The Mac OS nanokernel is an operating system kernel serving as the basis of most PowerPC based system software versions 7 through 9 of the classic Mac OS, predating Mac OS X. The initial revision of this software is a single tasking system which delegates most tasks to an emulator running the Motorola 68000 series (68K) version of the operating system. The second major revision supports multitasking, multiprocessing, and message passing, and would be more properly called a microkernel. Unlike the 68K-derived Mac OS kernel running within it, the PowerPC kernel exists in a protected memory space and executes device drivers in user mode.