In an effort to clarify Mark's article on the incompatibility between the IIvx and Apple's Unix operating system, A/UX 3.0 (TidBITS-157), Pythaeus writes:

The Mac IIvx does not run A/UX 3.0 for the same reason that System 6 does not run on the Quadra 700: the CPU was released long after the system software shipped, so the operating system (OS) can't possibly know about the addresses and capabilities of the new hardware. A/UX 3.0 does such a good job of making Unix look like a Macintosh that people forget the operating system is Unix, not MacOS. A/UX does not sit on top of System 7 (a la Tenon), but the other way around. It makes no sense to talk about A/UX being "System 7.1 compatible" once you understand this fact. Unless the hardware designers bend over backwards to maintain compatibility, you will always need a new OS (or new OS components) to support new hardware. The Mac IIvx was released after A/UX 3.0, and contains new ASICs (like its sound chip), so A/UX 3.0 has no clue what to do.