An OS microkernel developed at Carnegie Mellon University. It provides a low-level abstract layer onto which other operating systems can be ported. Operations that it handles are: interprocess communication, low-level I/O, memory management and scheduling. MacOS X runs a BSD-based "Mach server" atop the Mach microkernel. FSF's Hurd OS also runs atop Mach using a "multiple server" design.