neteffect-operatingsystems

8/6/2019 NetEffect-Operatingsystems

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Operating Systems www.neteffect.in

Operating System

An operating system (OS) is a set of system software programs in a computer that regulatethe ways application software programs use the computer hardware and the ways thatusers control the computer. For hardware functions such as input/output and memory spaceallocation, operating system programs act as an intermediary between application programsand the computer hardware, although application programs are usually executed directly bythe hardware. Operating Systems is also a field of study within Applied Computer Science.Operating systems are found on almost any device that contains a computer with multipleprogramsfrom cellular phones and video game consoles to supercomputers and webservers. Operating systems are two-sided platforms, bringing consumers (the first side) andprogram developers (the second side) together in a single market. Some popular modernoperating systems for personal computers include Microsoft Windows, Mac OS X, and Linux.

Because early computers were built to perform a series of single tasks, like a calculator,operating systems did not exist in their modern and more complex forms until the early1960s. Some operating system features were developed in the 1950s, such as programs

that could automatically run different programs in succession to speed up processing.Hardware features were added that enabled use of runtime libraries, interrupt, and parallelprocessing. When personal computers by companies such as Apple Inc., Atari, IBM andAmiga became popular in the 1980s, vendors adding operating system features that hadpreviously become widely used on mainframe and mini computers. Later many featuressuch as graphical user interface were developed specifically for personal computer operatingsystems.

An operating system can be divided into many different parts. One of the most importantparts is the kernel, which controls low-level processes that the average user usually cannotsee: it controls how memory is read and written, the order in which processes are executed,how information is received and sent by devices like the monitor, keyboard and mouse, anddeciding how to interpret information received by networks. The user interface is the part ofthe operating system that interacts with the computer user directly, allowing them tocontrol and use programs. The user interface may be graphical with icons and a desktop, ortextual, with a command line. Another similar feature is an Application programminginterface, which is a set of services and code libraries that let applications interact with oneanother, as well as the operating system itself. Depending on the operating system, manyof these components may not be considered an actual part. For example, Windowsconsiders its user interface to be part of the operating system, while many versions of Linuxdo not.

History

In the early 1950s, a computer could execute only one program at a time. Each user had

sole use of the computer and would arrive at a scheduled time with program and data onpunched paper cards and tape. The program would be loaded into the machine, and themachine would be set to work until the program completed or crashed. Programs couldgenerally be debugged via a front panel using toggle switches and panel lights. It is saidthat Alan Turing was a master of this on the early Manchester Mark 1 machine, and he was


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already deriving the primitive conception of an operating system from the principles of theUniversal Turing machine.Later machines came with libraries of software, which would be linked to a user's programto assist in operations such as input and output and generating computer code from human-readable symbolic code. This was the genesis of the modern-day operating system.However, machines still ran a single job at a time. At Cambridge University in England the

job queue was at one time a washing line from which tapes were hung with different coloredclothes-pegs to indicate job-priority.

Mainframes

Through the 1950s, many major features were pioneered in the field of operating systems,including batch processing, input/output interrupt, buffering, multitasking, spooling, runtimelibraries, and programs for sorting records in files. These features were included or notincluded in application software at the option of application programmers, rather than in aseparate operating system used by all applications. In 1959 the SHARE Operating Systemwas released as an integrated utility for the IBM 704 and IBM 709 mainframes.

During the 1960s, the concept of a single OS spanning an entire product line was crucial for

the success of System/360. IBM's current mainframe operating systems are distantdescendants of this original system and applications written for the OS/360 can still be runon modern machines. In the mid-'70s, the MVS, the descendant of OS/360 offered the firstimplementation of using RAM as a transparent cache for data.

OS/360 also pioneered the concept that the operating system keeps track of all of thesystem resources that are used, including program and data space allocation in mainmemory and file space in secondary storage, and file locking during update. When theprocess is terminated for any reason, all of these resources are re-claimed by the operatingsystem.

An alternative CP-67 system started a whole line of operating systems focused on theconcept of virtual machines.

Control Data Corporation developed the SCOPE operating system in the 1960s, for batchprocessing. In cooperation with the University of Minnesota, the KRONOS and later the NOSoperating systems were developed during the 1970s, which supported simultaneous batchand timesharing use. Like many commercial timesharing systems, its interface was anextension of the Dartmouth BASIC operating systems, one of the pioneering efforts intimesharing and programming languages.

