This article is from the MachTen & CodeBuilder FAQ, by Jaime Julca firstname.lastname@example.org with numerous contributions by others.
Even though MachTen gives the appearance of being just another Macintosh
application, MachTen and MacOS operate very much like co-resident operating
systems, sharing the processor. When MachTen has control, it pre-emptively
multitasks UNIX applications; when MacOS has control, it runs the Macintosh
applications using its cooperative sharing. There is a slide bar Control
Panel that lets you prioritize UNIX processing over Macintosh processing.