There was only one platform designed to be POSIX in all respects, the OSF/1 UNIX for the Alpha processor from Digital Equipment Corporation. It was first know

There was only one platform designed to be POSIX in all respects, the OSF/1 UNIX for the Alpha processor from Digital Equipment Corporation. It was first known as DEC OSF/1, then Digital UNIX, then Tru64. Previous to OSF/1, DEC had sold Ultrix, first on VAX, then on MIPS. https://en.wikipedia.org/wiki/Tru64_UNIX https://en.wikipedia.org/wiki/Tru64_UNIX The wiki says that OSF/1 was first released for MIPS, but that was before my time, and it wasn't supported for long. I did use Ultrix on MIPS DECStations in college. https://en.wikipedia.org/wiki/Ultrix https://en.wikipedia.org/wiki/Ultrix