In the late 1970s, Control Data and the University of Illinois developed the PLATO operatingsystem, which used plasma panel displays and long-distance time sharing networks. Platowas remarkably innovative for its time, featuring real-time chat, and multi-user graphicalgames. Burroughs Corporation introduced the B5000 in 1961 with the MCP, (Master Control

Program) operating system. The B5000 was a stack machine designed to exclusivelysupport high-level languages with no machine language or assembler, and indeed the MCPwas the first OS to be written exclusively in a high-level language ESPOL, a dialect ofALGOL. MCP also introduced many other ground-breaking innovations, such as being thefirst commercial implementation of virtual memory. During development of the AS400, IBMmade an approach to Burroughs to licence MCP to run on the AS400 hardware. Thisproposal was declined by Burroughs management to


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protect its existing hardware production. MCP is still in use today in the UnisysClearPath/MCP line of computers.UNIVAC, the first commercial computer manufacturer, produced a series of EXEC operatingsystems. Like all early main-frame systems, this was a batch-oriented system that managedmagnetic drums, disks, card readers and line printers. In the 1970s, UNIVAC produced theReal-Time Basic (RTB) system to support large-scale time sharing, also patterned after theDartmouth BASIC system.

General Electric and MIT developed General Electric Comprehensive Operating Supervisor(GECOS), which introduced the concept of ringed security privilege levels. After acquisitionby Honeywell it was renamed to General Comprehensive Operating System (GCOS).Digital Equipment Corporation developed many operating systems for its various computerlines, including TOPS-10 and TOPS-20 time sharing systems for the 36-bit PDP-10 classsystems. Prior to the widespread use of UNIX, TOPS-10 was a particularly popular system inuniversities, and in the early ARPANET community.

In the late 1960s through the late 1970s, several hardware capabilities evolved that allowedsimilar or ported software to run on more than one system. Early systems had utilizedmicroprogramming to implement features on their systems in order to permit different

underlying architecture to appear to be the same as others in a series. In fact most 360'safter the 360/40 (except the 360/165 and 360/168) were microprogrammedimplementations. But soon other means of achieving application compatibility were provento be more significant.

The enormous investment in software for these systems made since 1960s caused most ofthe original computer manufacturers to continue to develop compatible operating systemsalong with the hardware. The notable supported mainframe operating systems include:Burroughs MCP B5000,1961 to Unisys Clearpath/MCP, present.

IBM OS/360 IBM System/360, 1966 to IBM z/OS, present.IBM CP-67 IBM System/360, 1967 to IBM z/VM, present.

UNIVAC EXEC 8 UNIVAC 1108, 1967, to OS 2200 Unisys Clearpath Dorado, present.The first microcomputers did not have the capacity or need for the elaborate operatingsystems that had been developed for mainframes and minis; minimalistic operating systemswere 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 microcomputersand was closely imitated in MS-DOS, which became wildly popular as the operating systemchosen for the IBM PC (IBM's version of it was called IBM DOS or PC DOS), its successorsmaking Microsoft. In the 80's Apple Computer Inc. (now Apple Inc.) abandoned its popularApple II series of microcomputers to introduce the Apple Macintosh computer with aninnovative Graphical User Interface (GUI) to the Mac OS.

The introduction of the Intel 80386 CPU chip with 32-bit architecture and paging

capabilities, provided personal computers with the ability to run multitasking operatingsystems like those of earlier minicomputers and mainframes. Microsoft responded to this


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progress by hiring Dave Cutler, who had developed the VMS operating system for DigitalEquipment Corporation. He would lead the development of the Windows NT operatingsystem, which continues to serve as the basis for Microsoft's operating systems line. SteveJobs, a co-founder of Apple Inc., started NeXT Computer Inc., which developed the Unix-likeNEXTSTEP operating system. NEXTSTEP would later be acquired by Apple Inc. and used,along with code from FreeBSD as the core of Mac OS X.The GNU project was started by activist and programmer Richard Stallman with the goal ofa complete free software replacement to the proprietary UNIX operating system. While theproject was highly successful in duplicating the functionality of various parts of UNIX,development of the GNU Hurd kernel proved to be unproductive.

