This group gathers pioneers of software and its engineering whose primary contribution was an operating system, kernel or the system software that manages a computer hardware and resources, including time sharing systems, Unix and its descendants, microkernels, personal computer and mobile operating systems, and the file systems, shells and virtual memory schemes built into them. It follows the Operating Systems second-level heading of the ACM Computing Classification System under Software and its engineering. Their shared work is the software layer that runs a machine and manages its resources for every other program, as distinct from an application, a language or a network protocol.