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 file system (files, directories, namespace)

# Pipes and network connections (inter-process communication)