In 1991 Finnish computer science student Linus Torvalds, with cooperation from volunteersover the Internet, released the first version of the Linux kernel. It was soon merged withthe GNU userland and system software to form a complete operating system. Since then,the combination of the two major components has usually been referred to as simply"Linux" by the software industry, a naming convention which Stallman and the FreeSoftware Foundation remain opposed to, preferring the name "GNU/Linux" instead. TheBerkeley Software Distribution, known as BSD, is the UNIX derivative distributed by theUniversity of California, Berkeley, starting in the 1970s. Freely distributed and ported to

many minicomputers, it eventually also gained a following for use on PCs, mainly asFreeBSD, NetBSD and OpenBSD.

Examples of operating systems

Microsoft WindowsMicrosoft Windows is a family of proprietary operating systems most commonly used onpersonal computers. It is the most common family of operating systems for the personalcomputer, with about 90% of the market share. Currently, the most widely used version ofthe Windows family is Windows XP[citation needed], released on October 25, 2001. Thenewest version is Windows 7 for personal computers and Windows Server 2008 R2 forservers.It originated in 1981 as an add-on to the older MS-DOS operating system for the IBM PC.Released in 1985, Microsoft came to dominate the business world of personal computers,and went on to set a number of industry standards and commonplace applications.Beginning with Windows XP, all modern versions are based on the Windows NT kernel.Current versions of Windows run on x86 and x86-64 processors, although older versionssometimes supported other architectures.Windows is also used on servers, supporting applications such as web servers and databaseservers. In recent years, Microsoft has spent significant marketing and research &development money to demonstrate that Windows is capable of running any enterpriseapplication, which has resulted in consistent price/performance records (see the TPC) andsignificant acceptance in the enterprise market. However, its usage in servers is not aswidespread as personal computers, and here Windows actively competes against Linux andBSD for market share, while still capturing a steady majority by some accounts.


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Mac OS X

Mac OS X is a line of partially proprietary graphical operating systems developed, marketed,and sold by Apple Inc., the latest of which is pre-loaded on all currently shipping Macintoshcomputers. Mac OS X is the successor to the original Mac OS, which had been Apple'sprimary operating system since 1984. Unlike its predecessor, Mac OS X is a UNIX operatingsystem built on technology that had been developed at NeXT through the second half of the1980s and up until Apple purchased the company in early 1997.The operating system was first released in 1999 as Mac OS X Server 1.0, with a desktop-oriented version (Mac OS X v10.0) following in March 2001. Since then, six more distinct"client" and "server" editions of Mac OS X have been released, the most recent being MacOS X v10.6, which was first made available on August 28, 2009. Releases of Mac OS X arenamed after big cats; the current version of Mac OS X is "Snow Leopard".The server edition, Mac OS X Server, is architecturally identical to its desktop counterpartbut usually runs on Apple's line of Macintosh server hardware. Mac OS X Server includeswork group management and administration software tools that provide simplified access tokey network services, including a mail transfer agent, a Samba server, an LDAP server, adomain name server, and others.

Unix

Ken Thompson wrote B, mainly based on BCPL, which he used to write Unix, based on hisexperience in the MULTICS project. B was replaced by C, and Unix developed into a large,complex family of inter-related operating systems which have been influential in everymodern operating system (see History). The Unix-like family is a diverse group of operatingsystems, with several major sub-categories including System V, BSD, and GNU/Linux. Thename "UNIX" is a trademark of The Open Group which licenses it for use with any operatingsystem that has been shown to conform to their definitions. "Unix-like" is commonly used torefer to the large set of operating systems which resemble the original Unix.Unix-like systems run on a wide variety of machine architectures. They are used heavily forservers in business, as well as workstations in academic and engineering environments.Free Unix variants, such as GNU/Linux and BSD, are popular in these areas.Some Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that vendor'shardware. Others, such as Solaris, can run on multiple types of hardware, including x86servers and PCs. Apple's Mac OS X, a hybrid kernel-based BSD variant derived fromNeXTSTEP, Mach, and FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS.Unix interoperability was sought by establishing the POSIX standard. The POSIX standardcan be applied to any operating system, although it was originally created for various Unixvariants.


