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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.