from:
http://en.wikipedia.org/wiki/RSTS/E
"Mentec would later publicly grant a no-cost license to anyone for
non-commercial
hobby purpose use of the operating system software developed for the
PDP-11,
and as the result of the release (for non-commercial use) of a PDP-11
emulator
for the IBM-PC, as well as image copies of disk drives of RSTS which
exist on
the Internet, it is possible for a hobbyist to run RSTS/E on a
personal computer
in software emulation as fast or faster than it originally ran on the
real hardware."
It's on the web, so it must be true!