Enterprise realtime and cooperative development

Yeah, but a system with a 3 hour hard realtime bound can in practice be 
implemented with pencil and paper in many cases: virtually any normal OS 
could handle bounds like that (OK, so it couldn't *guarantee*, but it 
could push the probability down arbitrarily far, farther down than, say, 
the likelihood of hardware failure in that timespan).

Hell, I've implemented hard-realtime hardware control systems with 3s 
bounds using mlock()ed processes on ordinary Unix platforms. (Your only 
problem then is priority inversion, and the easiest solution to that is to 
dictate what is running on the box, which is generally 'not a lot' for 
such systems, and to take care with the locking design of one's own 
program, if it includes processes running at different priorities.)

The only time realtime stuff is interesting for the OS is when priority 
inversion really is an issue or when the realtime bounds are quite tight.