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An operating system (OS) is a computer program that manages the hardware and software resources of a computer. At the foundation of all system software, the OS performs basic tasks such as controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating networking, and managing files. It also may provide a graphical user interface for higher level functions.
Every program running on a computer, be it background services or applications, is a process. As long as a von Neumann architecture is used to build computers, only one process per CPU can be run at a time. Older microcomputer OS such as MS-DOS did not attempt to bypass this limit, with the exception of interrupt processing, and only one process could be run under them (although DOS itself featured TSR as a very partial and not too easy to use solution). Mainframe operating systems have had multitasking capabilities since the early 1960's. Modern operating systems enable concurrent execution of many processes at once via multitasking even with one CPU. Process management is an operating system's way of dealing with running multiple processes. Since most computers contain one processor with one core, multitasking is done by simply switching processes quickly. Depending on the operating system, as more processes run, either each time slice will become smaller or there will be a longer delay before each process is given a chance to run. Process management involves computing and distributing CPU time as well as other resources. Most operating sytems allow a process to be assigned a priority which affects its allocation of CPU time. Interactive operating systems also employ some level of feedback in which the task with which the user is working receives higher priority. Interrupt driven processes will normally run at a very high priority. In many systems there is a background process, such as the System Idle Process in Windows, which will run when no other process is waiting for the CPU.
According to Parkinson's law "Data expands to fill the space available for storage". Programmers prefer a memory of infinite size and infinite speed. Nowadays most of the computer's memory is arranged in a hierarchical manner, starting from the fastest registers, CPU cache, RAM and disk storage. The memory manager in an OS coordinates the memories by tracking which one is available, which is to be allocated or deallocated and how to swap between the main memory and secondary memories. This activity, which is usually referred to as virtual memory management, greatly increases the amount of memory available for each process. There is a speed penalty associated with swapping RAM to disk or other slower storage. If running processes require significantly more RAM than is available, either because one process requires a large amount of RAM or two (or more) processes each require less RAM than is available in the system but multitasking results in constant swapping of each process to slower storage, leading to thrashing.
Another important part of memory management activity is managing virtual addresses. If multiple processes are in memory at once, they must be prevented from interfering with each other's memory (unless there is an explicit request to share a limited amount of memory, in controlled ways). This is achieved by having separate address spaces. Each process sees the whole virtual address space (typically from address 0 up to the maximum size of virtual memory) as uniquely assigned to it (ignoring the fact that some areas are OS reserved). The operating system maintains tables to match virtual addresses to physical addresses. This process is called "paging".
The operating system tracks all memory used by each process so that when a process terminates, all memory used by that process will be available for other processes.
Disk and file systems
Operating systems have a variety of native file systems. Linux has a greater range of native file systems, those being: ext2, ext3, ReiserFS, Reiser4, GFS, GFS2, OCFS, OCFS2, NILFS and Google File System. Linux also has full support for XFS and JFS, along with the FAT file systems, and NTFS. Windows, on the other hand, has limited file system support which only includes: FAT12, FAT16, FAT32, and NTFS. The NTFS file system is the most efficient and reliable of the four Windows systems. All the FAT systems are older than NTFS and have limitations on the partition and file size that can cause a variety of problems.
For most of the above file systems, there are two ways it can be allocated. Each system can be journaled or non-journaled, journaled being the safer alternative under the circumstances of a system crash. If a system comes to an abrupt stop in a system crash scenario, the non-journaled system will need to undergo an examination from the system check utilities, whereas the journaled file systems recovery is automatic. Microsoft's NTFS is journaled along with most Linux file systems including ext3, reiserfs and JFS (except ext2).
Most modern file systems are made up of similar directories and subdirectories. Along with the operating systems file system similarities, there are subtle differences. Microsoft separates its directories with a backslash and its file names are case insensitive whereas Unix/Linux-derived operating systems use the forward slash and their file names generally are case sensitive.
Most current operating systems are capable of using the TCP/IP networking protocols. This means that one system can appear on a network of the other and share resources such as files, printers, and scanners.
Many operating systems also support one or more vendor-specific legacy networking protocols as well, for example, SNA on IBM systems, DECnet on systems from Digital Equipment Corporation, and Microsoft-specific protocols on Windows. Specific protocols for specific tasks may also be supported such as NFS for file access.
Many operating systems include some level of security. Security is based on the two ideas that:
- The operating system provides access to a number of resources, directly or indirectly, such as files on a local disk, privileged system calls, personal information about users, and the services offered by the programs running on the system;
- The operating system is capable of distinguishing between some requesters of these resources who are authorized (allowed) to access the resource, and others who are not authorized (forbidden). While some systems may simply distinguish between "privileged" and "non-privileged", systems commonly have a form of requester identity, such as a user name. Requesters, in turn, divide into two categories:
- Internal security: an already running program. On some systems, a program once it is running has no limitations, but commonly the program has an identity which it keeps and is used to check all of its requests for resources.
