According to a prevailing myth, if you scratch a POSIX operating system, you'll find UNIX beneath the surface! A POSIX OS is therefore too large and unsuitable for embedded systems.
The fact, however, is that POSIX is not UNIX. Although the POSIX standards are rooted in existing UNIX practice, the POSIX working groups explicitly defined the standards in terms of “interface, not implementation.”
Thanks to the precise specification within the standards, as well as the availability of POSIX test suites, nontraditional OS architectures can provide a POSIX API without adopting the traditional UNIX kernel. Compare any two POSIX systems and they'll look very much alike—they'll have many of the same functions, utilities, etc. But when it comes to performance or reliability, they may be as different as night and day. Architecture makes the difference.
Despite its decidedly non-UNIX architecture, QNX Neutrino implements the standard POSIX API. By adopting a microkernel architecture, the OS delivers this API in a form easily scaled down for realtime embedded systems or incrementally scaled up, as required.