fast os switching... 1678

That was me, and yes, I also know about it from having been a happy user at a long-ago place of employment.

Nice description, and consistent with how I understand the system to work, more or less reasoning from first principles (or something along those lines).

However, it seems like there would be a fair amount of overhead if all privileged instructions executed by "guest" O-S's (the ones running "under" CP) generated interrupts. I'm not clear on details, but I believe there were-are hardware features that allow some of this overhead to be bypbutted. Googling for "hardware buttist" and some form of the operating system's name (VM-CP, VM-370, z-VM) ought to turn up useful references, for anyone who wants to pursue it. A lot of the ones I found seem to be from the extensive archives of one of the regular posters in alt.folklore.computers, Lynn Wheeler. Lots of good stuff there, if sometimes a little hard to follow.

Sing it, brother! that was exactly the appeal to me as well, in about the same time frame -- well, that and the fact that if your new stuff didn't work, there were more debugging facilities under VM than on real hardware. But running MVS (large mainframe O-S of the era, for those who don't know this environment) with TSO ("time-sharing option" -- support for interactive users) under VM on a heavily-loaded system .... Not fast. Very much "not fast".

-- B. L. Mbuttingill ObDisclaimer: I don't speak for my employers; they return the favor.