Operating systems: principles and functions

The operating system of a computer, or of a set of computers connected in a network, is a set of programs that perform two main functions:

• allocate hardware and software resources to meet the needs of application programs ;

• present applications with an interface better suited to their needs than that provided by the hardware, while concealing the complexity of the underlying infrastructure.

It is more accurate to speak of a set of interfaces: users and applications can interact with different layers of the operating system. At the highest level, the interface takes the form of a symbolic or graphical command language; the interpreter of this language is usually outside the operating system itself, and can therefore be replaced or modified. At a lower level is the main interface, made up of a set of "system calls", which enable applications to access the services provided by the system. Finally, some operating systems (microkernel, component-based) have a modular structure that allows access to several different levels of functions, more or less similar to the physical machine.

The evolution of operating systems is governed by both hardware architectures and application requirements. The highlights of the 2000s can be summarized as follows:

• gradual transition from "proprietary" systems (associated with a particular machine architecture, providing a specific interface) to "open" systems (portable on a wide range of machines and featuring standardized interfaces, to facilitate the transport and interconnection of applications); this evolution is linked to the growing spread of free software (GNU project, Free Software Foundation), a prominent example of which is the Linux system;

• the transition from centralized to distributed computing, in which the operating system takes into account the distribution of hardware resources, data and applications, while concealing this distribution from users;

• rapid development of embedded systems, embedded in a variety of objects and environments (vehicles, production sites, smart cards, medical equipment, various sensors), communicating with the physical world, and subject to tight constraints in terms of reaction time, hardware resources and energy consumption. The interconnection of these systems has given rise to the "Internet of Things" and its many potential applications;

• the widespread use of new mobile devices such as smartphones and tablets, which are driving new uses and applications in a wide range of fields;

• development of virtualization techniques,...