PRIMOS was written in Fortran IV and assembler. It was an operating system descended from Multics.
The source code is available here: http://www.andrewpetermarlow.co.uk/goodies/Rev_19p2p3.tar.bz2
There is an emulator that was announced some time ago already: http://groups.google.es/group/comp.sys.prime/browse_thread/thread/e4ca58eb664f9299/05c8b40bc198db8c?lnk=gst&q=prime+emulator#05c8b40bc198db8c
Note: I hope Pacolinux completes this article 😀