What unique characteristics do mainframe operating systems have?

Answer 1

I'm not an expert but I had some experience with an AS400 and what I learned was it operates on a reduced instruction set which is hard coded into the hardware. This means that any input is directly handled instead of first being interpreted and recompiled as pc's do. They also use larger data chunks, like 64 and 128 bits. Hope that somewhat answers you question.

Answer 2

Most "mainframes" (the term is really replaced by servers) run some flavor of Unix. Unix was originally proprietary, so Linus Torvald rewrote his own version it and licensed it as open source - hence GNU/ Linux. MacOS also descended from Unix, more specifically BSD. The main differences are really syntactical if you want to think about it that way. They are all functional for almost any user needs (with the exception of gaming). You can use a "desktop" OS as a server (I do it with dekstop distros of linux) - but you might be forced to restart more often, since Server OSes usually only force restart on critical security updates. So the only real "fundamental" difference is this: Open source vs. Proprietary. You have MacOS and Windows which don't allow customization of the OS because their source code is pre-compiled, and then you have things like Unix, Linux, and FreeBSD which can be toyed with and tweaked for efficiency to your hearts content.