the master control program that coordinates the operations of the hardware and software elements of a computer so that they perform quickly and efficiently. An operating system, or OS, may be embodied in both software and “firmware” (integrated circuits programmed with instructions). In personal computers with a hard disk, the operating system software generally resides on the hard disk, from which it is sent to the computer’s random-access memory (RAM) as needed; the firmware is the computer’s read-only memory (ROM; see Integrated Circuits). When such a personal computer is turned on, or “booted” (“boot” is short for “bootstrap,” a term derived from the expression “pulling oneself up by one’s own bootstraps”), the loading of operating system files into RAM is initiated by instructions contained in the BIOS (basic input/output system), located in ROM.

Early computer operating systems (those in service about 1955–65), usually known as executives or monitors, ran only one program at a time and were inefficient because most of the hardware was idle much of the time. For example, no computation could take place during printing. Further, earlier operating systems typically required that the user learn a difficult special language, known as a Job Control Language (JCL) to run each program. Modern computer operating systems tend to be “multitasking”—capable of performing two or more processes at the same time. They automatically allocate resources according to complex rules. Furthermore, they let users run programs via a graphical user interface (GUI, pronounced gooey) featuring visual elements on the monitor screen such as icons, easy-to-understand or “user-friendly” words, and pictures. With a GUI, a hand-guided pointing device such as a mouse can be used to issue commands to the operating system—the user moves the mouse to position a cursor on the appropriate element and then clicks the mouse.

The Operating System’s Role.

An operating system provides the display on the computer monitor with text or graphics; interprets commands issued by the user; shares the hardware among processes such as user programs; allows processes to share data and controls their sharing; keeps track of data and program files; handles errors committed by the system or by applications (individual programs); enables the computer to access input and output devices such as keyboards, joysticks, monitors, printers, and modems; and, in multiuser systems, schedules resources among users as well as processes.

Word processing on a typical desktop or notebook personal computer offers a representative example of the role of an operating system: (1) The user requests word-processing software; the operating system accesses hard-disk storage and loads the word processor program into the computer’s RAM. (2) The user requests a file to edit; the operating system locates the desired file and places it in a buffer area for processing. (3) After editing the file, the user executes the command to save the file; the operating system stores the revised file on the hard disk, perhaps automatically saving the old version into a “backup” copy in case the user has a change of mind. (4) The user requests a printed copy of the file; the operating system loads the printer driver software into RAM and initiates the print job. If the printer is slow, or the document is large, the print job may be temporarily placed on the hard disk and sent to the printer as it requests more output from the word-processing program. (5) The user exits the word-processing software; the operating system may remove the word processor from RAM in order to free up space for the next task. Through all of this, the operating system allocates and manages all the computer’s resources—keyboard, mouse, screen, printer, disks, central processing unit, and memory.

The operating system relies on special instructions within the computer’s instruction set (basic commands). These instructions allow only the operating system to access peripheral (input/output or I/O) devices and memory resources. It is also subject to interrupts (requests from programs or peripheral devices that force it to perform a specific task, such as sending or receiving data).

Major Operating Systems.

Operating systems may be preinstalled on a computer by the manufacturer or purchased from a vendor and then installed on the computer. Some are intended for large, mainframe computers, others for smaller minicomputers, others for small desktop or notebook machines, and still others for “handheld” models; some are for groups of computers joined together in a network; some are usable only on specific platforms (computer architectures that employ the same instruction set, memory management, and resource control patterns), while others are more or less platform-independent and can run on diverse computers with various architectures. Most operating systems are originally developed for specific computer products. Without adaptation, they cannot function on architectures other than the one they were designed for, and they generally can use only software that was developed expressly for them.

UNIX.

The major platform-independent operating system is UNIX, originally developed in 1969 and the early 1970s by Ken Thompson (1943–    ) and Dennis Ritchie (1941–    ) at Bell Laboratories, then the research arm of the AT&T Corp. UNIX is a powerful multitasking operating system that accommodates multiple users. In its early years it required all users to type complicated text-based commands, but in the late 1980s a set of programs called the X Window System was introduced that provided it with an optional GUI.

Legal constraints initially prevented AT&T from marketing UNIX as a commercial product, and the company distributed it to other organizations under license. Many of these organizations made extensive modifications in the operating system, adapting it for a variety of computer platforms. Several companies produced their own proprietary variants. At the beginning of the 21st century, versions of UNIX were in use on platforms ranging from mainframes to desktop computers and even smaller machines.

