A real time system can be defined as a "system capable of guaranteeing timing requirements of the processes under its control". Real time system can be broadly classified into soft real time systems and hard real time systems. In hard real time systems, the completion of an operation after its deadline is considered useless - ultimately, this may lead to a critical failure of the complete system. A soft real-time system on the other hand will tolerate such latency, and may respond with decreased service quality (e.g., dropping frames while displaying a video).
RTAI is not a real time operating system, such as RTLinux, VXworks or pSOS. It is based on the Linux kernel, providing the ability to make it fully pre-emptable. RTAI offers the same services of the Linux kernel core, adding the features of an industrial real time operating system. It consists basically of an interrupt dispatcher: RTAI mainly traps the peripherals interrupts and if necessary re-routes them to Linux. It is not an intrusive modification of the kernel; it uses the concept of HAL (hardware abstraction layer) to get information from Linux and to trap some fundamental functions. RTAI considers Linux as a background task running when no real time activity occurs.
How to install RTAI in Linux:
RTAI is module oriented approach. To understand about the linux loadable kernel modules read this post.
http://embeddedbox.blogspot.com/2008/02/linux-loadable-kernel-moduels.html
The installation for ubuntu hardy is explained very well here : https://woc.uc.pt/deec/getFile.do?tipo=2&id=5690 . Basically the installation for any distribution, this procedure is same.
These are some good links about RTAI:
1. https://www.rtai.org/
2. http://rtportal.upv.es/comparative/rtl_vs_rtai.html
3. http://www.captain.at/rtai.php
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