> 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).
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).
https://en.wikipedia.org/wiki/Mac_OS_nanokernel
https://en.wikipedia.org/wiki/Multiprocessing_Services