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BSD and its descendants

A subgroup of the Unix family is the Berkeley Software Distribution family, which includesFreeBSD, NetBSD, and OpenBSD. These operating systems are most commonly found onwebservers, although they can also function as a personal computer OS.The internet owes much of its existence to BSD, as many of the protocols now commonlyused by computers to connect, send and receive data over a network were widelyimplemented and refined in BSD. The world wide web was also first demonstrated on anumber of computers running an OS based on BSD called NextStep.

BSD has its roots in Unix. In 1974, University of California, Berkeley installed its first Unixsystem. Over time, students and staff in the computer science department there beganadding new programs to make things easier, such as text editors. When Berkely receivednew VAX computers in 1978 with Unix installed, the school's undergraduates modified Unixeven more in order to take advantage of the computer's hardware possibilities. The DefenseAdvanced Research Projects Agency of the US Department of Defense took interest, anddecided to fund the project. Many schools, corporations, and government organizations tooknotice and started to use Berkeley's version of Unix instead of the official one distributed byAT&T. Steve Jobs, upon leaving Apple Inc. in 1985, formed NeXT Inc., a company that

manufactured high-end computers running on a variation of BSD called NeXTSTEP. One ofthese computers was used by Tim Berners-Lee as the first webserver to create the WorldWide Web.

Developers like Keith Bostic encouraged the project to replace any non-free code thatoriginated with Bell Labs. Once this was done, however, AT&T sued. Eventually, after twoyears of legal disputes, the BSD project came out ahead and spawned a number of freederivatives, such as FreeBSD and NetBSD. However, the two year wait had set the stage fortwo projects that would ultimately eclipse both BSD and Unix: GNU and Linux.

Plan 9

Ken Thompson, Dennis Ritchie and Douglas McIlroy at Bell Labs designed and developed theC programming language to build the operating system Unix. Programmers at Bell Labswent on to develop Plan 9 and Inferno, which were engineered for modern distributedenvironments. Plan 9 was designed from the start to be a networked operating system, andhad graphics built-in, unlike Unix, which added these features to the design later. It iscurrently released under the Lucent Public License. Inferno was sold to Vita Nuova Holdingsand has been released under a GPL/MIT license.

Linux

Linux is a generic name for a member in a family of operating systems that can be found onanything from supercomputers to wristwatches. Because its components are open source,anyone can read and modify its code. Because of this, it has been modified for a huge

variety of electronics. It is used on only 0.5-2% of all personal computers, but it is a silentgiant in the world of cell phones, servers, and embedded systems. Linux has supersededUnix in most places, and is used on the 10 most powerful supercomputers in the world.


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The GNU project is a mass collaboration of programmers who seek to create a completelyfree and open operating system that was similar to Unix but with completely original code.It was started in 1983 by Richard Stallman, and is responsible for many of the parts of mostLinux variants. For this reason, Linux is often called GNU/Linux. Thousands of pieces ofsoftware for virtually every operating system are licensed under the GNU General PublicLicense. Meanwhile, the Linux kernel began as a side project of Linus Torvalds, a universitystudent from Finland. In 1991, Torvalds began work on it, and posted information about hisproject on a newsgroup for computer students and programmers. He received a wave ofsupport and volunteers who ended up creating a full-fledged kernel. Programmers from GNUtook notice, and members of both projects worked to integrate the finished GNU parts intothe linux kernel in order to create a full-fledged operating system.

Google Chrome OS

Chrome is an operating system based on the Linux kernel and designed by Google. It iscurrently in development, and is targeted towards use in specific types of netbooks. Chrometargets computer users that spend most of their time on the internetit is technically only aweb browser with no other applications, and relies on internet applications used in the webbrowser to accomplish tasks such as word processing and media viewing.

Other

Older operating systems which are still used in niche markets include OS/2 from IBM andMicrosoft; Mac OS, the non-Unix precursor to Apple's Mac OS X; BeOS; XTS-300. Some,most notably RISC OS, MorphOS and AmigaOS 4 continue to be developed as minorityplatforms for enthusiast communities and specialist applications. OpenVMS formerly fromDEC, is still under active development by Hewlett-Packard. Yet other operating systems areused almost exclusively in academia, for operating systems education or to do research onoperating system concepts. A typical example of a system that fulfills both roles is MINIX,while for example Singularity is used purely for research.

