The operating system is a program that acts as an intermediary between a computer user and the computer hardware. We shall use both the abbreviation OS and the complete denomination operating system. An operating system has the following main objectives: first it has to execute user programs and facilitate troubleshooting for users; secondly it has to make the computing system easy to use; and third it has to use efficiently the hardware of the computing system [1], [2]. The operating system offers services to both users and developers who make possible the operation of a computer without the need of employing low-level hardware controls, which are difficult run. The operating system provides relatively uniform interfaces to access a wide range of devices the computer interacts with, from input/output devices, such as printers or digital cameras, to wire or wireless network components that ensure communication among computers. The operating system allows users to create, manage and organize different types of files [11]. Most modern operating systems provide graphical interfaces (GUI – Graphical User Interface) to ensure the relatively easy usage for customers [6], [9], [10], [18].

II. OPERATING SYSTEMS

Operating systems usually provided a set of APIs, named system calls. However safer operating systems have been adopted recently. Currently we have multithreading and multicore architectures integrated into our operating systems. Modern operating systems for mobile devices support many media formats: audio, video, image, and may use video cameras, touchscreens, GPS, accelerometers, accelerated 3D graphics and other types of sensors. System calls have changed so dramatically in the past two years that they actually became irrelevant. This change may be attributed to two companies, Apple and Google, which are real innovation forces. With the launch of the iPhone by Apple a new paradigm has been introduced on the market, i.e. the one where the keywords are controlled user interfaces and application stores. Google followed this example with the introduction of the Android operating system, which was quickly adopted by both application developers and users. This caused a major change in the way we think about operating systems, [8], [12], [13], [14], [15].