system or independent
of the file system. When a program writes to the disk, it will not incur the overhead of a general-purpose file
and directory manager.
Memory management may be more wasteful but faster to execute in a real-time system. Rather than tailor
the amount of memory allocated to a process, the OS may simply dispatch a process to a fixed, large block of
main memory, thus reducing the setup time for the process.
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