Components

The components of an operating system all exist in order to make the different parts of acomputer work together. All softwarefrom financial databases to film editorsneeds to gothrough the operating system in order to use any of the hardware, whether it be as simpleas a mouse or keyboard or complex as an internet connection.The user interfaceAn example of the command line. Each command is typed out after the 'prompt', and thenits output appears below, working its way down the screen. The current command prompt isat the bottom.An example of a graphical user interface. Programs take the form of images on the screen,and the files, folders, and applications take the form of icons and symbols. A mouse is usedto navigate the computer.


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Every computer that receives some sort of human input needs a user interface, which allowsa person to interact with the computer. While devices like keyboards, mice andtouchscreens make up the hardware end of this task, the user interface makes up thesoftware for it. The two most common forms of a user interface have historically been theCommand-line interface, where computer commands are typed out line-by-line, and theGraphical user interface, where a visual environment (most commonly with windows,buttons, and icons) is present.

Graphical User Interface

Most of the modern computer systems support graphical user interfaces (GUI), and ofteninclude them. In some computer systems, such as the original implementations of MicrosoftWindows and the Mac OS, the GUI is integrated into the kernel.While technically a graphical user interface is not an operating system service, incorporatingsupport for one into the operating system kernel can allow the GUI to be more responsiveby reducing the number of context switches required for the GUI to perform its outputfunctions. Other operating systems are modular, separating the graphics subsystem fromthe kernel and the Operating System. In the 1980s UNIX, VMS and many others hadoperating systems that were built this way. GNU/Linux and Mac OS X are also built this way.

Modern releases of Microsoft Windows such as Windows Vista implement a graphicssubsystem that is mostly in user-space, however versions between Windows NT 4.0 andWindows Server 2003's graphics drawing routines exist mostly in kernel space. Windows 9xhad very little distinction between the interface and the kernel.Many computer operating systems allow the user to install or create any user interface theydesire. The X Window System in conjunction with GNOME or KDE is a commonly foundsetup on most Unix and Unix-like (BSD, GNU/Linux, Solaris) systems. A number of Windowsshell replacements have been released for Microsoft Windows, which offer alternatives tothe included Windows shell, but the shell itself cannot be separated from Windows.Numerous Unix-based GUIs have existed over time, most derived from X11. Competitionamong the various vendors of Unix (HP, IBM, Sun) led to much fragmentation, though aneffort to standardize in the 1990s to COSE and CDE failed for the most part due to variousreasons, eventually eclipsed by the widespread adoption of GNOME and KDE. Prior to freesoftware-based toolkits and desktop environments, Motif was the prevalent toolkit/desktopcombination (and was the basis upon which CDE was developed).Graphical user interfaces evolve over time. For example, Windows has modified its userinterface almost every time a new major version of Windows is released, and the Mac OSGUI changed dramatically with the introduction of Mac OS X in 1999.

The kernel

A kernel connects the application software to the hardware of a computer.With the aid of the firmware and device drivers, the operating system provides the mostbasic level of control over all of the computer's hardware devices. It manages memoryaccess for programs in the RAM, it determines which programs get access to which

hardware resources, it sets up or resets the CPU's operating states for optimal operation at


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all times, and it organizes the data for long-term non-volatile storage with file systems onsuch media as disks, tapes, flash memory, etc.

Program execution

The operating system acts as an interface between an application and the hardware. Theuser interacts with the hardware from "the other side". The operating system is a set ofservices which simplifies development of applications. Executing a program involves thecreation of a process by the operating system. The kernel creates a process by assigningmemory and other resources, establishing a priority for the process (in multi-taskingsystems), loading program code into memory, and executing the program. The programthen interacts with the user and/or other devices and performs its intended function.

