alt Hacker News> You are describing attributes of the NeXT-based OS X but calling it MacOS 9.
You're mistaken.
> MacOS 9 [had] no preemption
Classic Mac OS has had vertical retrace (VBL) interrupts since System 1.0 (which are a form of preemption). But suppose we disregard that.
>> MacOS 9 had a preemptively scheduled kernel.
The GP is correct. Applications run cooperatively within the Blue task, but the Blue task itself runs preemptively with other MP tasks (which an application can create).
I know about the nanokernel, yes. Without following the links, Gary Davidian (is that spelled right?) did amazing work.
Very very little used it. About the only classic app I ever heard of that could even use pre-emption and background execution were some demanding Photoshop filters. Enough that Daystar Digital built multiprocessor Mac clones just for that alone:
https://lowendmac.com/1995/daystar-genesis-mp/
Within the MacOS kernel it is as coop as ever, and there were no different OSes inside virtual machines. No, Tenon MachTen does not count.