Two principal variants that emerged early on were BSD (for Berkeley Software Distribution), developed at the University of California at Berkeley, and System V, developed by AT&T and others. Some of the better-known UNIX variants include two used on personal computers: Linux and the Apple Macintosh operating system known as OS X (“X” in this case being the Roman numeral for ten). Several proprietary variants originally developed for more powerful computers, such as “workstations” used by engineers and designers, are also rather widely used—among them Solaris, from Sun Microsystems, Inc., and AIX, from the International Business Machines Corp. (IBM). Also noteworthy is GNU, a UNIX-like operating system and related software sponsored by the Free Software Foundation, which was founded in 1983. (“GNU” is a recursive acronym for “GNU’s Not UNIX.”)

As variants of UNIX, many of them rather incompatible, proliferated, efforts were made toward standardization. The rights to the UNIX trademark were obtained around 1994 by the X/Open standards group, which later became an industry consortium known as the Open Group. Only operating systems that conform to the Open Group’s specifications are permitted to bear the trademarked name UNIX. These specifications include a UNIX-based standard set of interfaces called POSIX (Portable Operating System Interface) that is sponsored by the Institute of Electrical and Electronics Engineers (ISEE). Programs adhering to the POSIX standard can be readily adapted, without fundamental alterations in code, for use on any POSIX-supporting platform.

DEC machines.

The Virtual Address eXtension (VAX) series of medium-size computers manufactured by the Digital Equipment Corp. (DEC) beginning in the late 1970s came into wide use in businesses, factories, and universities. (DEC was acquired in 1998 by the Compaq Computer Corp., which merged with the Hewlett-Packard Company in 2002.) A variant of UNIX called ULTRIX was developed for VAX machines, but DEC also introduced a proprietary operating system called the Virtual Memory System (VMS). VMS provided a text-based interface for multiple users. It could handle files with a high degree of security (privacy) and accommodated a wide variety of peripheral devices such as terminals, disks, tapes, and printers. In addition, it could easily be connected to a network of many other computer systems. VMS functioned well with a wide variety of programming languages, such as COBOL, FORTRAN, Pascal, and C. It also supported large databases and many other products, such as accounting systems and Computer-Aided Design/Computer-Aided Manufacture (CAD/CAM) systems, provided by outside vendors. DEC eventually phased out its VAX line of computers in favor of machines using the company’s more advanced Alpha microprocessor. At the same time, it introduced OpenVMS, a VMS-based operating system that was called “Open” because it incorporated support for the POSIX standard.

IBM mainframes.

IBM developed a system of “host” and “client” operating systems for its 360/370/390 mainframe computers. The host operating system, known as VM (for Virtual Machine), supplied the basic functions on which other operating systems (clients) could run. A typical client system was the Conversational Monitor System (CMS), which dealt with creating, updating, and managing files, as well as compiling and loading programs. CMS worked in tandem with the Control Program (CP), which handled hardware devices required to store files (disks and tapes), work on files (terminals and memory), and output files (printers and terminals). VM/CMS was particularly popular around the 1980s for large-scale commercial and scientific computations.

The common IBM mainframe and large-server operating system known as Multiple Virtual Storage (MVS) may be run as a client under VM. MVS found extensive application in enterprises with large databases and substantial operations involving batch processing (the processing of a large group of data or files at one time). It is the basis of the OS/390 operating system used on IBM’s S/390 line of mainframes. OS/390 in turn provided the foundation for the z/OS operating system used on IBM’s zSeries 900 (z900) line of large servers.

Apple personal computers.

The first generally available GUI was provided by the operating system developed by the Apple Computer Company for its Macintosh desktop computer, introduced in 1984. (A less capable GUI was used in an Apple machine called the Lisa that went on the market the preceding year; the Lisa failed to enjoy commercial success.) The Macintosh operating system grew out of research done at Xerox Corp.’s Palo Alto Research Center in the early 1970s. Its GUI was a radical departure from the text-oriented operating systems that preceded it. The Macintosh operating system was radically revamped for the UNIX-based version called OS X, which was released in 2001. Apple Macintosh computers have used microprocessors produced by the Motorola Corp. and IBM.

Personal computers with Intel-type microprocessors.

Most personal computers use a microprocessor belonging to the Intel Corp. series that began with the 8086/88 and evolved into the 80286, 80386, 80486, Pentium, Pentium Pro, Pentium II, Celeron, Pentium III, and Pentium 4—or a similar microprocessor produced by such competitors as Advanced Micro Devices, Inc., (AMD) and VIA Technologies, Inc. A variety of operating systems have been developed for this type of microprocessor in its various models. The first Intel-based operating system was the Disk Operating System (DOS) that Microsoft Corp. developed for the IBM PC in 1980 on the basis of an existing operating system designed by programmer Tim Paterson (1957?–    ) called QDOS (Quick and Dirty Operating System). Microsoft’s adaptation of QDOS as sold by IBM was known as PC-DOS. A nearly identical version was licensed by Microsoft to other companies under the name MS-DOS. Microsoft’s DOS was a simple single-tasking operating system designed for a single user and employing a text interface. As more advanced hardware and larger memories became available, newer versions of DOS appeared, but they always maintained “upward compatibility” so that old programs generally could run on the newer systems.

