RTLINUX

One of the best-known realtime Linux implementations; the development was sponsored by Finite State Machine Labs (FSMLabs), who now sell RTLinux and related products. RTLinux is available under the GPL.
RTLinux claims genuine hard realtime, via an RT kernel which runs the realtime processes, and runs the Linux kernel as its least-priority thread. Processes not requiring realtime response are written as standard Linux apps. Realtime processes are implemented either as realtime threads or interrupt handlers, and are controlled by the RT kernel. The RTLinux project claims a typical 15usec response time to an interrupt assertion on the x86 platform, with an admonishment to compare the worst-case response time (less than 30ms on a 486-33, "near the hardware limit"). Realtime threads follow the POSIX realtime conventions for pthreads. RTLinux is available on x86 (incl. SMP), PPC and Alpha; floating-point support within realtime threads requires the availability of hardware FP, currently working on the x86 and PPC architectures. There appears to be no ARM support.