Mac OS nanokernel

The Mac OS nanokernel is an extremely simple 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 an extremely simple, 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.

The nanokernel is very different from the Copland OS microkernel, although they were created in succession with similar goals.

The original nanokernel, and the tightly integrated Mac 68k emulator, were written by emulation consultant Gary Davidian. Its main purpose is to allow the existing Motorola 68K version of the operating system to run on new hardware. As such, the normal state of the system is to be running 68K code. The operating system does little until activated by an interrupt, which is quickly mapped to its 68K equivalent within the virtual machine.

Other tasks may include switching back to PowerPC mode, if necessary, upon completion of the interrupt handler, and mapping the Macintosh virtual memory system to the PowerPC hardware. However, as the software is little documented, these might instead be handled by the emulator running in user mode.

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