They tend to be made for specific processors/microcontrollers, and a lot of them can be very expensive.
By the way, I just realized that you're asking for real time linux distros and not embedded linux distros. I seem to have misread the first time. A lot of the embedded distros I mentioned are not necessarily strict real time.
I thought Linux is supposed to be Free and Open-source all around...how is it that companies charge money for it ?
I'm not too familiar with the subject, can you explain a little more...
I think the 2 versions that you mentioned aren't true Linux.
It's what's called a microkernel...while true Linux is considered to be a "Monolithic Kernel"
This is what I asked about:
I thought Linux is supposed to be Free and Open-source all around...how is it that companies charge money for it ?
I'm not too familiar with the subject, can you explain a little more...
I don't have specific examples in mind for embedded real time platforms, but for desktop environments for example, Red Hat Enterprise is not free. You tend to pay for official technical support I believe.
You can read more about uclinux of great flexibility.. there are different ports for different processors available online. . But note that there is not strong realtime operation in linux