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OS X is based upon the NeXTSTEP/OPENSTEP OSes developed in the 1980s and 1990s by NeXT Inc., which Apple purchased in 1996.

Mac OS X consists of a Mach/BSD-based kernel, userland primarily based on FreeBSD, and additional frameworks (written in C, C++ and Objective-C) providing user interface and application-level services.

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