Operating systems can be viewed from two viewpoints: resource managers and extended machines. In the resource-manager view, the operating system’s job is to manage the different parts of the system efficiently. In the extended-machine view, the system provides the users with abstractions that are more convenient to use than the actual machine. These include processes, address spaces, and files.

Operating systems have a long history, starting from the days when they replaced the operator, to modern multiprogramming systems. Highlights include early batch systems, multiprogramming systems, and personal computer systems.

Since operating systems interact closely with the hardware, some knowledge of computer hardware is useful to understanding them. Computers are built up of processors, memories, and I/O devices. These parts are connected by buses.

Operating systems can be structured as monolithic, layered, microkernel/client-server, virtual machine, or exokernel/unikernel systems. Regardless, the basic concepts on which they are built are processes, memory management, I/O management, the file system, and security. The main interface of an operating system is the set of system calls that it can handle. These tell us what it really does.