Interrupts

Interrupts are central to operating systems, as they provide an efficient way for theoperating system to interact with and react to its environment. The alternativehaving theoperating system "watch" the various sources of input for events (polling) that requireactioncan be found in older systems with very small stacks (50 or 60 bytes) but fairly

unusual in modern systems with fairly large stacks. Interrupt-based programming is directlysupported by most modern CPUs. Interrupts provide a computer with a way of automaticallysaving local register contexts, and running specific code in response to events. Even verybasic computers support hardware interrupts, and allow the programmer to specify codewhich may be run when that event takes place.When an interrupt is received, the computer's hardware automatically suspends whateverprogram is currently running, saves its status, and runs computer code previouslyassociated with the interrupt; this is analogous to placing a bookmark in a book in responseto a phone call. In modern operating systems, interrupts are handled by the operatingsystem's kernel. Interrupts may come from either the computer's hardware or from therunning program.When a hardware device triggers an interrupt, the operating system's kernel decides how todeal with this event, generally by running some processing code. The amount of code beingrun depends on the priority of the interrupt (for example: a person usually responds to asmoke detector alarm before answering the phone). The processing of hardware interruptsis a task that is usually delegated to software called device driver, which may be either partof the operating system's kernel, part of another program, or both. Device drivers may thenrelay information to a running program by various means.A program may also trigger an interrupt to the operating system. If a program wishes toaccess hardware for example, it may interrupt the operating system's kernel, which causescontrol to be passed back to the kernel. The kernel will then process the request. If aprogram wishes additional resources (or wishes to shed resources) such as memory, it willtrigger an interrupt to get the kernel's attention.Protected mode, supervisor mode, and virtual modes


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Privilege rings for the x86 available in protected mode. Operating systems determine whichprocesses run in each mode.

Modern CPUs support multiple modes of operation. CPUs with this capability use at least twomodes: protected mode and supervisor mode. The supervisor mode is used by theoperating system's kernel for low level tasks that need unrestricted access to hardware,such as controlling how memory is written and erased, and communication with devices likegraphics cards. Protected mode, in contrast, is used for almost everything else. Applicationsoperate within protected mode, and can only use hardware by communicating with thekernel, which controls everything in supervisor mode. CPUs might have other modes similarto protected mode as well, such as the virtual modes in order to emulate older processortypes, such as 16-bit processors on a 32-bit one, or 32-bit processors on a 64-bit one.

When a computer first starts up, it is automatically running in supervisor mode. The firstfew programs to run on the computer, being the BIOS, bootloader and the operating systemhave unlimited access to hardware - and this is required because, by definition, initializing aprotected environment can only be done outside of one. However, when the operatingsystem passes control to another program, it can place the CPU into protected mode.In protected mode, programs may have access to a more limited set of the CPU's

instructions. A user program may leave protected mode only by triggering an interrupt,causing control to be passed back to the kernel. In this way the operating system canmaintain exclusive control over things like access to hardware and memory.The term "protected mode resource" generally refers to one or more CPU registers, whichcontain information that the running program isn't allowed to alter. Attempts to alter theseresources generally causes a switch to supervisor mode, where the operating system candeal with the illegal operation the program was attempting (for example, by killing theprogram).

Memory management

Among other things, a multiprogramming operating system kernel must be responsible formanaging all system memory which is currently in use by programs. This ensures that aprogram does not interfere with memory already used by another program. Since programstime share, each program must have independent access to memory.Cooperative memory management, used by many early operating systems assumes that allprograms make voluntary use of the kernel's memory manager, and do not exceed theirallocated memory. This system of memory management is almost never seen anymore,since programs often contain bugs which can cause them to exceed their allocated memory.If a program fails it may cause memory used by one or more other programs to be affectedor overwritten. Malicious programs, or viruses may purposefully alter another program'smemory or may affect the operation of the operating system itself. With cooperativememory management it takes only one misbehaved program to crash the system.


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Memory protection enables the kernel to limit a process' access to the computer's memory.Various methods of memory protection exist, including memory segmentation and paging.All methods require some level of hardware support (such as the 80286 MMU) which doesn'texist in all computers.In both segmentation and paging, certain protected mode registers specify to the CPU whatmemory address it should allow a running program to access. Attempts to access otheraddresses will trigger an interrupt which will cause the CPU to re-enter supervisor mode,placing the kernel in charge. This is called a segmentation violation or Seg-V for short, andsince it is both difficult to assign a meaningful result to such an operation, and because it isusually a sign of a misbehaving program, the kernel will generally resort to terminating theoffending program, and will report the error.Windows 3.1-Me had some level of memory protection, but programs could easilycircumvent the need to use it. A general protection fault would be produced indicating asegmentation violation had occurred, however the system would often crash anyway.

