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MACH is the the basic OS layer NeXT uses for NEXTSTEP. It is a micro kernel, which means it is extensible at runtime. Micro kernel often stands for a small kernel size, too, but due to the compatibility to BSD 4.3 MACH is currently about 1MB in size.
Features of MACH are: loadable kernel services (extensions during runtime), different scheduling algorithms, an advanced messaging system, an advanced memory allocation mechanism (copy on demand, world wide message broadcasting), true multitasking, multi threading and BSD compatibility.