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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.
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.