GNU Hurd
Mach () is a kernel developed at Carnegie Mellon University by Richard Rashid and Avie Tevanian and derived originally from 4.3BSD to support operating system research, primarily distributed and parallel computing. Mach is often mentioned as one of the earliest examples of a microkernel. However, not all versions of Mach are microkernels. Mach's derivatives are the basis of the operating system kernel in GNU Hurd and of Apple's XNU kernel used in macOS, iOS, iPadOS, tvOS, and watchOS.