The computer's master control program. When a computer is turned on, a small "boot program" loads the operating system. Although additional system modules may be loaded as needed, the main part, known as the "kernel" resides in memory (RAM) at all times.
The operating system (OS) sets the standards for all application programs that run in the computer. Applications "talk to" the operating system for all user interface and file management operations. Also called an "executive" or "supervisor," an operating system performs the following functions.
All graphics based today, the user interface includes the windows, menus and method of interaction between the user and the computer. Prior to graphical user interfaces (GUIs), all operations were performed by typing in commands. However, command-line interfaces are not extinct. In fact, they are included in all major operating systems, and technical operations are commonly executed from the command line by programmers and administrators. Command line statements often get the job done more efficiently than with a GUI, and they are easily grouped in a file and executed all at once (see batch file
Operating systems may support optional interfaces. Although the overwhelming majority of people work with the default interface, different "shells" offer variations of functionality, and "skins" provide different appearances. See GUI
Job management controls the time and sequence that applications are run. Common in the server and mainframe environment, IBM's job control language (JCL) was developed decades ago to schedule the daily work. The execution of short scripts at specific times throughout the day or at the end of the month is common in Unix/Linux servers. In a desktop environment, batch files and PowerShell scripts for Windows or shell scripts for Mac can be written to perform a sequence of operations. See batch file
and shell script
Multitasking, which is the ability to simultaneously execute multiple programs, is available in all operating systems today. Critical in the server and mainframe environment, applications can be prioritized to run faster or slower depending on their purpose. In the desktop world, multitasking is more often than not "task switching," which keeps applications open so users can bounce back and forth among them. See multitasking
Data management keeps track of the data in storage (disk or SSD). The application program deals with data by file name and a particular location within the file. The operating system's file system knows where the data are physically stored (which sectors), and interaction between the application and operating system is through the programming interface (API). When an application needs to retrieve or save data, it makes a call to the file system, which is in charge of actually opening, reading, writing and closing files. See solid state drive
and file system
Device management controls peripheral devices by sending them commands in their proprietary machine language. The software routine that deals with each device is called a "driver," and the OS requires drivers for each of the peripherals attached to the computer. When a different type of peripheral is attached, that device's driver must be added to the operating system if not previously installed. See driver
Operating systems provide password protection to keep unauthorized users out of the system. Activity logs are maintained, which may provide time accounting for billing purposes. They generally provide backup and recovery routines for starting over in the event of a system failure.
In the 1950s, programmers wrote their own input/output routines to read and write magnetic tape. When magnetic disks came on the scene several years later, it became essential to have a separate program to manage them as data were added and deleted. In addition, running more than one application at a time (timesharing and multitasking) required a control program to keep track of everything. Thus, the operating system was born. Today, most all computing devices use an operating system, the exception being a device with custom hardware (see ASIC
Common Operating Systems
The primary computer operating systems in use are Windows Server, Windows XP, 7, 8 and 10, macOS, the many versions of Linux and Unix, IBM i (from the midrange AS/400) and z/OS (IBM mainframes). DOS is still used for some applications, and there are several more (see real-time system
and embedded system
Mobile operating systems provide almost the same functions as the OS in a desktop computer or server. Smartphones and tablets use Apple's iOS, Google's Android, BlackBerry's QNX and Microsoft's Windows 10 Mobile.
The operating system is the master control program in the computer.
Drivers and Peripherals
Operating System and Applications
The operating system communicates with the computer's peripherals via the software drivers for the devices. Older PCs had other drivers (see legacy driver
When running, applications constantly command the operating system to display information on screen and perform file read/write operations.