- External security: a new request from outside the computer, such as a login at a connected console or some kind of network connection. To establish identity there may be a process of authentication. Often a username must be quoted, and each username may have a password. Other methods of authentication, such as magnetic cards or biometric data might, be used instead. In some cases, especially connections from the network, resources may be accessed with no authentication at all.
In addition, to the allow/disallow model of security, a system with a high level of security will also offer auditing options. These would allow tracking of requests for access to resources (such as, "who has been reading this file?").
Security of operating systems has long been a concern because of highly sensitive data held on computers, both of a commercial and military nature. The United States Government Department of Defense (DoD) created the Trusted Computer System Evaluation Criteria (TCSEC), which is a standard that sets basic requirements for assessing the effectiveness of security. This became of vital importance to operating system makers, because the TCSEC was used to evaluate, classify and select computer systems being considered for the processing, storage and retrieval of sensitive or classified information.
Internal security can be thought of as protecting the computer's resources from the programs concurrently running on the system. Most operating systems set programs running natively on the computer's processor, so the problem arises of how to stop these programs doing the same task and having the same privileges as the operating system (which is after all just a program too). Processors used for general purpose operating systems generally have a hardware concept of privilege. Generally less privileged programs are automatically blocked from using certain hardware instructions, such as those to read or write from external devices like disks. Instead, they have to ask the privileged program (operating system) to read or write. The operating system therefore gets the chance to check the program's identity and allow or refuse the request.
An alternative strategy, and the only strategy available where the operating system and user programs have the same hardware privilege, is the operating system not running user programs as native code, but instead either emulates a processor or provides a host for a p-Code based system such as Java.
Internal security is especially relevant for multi-user systems; it allows each user of the system to have private files that the other users cannot tamper with or read. Internal security is also vital if auditing is to be of any use, since a program can potentially bypass the operating system, inclusive of bypassing auditing.
Typically an operating system offers (hosts) various services to other network computers and users. These services are usually provided through ports or numbered access points beyond the operating systems network address. Typically services include offerings such as file sharing, print services, email, web sites, and file transfer protocols. At the front line of security are hardware devices known as firewalls. At the operating system level, there are various software firewalls. A software firewall is configured to allow or deny traffic to a service running on top of the operating system. Therefore, one can install and be running an insecure service, such as Telnet or FTP, and not have to be threatened by a security breach because the firewall would deny all traffic trying to connect to the service on that port.
Graphical user interfaces
Today, most modern operating systems contain Graphical User Interfaces (GUIs, pronounced goo-eez). A few older operating systems tightly integrated the GUI to the kernel—for example, the original implementations of Windows and Mac OS. More modern operating systems are modular, separating the graphics subsystem from the kernel (as is now done in Linux, and Mac OS X, and to a limited extent Windows).
Many operating systems allow the user to install or create any user interface they desire. The X Window System in conjunction with GNOME or KDE is a commonly found setup on most Unix and Unix derivative (BSD, Linux, Minix) systems. However, some operating systems do not give such a flexible GUI, such as Windows—these operating systems require the use of software to modify the existing GUI, and more often than not, they are only able to change simple attributes such as menu style, colours, etc.
GUIs tend to change with time. For example, Windows has modified its GUI every time a new major version of Windows is released and the Mac OS GUI changed dramatically with the introduction of Mac OS X.
A device driver is a specific type of computer software developed to allow interaction with hardware devices. Typically this constitutes an interface for communicating with the device, through the specific computer bus or communications subsystem that the hardware is connected to, providing commands to and/or receiving data from the device, and on the other end, the requisite interfaces to the operating system and software applications. It is a specialized hardware-dependent computer program which is also operating system specific that enables another program, typically an operating system or applications software package or computer program running under the operating system kernel, to interact transparently with a hardware device, and usually provides the requisite interrupt handling necessary for any necessary asynchronous time-dependent hardware interfacing needs.
The key design goal of device drivers is abstraction. Every model of hardware (even within the same class of device) is different. Newer models also are released by manufacturers that provide more reliable or better performance and these newer models are often controlled differently. Computers and their operating systems cannot be expected to know how to control every device, both now and in the future. To solve this problem, OSes essentially dictate how every type of device should be controlled. The function of the device driver is then to translate these OS mandated function calls into device specific calls. In theory a new device, which is controlled in a new manner, should function correctly if a suitable driver is available. This new driver will ensure that the device appears to operate as usual from the operating systems' point of view for any person.
The first computers did not have operating systems. By the early 1960s, commercial computer vendors were supplying quite extensive tools for streamlining the development, scheduling, and execution of jobs on batch processing systems. Examples were produced by UNIVAC and Control Data Corporation, amongst others.
