Explain the layered aspect of a UNIX system. What are the layers? What does it mean to say they are layers?

A UNIX system has essentially three main layers: ? The hardware ? The operating system kernel ? The user-level programs The kernel hides the system?s hardware underneath an abstract, high-level programming interface. It is responsible for implementing many of the facilities that users and user-level programs take for granted. The kernel assembles all of the following UNIX concepts from lower-level hardware features: ? Processes (time-sharing, protected address space) ? Signals and semaphores ? Virtual Memory (swapping, paging, and mapping) ? The filesystem (files, directories, namespace) ? Pipes and network connections (inter-process communication)