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An operating system (OS) is a software program that manages the hardware and software resources of a computer. A key component of system software, the OS performs basic tasks, such as controlling and allocating memory, prioritizing the processing of instructions, controlling input and output devices, facilitating networking, and managing files.

Modern general-purpose computers, including personal computers and mainframes, have an operating system to run other programs, such as application software. Examples of operating systems for personal computers include Microsoft Windows, Linux, Mac OS (Darwin), and Unix.

The main advantages of an operating system include:

  • Allows multiple programs to run concurrently.
  • Simplifies the programming of application software because the program does not have to manage the hardware. The operating systems manages all hardware and the interaction of software. It also gives the program a high level interface to the hardware and ways of interacting with other programs.
The lowest level of any operating system is its kernel. This is the first layer of software loaded into memory when a system boots or starts up. The kernel provides access to various common core services to all other system and application programs. These services include, but are not limited to: task scheduling, memory management, disk access, and access to hardware devices.

As well as the kernel, an operating system is often distributed with system software that manages a graphical user interface (although Windows and Macintosh have integrated these programs into the operating system), as well as utility programs for tasks such as managing files and configuring the operating system. Often times distributed with operating systems are application software that does not directly relate to the operating system's core function, but which the operating system distributor finds advantageous to supply with the operating system.

The delineation between the operating system and application software is not precise, and is occasionally subject to controversy. From commercial or legal points of view, the delineation can depend on the contexts of the interests involved. For example, one of the key questions in the United States v. Microsoft antitrust trial was whether Microsoft's web browser was part of its operating system, or whether it was a separable piece of application software.

Like the term "operating system" itself, the question of what exactly the "kernel" should manage is subject to some controversy, with debates over whether things like file systems should be included in the kernel. Various camps advocate microkernels, monolithic kernels, and so on.

Operating systems are used on most, but not all, computer systems. The simplest computers, including the smallest embedded systems and many of the first computers did not have operating systems. Instead, they relied on the application programs to manage the minimal hardware themselves, perhaps with the aid of libraries developed for the purpose. Commercially-supplied operating systems are present on virtually all modern devices described as computers, from personal computers to mainframes, as well as mobile computers such as PDAs and mobile phones.


Process management

Every action on a computer, be it background services or applications, is run inside a process. As long as von Neumann's architecture is used to build computers, only one process per CPU can be run at a time. Older OS such as MS-DOS did not attempt any artifacts to bypass this limit and in fact only one process could be run under them (although DOS itself featured TSR as a very partial and not too easy to use solution). Modern operating systems are able to simulate execution of many processes at once (multi-tasking) even under a single CPU. Process management is an operating system's way of dealing with running multiple processes. Since most computers contain one processor with one core, multi-tasking is accomplished by simply switching processes quickly. As a user runs more processes, all timeshares become smaller. On many systems, this can eventually lead to problems such as skipping of audio or jittery mouse movement (this is called thrashing, a state in which OS related activity becomes the only thing a computer does). Process management involves the computation and distribution of "timeshares". Most operating systems allow a process to be assigned a process priority which impacts its timeshare. Interactive operating systems also employ some level of feedback in which the task with which the user is working receives a priority boost

Memory management

According to Parkinson's law "Programs expand to fill the memory available to hold them". Thus the programmers like a memory of infinite size and infinite speed. Nowadays most of the computer's memory is arranged in a hierarchical manner, starting from fastest registers, cache, RAM, disk storage, and perhaps even magnetic tape. The memory manager in a 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 a process (4Gb for a typical Pentium PC, even if the physical RAM available is just 256Mb). This however comes at a speed penalty which is usually low, but can become very high in extreme cases and, again, lead to thrashing.

Another important part of memory management activity is managing virtual addresses, with help from the CPU. 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 for a limited amount of memory and in controlled ways). This is achieved by having separate address spaces. Each process in fact 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). What actually happens is that the CPU stores some tables to match virtual addresses to physical addresses. This is why on Windows NT most resource sharing by memory pointers does not work: a pointer can refer to a GDI object on some process but be dangling on another!

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 read support for 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 recovery. 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 where as the journaled file systems recovery is automatic. Microsoft's NTFS is journaled along with most Linux file systems, except ext2, but including ext3, reiserfs and JFS.

Every file system is made up of similar directories and subdirectories. Along with the operating systems file system similarities there are the subtle differences. Microsoft separates its directories with a back slash and its file names aren't case sensitive whereas Unix-derived operating systems (including Linux) use the forward slash and their file names generally are case sensitive.


Most operating systems are capable of using the now-universal 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.


Security as it pertains to the operating system is the ability to authenticate users prior to access, categorize the level of access the user has, and limit access based on a policy placed by administration. 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 gooeys). 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 Mac OS X and all Windows versions based on Windows NT)

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.

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.

Device drivers

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.


The first computers did not have operating systems. However, software tools for managing the system and simplifying the use of hardware appeared very quickly afterwards, and gradually expanded in scope. 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. 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. One notable early 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 a Graphical user interface 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) typically 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 has been ported to a few other CPUs (DEC Alpha and MIPS Magnum). Since the early 1990s the choice for personal computers has largely been limited to the Microsoft Windows family and the Unix-like family, 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, but typically more similar to Unix than Windows.

  • Personal computers
  • IBM PC compatible - Microsoft Windows and smaller Unix-variants (like Linux and BSD)
  • Apple Macintosh - Mac OS X, Windows, Linux and BSD
  • Mainframes - A number of unique operating systems; sometimes Linux and other Unix variants.
  • Embedded systems - a variety of dedicated operating systems and limited versions of Linux or other 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.

Microsoft Windows

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 computers, 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 this is thought to be dwindling due to the increase of interest in open source operating systems. 1 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 MS_WIN_XP Service Pack 2 released on August 6, 2004.

Microsoft is currently developing its next generation of Windows Platform named Windows Vista (formerly Windows Longhorn), which has yet to be released and boasts some impressive new functionality particularly in security and network adminstration. The as-yet-unreleased software also boasts a completely new front-end known as Windows-Black-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 the Windows-like OS/2 from IBM; Mac OS, the non-Unix precursor to Apple's Mac OS X; BeOS; RISC OS; XTS-300; and AmigaOS.

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.

source: wikipedia

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