Through the 1960s, several major concepts were developed, driving the development of operating systems. The development of the IBM System/360 produced a family of mainframe computers available in widely differing capacities and price points, for which a single operating system OS/360 was planned (rather than developing ad-hoc programs for every individual model). This concept of a single OS spanning an entire product line was crucial for the success of System/360 and, in fact, IBM's current mainframe operating systems are distant descendants of this original system; applications written for the OS/360 can still be run on modern machines. OS/360 also contained another important advance: the development of the hard disk permanent storage device (which IBM called DASD). Another key development was the concept of time-sharing: the idea of sharing the resources of expensive computers amongst multiple computer users interacting in real time with the system. Time sharing allowed all of the users to have the illusion of having exclusive access to the machine; the Multics timesharing system was the most famous of a number of new operating systems developed to take advantage of the concept.
Multics, particularly, was an inspiration to a number of operating systems developed in the 1970s, notably Unix by Dennis Richie and Ken Thompson. Another commercially-popular minicomputer operating system was VMS.
The first microcomputers did not have the capacity or need for the elaborate operating systems that had been developed for mainframes and minis; minimalistic operating systems were developed, often loaded from ROM and known as Monitors. One notable early disk-based operating system was CP/M, which was supported on many early microcomputers and was largely cloned in creating MS-DOS, which became wildly popular as the operating system chosen for the IBM PC (IBM's version of it was called IBM-DOS or PC-DOS), its successors making Microsoft one of the world's most profitable companies. The major alternative throughout the 1980s in the microcomputer market was Mac OS, tied intimately to the Apple Macintosh computer.
By the 1990s, the microcomputer had evolved to the point where, as well as extensive GUI facilities, the robustness and flexibility of operating systems of larger computers became increasingly desirable. Microsoft's response to this change was the development of Windows NT, which served as the basis for Microsoft's entire operating system line starting in 1999. Apple rebuilt their operating system on top of a Unix core as Mac OS X, released in 2001. Hobbyist-developed reimplementations of Unix, assembled with the tools from the GNU Project, also became popular; versions based on the Linux kernel are by far the most popular, with the BSD derived UNIXes holding a small portion of the server market.
The growing complexity of embedded devices has led to increasing use of embedded operating systems.
Modern operating systems usually have a Graphical user interface (GUI) which uses a pointing device such as a mouse or stylus for input in addition to the keyboard. Older models and Operating Systems not designed for direct-human interaction (such as web-servers) generally use a Command line interface (or CLI) typically with only the keyboard for input. Both models are centered around a "shell" which accepts and processes commands from the user (eg. clicking on a button, or a typed command at a prompt). The choice of OS may depend on the hardware architecture, specifically the CPU, with only Linux and BSD running on almost any CPU. Windows NT, which is no longer supported, was ported to the DEC Alpha and MIPS Magnum. Since the early 1990s, the choice for personal computers has largely been limited to the Microsoft Windows family, the Unix-like family and Linux, of which Linux and Mac OS X are becoming the major alternatives. Mainframe computers and embedded systems use a variety of different operating systems, many with no direct connection to Windows or Unix.
- IBM PC compatible - Microsoft Windows, Unix variants, and Linux variants.
- Apple Macintosh - Mac OS X (a Unix variant), Windows (x86 variants only), Linux and BSD
The earliest operating systems were developed for mainframe computer architectures in the 1960's. The enormous investment in software for these systems caused most of the original computer manufacturers to continue to develop hardware and operating systems that are compatible with those early operating systems. Those early sytems pioneered many of the features of modern operating systems. Mainframe operating systems that are still supported include:
- Burroughs MCP-- B5000,1961 to Unisys Clearpath/MCP, present.
- IBM OS/360 -- IBM System/360, 1964 to IBM zSeries, present
- UNIVAC EXEC 8 -- UNIVAC 1108, 1964, to Unisys Clearpath IX, present.
Modern mainframes typically also run Linux or Unix variants.
Embedded systems use a variety of dedicated operating systems and limited versions of Linux or other operating systems. In some cases, the "operating system" software is directly linked to the application to produce a monolithic special-purpose program. In the simplest embedded systems, there is no distinction between the OS and the application. Embedded systems that have certain time requirements are known as Real-time operating systems.
The Unix-like family is a diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "UNIX" is a trademark of The Open Group which licenses it for use with any operating system that has been shown to conform to their definitions. "Unix-like" is commonly used to refer to the large set of operating systems which resemble the original Unix.
Unix systems run on a wide variety of machine architectures. They are used heavily as server systems in business, as well as workstations in academic and engineering environments. Free software Unix variants, such as Linux and BSD, are increasingly popular. They are used in the desktop market as well, for example Ubuntu, but mostly by hobbyists.
