Jim: MachTen is a Mach-based environment that runs as an application under the classic Mac OS (I run it under OS 9.2.2, but it will happily run under Systems as old as 7.x). Much like Mac OS X, Mach operates as the system's kernel and a BSD userland (in this case based on BSD4.4) sits on top; unlike OS X, however, the Mach kernel runs as a regular Mac application and implements a virtual machine which manages the pre-emptive multitasking of processes running within MachTen. In effect, it's OS X turned inside out: in OS X Tiger, the classic MacOS runs as an app within Mach, whereas under MachTen, Mach runs as an app within the classic MacOS.
Jim: http://www.floodgap.com/retrotech/machten/
James: So basically it's Copland before all the devs left and they released OS 8 instead.