On the basis of DOS, Microsoft developed a GUI system known as Windows. Like the Macintosh system, Windows supports a pointing device such as a mouse and an icon-and-menu display where actions are initiated through mouse clicks and a minimal number of keyboard entries. In its initial versions Windows was actually an application that ran on top of DOS. Windows was radically revamped in later versions but retained the ability to run some older DOS–based commands and applications. The first version to gain wide popularity was Windows 3.0, released in 1990. It was followed by Windows 3.1; Windows 3.11; Windows 95, which introduced Internet access; Windows 98; and, in 2000, Windows Me (Millennium Edition). Meanwhile, Microsoft developed a non–DOS-based operating system for businesses and other heavy-duty users requiring more robust and secure computers with better multiuser and networking capabilities. The initial version, called Windows NT (the “NT” reportedly stood for “New Technology”), was released in 1993; versions for DEC’s Alpha chip and certain other processors followed. Windows NT was succeeded in 1999 by Windows 2000, which featured a GUI similar to that of Windows 98, and in 2001 by Windows XP (for “experience”), whose enhancements included a more customizable desktop. Windows XP came in two versions: a full-powered version called Windows XP Professional and a trimmed-down version called Windows XP Home, which supplanted Windows Me as Microsoft’s principal personal computer operating system for consumers.

Another GUI operating system for personal computers with Intel-type processors is Operating System 2 (OS/2), which like Windows NT was designed to work well in a multiuser server in a network, not just in a single-user computer. Developed by IBM (initially with the joint participation of Microsoft), OS/2 was the first multitasking operating system designed for Intel-type, or “IBM-compatible,” personal computers. It was initially released in 1987 in a text-based version and was provided with a GUI the following year. An enhanced version called OS/2 Warp saw its first release in 1994.

Several UNIX-derived operating systems, such as Solaris, have been adapted for Intel-type microprocessors. By far the most popular UNIX variant run on Intel-type machines is Linux, a POSIX-compliant operating system that was originally developed specifically for the Intel platform. It is named after Linus Torvalds (1969–    ), who wrote its core, or kernel, in 1991 while a student at the University of Helsinki in Finland. In contrast to Windows, OS/2, and other proprietary systems, Linux is open-source software; it is available to the public for free via the Internet (it is also distributed commercially by private companies), and its continuing development is carried out by volunteers, under the overall guidance of Torvalds. Versions of Linux have been developed for a variety of other platforms, including Sun’s Sparc microprocessor, the Alpha chip, and IBM’s S/390.

Handhelds.

Microsoft applied its Windows brand name to other operating systems that it developed, including those for the small computers known as personal digital assistants (PDAs) and “handhelds,” for cable TV set-top boxes, and for certain other electronic devices. The GUI provided by the systems resembles the Windows used with personal computers. The first version of the company’s Windows CE (“CE” is said to derive from “Consumer Electronics”) was introduced in 1996. Windows CE underlies Microsoft’s Pocket PC operating system, first released in 2000.

Palm, Inc., developed a proprietary GUI operating system called the Palm OS specifically for its popular PDAs, the first of which, the PalmPilot, went on the market in 1996. Another frequently encountered operating system used in handhelds, mobile phones, and the like is the Symbian OS from Symbian Ltd. Previously known as EPOC, it was originally introduced by Psion P.L.C.

Network Operating Systems.

Computers are often interconnected in networks for ease of file and software sharing. Such networks may be controlled by an auxiliary operating system, such as Novell NetWare, or by a computer operating system that contains special provisions for networking, such as UNIX or Windows NT and its successors. The network operating system permits users’s individual computers to share peripheral devices and communicate with the network’s server computers, which may store data as well as support various network services needed by the users. The individual computers also have their own operating systems, which may differ from each other; users need to enter explicit commands in order to gain access to files on other machines in the network.

Researchers are exploring an alternative way of networking computers that uses a so-called distributed operating system. In this approach, the computers share a common operating system; a process can run on any machine in the network and access data on any of the networked machines. In contrast to a traditional network, the computers are connected seamlessly, so that the network looks to users like a single computer—that is, users are not aware of the existence of multiple computers. Examples of distributed operating systems include Amoeba, developed at the Free University of Amsterdam in the Netherlands, and Sprite, developed at the University of California at Berkeley.

For further information on this topic, see the Bibliography, sections 541. Computer history–542. Computer applications.