Some Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that vendor's proprietary hardware. Others, such as Solaris, can run on both proprietary hardware and on commodity x86 PCs. Apple's Mac OS X, a microkernel BSD variant derived from NeXTSTEP, Mach, and FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS. Over the past several years, free Unix systems have supplanted proprietary ones in most instances. For instance, scientific modeling and computer animation were once the province of SGI's IRIX. Today, they are dominated by Linux-based or Plan 9 clusters. 
The team at Bell Labs that designed and developed Unix went on to develop Plan 9 and Inferno, which were designed for modern distributed environments. They had graphics built-in, unlike Unix counterparts that added it to the design later. Plan 9 did not become popular because, unlike many Unix distributions, it was not originally free. It has since been released under Free Software and Open Source Lucent Public License, and has an expanding community of developers. Inferno was sold to Vita Nuova and has been released under a GPL/MIT license.
The Microsoft Windows family of operating systems originated as a graphical layer on top of the older MS-DOS environment for the IBM PC. Modern versions are based on the newer Windows NT core that first took shape in OS/2 and borrowed from OpenVMS. Windows runs on 32-bit and 64-bit Intel and AMD processors, although earlier versions also ran on the DEC Alpha, MIPS, and PowerPC architectures (some work was done to port it to the SPARC architecture).
As of 2004, Windows held a near-monopoly of around 90% of the worldwide desktop market share, although some predict this to dwindle due to the increased interest in open source operating systems.  It is also used on low-end and mid-range servers, supporting applications such as web servers and database servers. In recent years, Microsoft has spent significant marketing and R&D money to demonstrate that Windows is capable of running any enterprise application (see the TPC article).
The most recent addition to the Microsoft Windows family is Microsoft Windows XP, released on October 25, 2001. The latest stable release is Windows XP Service Pack 2, released on August 6, 2004.
Microsoft is currently releasing a new version of Windows, Windows Vista (formerly codenamed "Longhorn"), which contains new functionality, particularly in security, network administration, and digital rights management. The GUI has been updated, as well; In addition to a new look for its standard 2D desktop interface, systems with sufficient processing power can use a 3D interface called Aero Glass.
Mainframe operating systems, such as IBM's z/OS, and embedded operating systems such as VxWorks, eCos, and Palm OS, are usually unrelated to Unix and Windows, except for Windows CE, Windows NT Embedded 4.0 and Windows XP Embedded which are descendants of Windows, and several *BSDs, and Linux distributions tailored for embedded systems. OpenVMS from Hewlett-Packard (formerly DEC), is still under active development.
Older operating systems which are still used in niche markets include OS/2 from IBM; Mac OS, the non-Unix precursor to Apple's Mac OS X; BeOS; XTS-300.
Popular prior to the Dot COM era, operating systems such as AmigaOS and RISC OS continue to be developed as minority platforms for enthusiast communities and specialist applications.
Research and development of new operating systems continues. GNU Hurd is designed to be backwards compatible with Unix, but with enhanced functionality and a microkernel architecture. Microsoft Singularity is a research project to develop an operating system with better memory protection based on the .Net managed code model.
- Operating systems category
- History of operating systems
- List of operating systems
- Comparison of operating systems
- Comparison of open source operating systems
- Comparison of Linux distributions
- Comparison of BSD operating systems
- Comparison of kernels
- Operating systems timeline
- Important publications in operating systems
- Hollywood operating system - computer clichés in movies and television
- Monolithic Kernel – Microkernel – Nanokernel – Exokernel – Virtual machine – System call
- Asymmetric and Symmetric Multiprocessing (SMP) – Clustering – Distributed computing
- Real-time operating system – Time-sharing – Multitasking – Embedded system – Single-user – Multi-user
- Orthogonally persistent capabilities versus access control lists
- Object-oriented operating system
- Disk operating systems
- Hard disk drive partitioning
- LiveCD OS - Operating Systems bootable from a CD without need of hard disk installation.
- Operating system advocacy
- OS-tan (Personification of operating systems)
- Deitel, Harvey M.; Deitel, Paul; Choffnes, David (2004). Operating Systems. Upper Saddle River, NJ: Pearson/Prentice Hall. ISBN 0-13-182827-4.
- Silberschatz, Abraham; Galvin, Peter Baer; Gagne, Greg (2004). Operating System Concepts. Hoboken, NJ: John Wiley & Sons. ISBN 0-471-69466-5.
- Tanenbaum, Andrew S.; Woodhull, Albert S. (2006). Operating Systems. Design and Implementation. Upper Saddle River, N.J.: Pearson/Prentice Hall. ISBN 0-13-142938-8.
Wiktionary, the free dictionary.
- Operating systems at dmoz.org
- Multics History and the history of operating systems