Virtual memory

Many operating systems can "trick" programs into using memory scattered around the harddisk and RAM as if it is one continuous chunk of memory called virtual memory.

The use of virtual memory addressing (such as paging or segmentation) means that thekernel can choose what memory each program may use at any given time, allowing theoperating system to use the same memory locations for multiple tasks.If a program tries to access memory that isn't in its current range of accessible memory,but nonetheless has been allocated to it, the kernel will be interrupted in the same way as itwould if the program were to exceed its allocated memory. (See section on memorymanagement.) Under UNIX this kind of interrupt is referred to as a page fault.When the kernel detects a page fault it will generally adjust the virtual memory range of theprogram which triggered it, granting it access to the memory requested. This gives thekernel discretionary power over where a particular application's memory is stored, or evenwhether or not it has actually been allocated yet.In modern operating systems, memory which is accessed less frequently can be temporarilystored on disk or other media to make that space available for use by other programs. Thisis called swapping, as an area of memory can be used by multiple programs, and what thatmemory area contains can be swapped or exchanged on demand.

Multitasking

Multitasking refers to the running of multiple independent computer programs on the samecomputer; giving the appearance that it is performing the tasks at the same time. Sincemost computers can do at most one or two things at one time, this is generally done viatime-sharing, which means that each program uses a share of the computer's time toexecute.An operating system kernel contains a piece of software called a scheduler whichdetermines how much time each program will spend executing, and in which order


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execution control should be passed to programs. Control is passed to a process by thekernel, which allows the program access to the CPU and memory. Later, control is returnedto the kernel through some mechanism, so that another program may be allowed to use theCPU. This so-called passing of control between the kernel and applications is called acontext switch.

An early model which governed the allocation of time to programs was called cooperativemultitasking. In this model, when control is passed to a program by the kernel, it mayexecute for as long as it wants before explicitly returning control to the kernel. This meansthat a malicious or malfunctioning program may not only prevent any other programs fromusing the CPU, but it can hang the entire system if it enters an infinite loop.Modern operating systems extend the concepts of application preemption to device driversand kernel code, so that the operating system has preemptive control over internal run-times as well.

The philosophy governing preemptive multitasking is that of ensuring that all programs aregiven regular time on the CPU. This implies that all programs must be limited in how muchtime they are allowed to spend on the CPU without being interrupted. To accomplish this,modern operating system kernels make use of a timed interrupt. A protected mode timer is

set by the kernel which triggers a return to supervisor mode after the specified time haselapsed. (See above sections on Interrupts and Dual Mode Operation.)On many single user operating systems cooperative multitasking is perfectly adequate, ashome computers generally run a small number of well tested programs. Windows NT wasthe first version of Microsoft Windows which enforced preemptive multitasking, but it didn'treach the home user market until Windows XP, (since Windows NT was targeted atprofessionals.)

Disk access and file systems

Filesystems allow users and programs to organize and sort files on a computer, oftenthrough the use of directories (or "folders")Access to data stored on disks is a central feature of all operating systems. Computers storedata on disks using files, which are structured in specific ways in order to allow for fasteraccess, higher reliability, and to make better use out of the drive's available space. Thespecific way in which files are stored on a disk is called a file system, and enables files tohave names and attributes. It also allows them to be stored in a hierarchy of directories orfolders arranged in a directory tree.

Early operating systems generally supported a single type of disk drive and only one kind offile system. Early file systems were limited in their capacity, speed, and in the kinds of filenames and directory structures they could use. These limitations often reflected limitationsin the operating systems they were designed for, making it very difficult for an operatingsystem to support more than one file system.While many simpler operating systems support a limited range of options for accessing

storage systems, operating systems like UNIX and GNU/Linux support a technology knownas a virtual file system or VFS. An operating system like UNIX supports a wide array of


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storage devices, regardless of their design or file systems to be accessed through a commonapplication programming interface (API). This makes it unnecessary for programs to haveany knowledge about the device they are accessing. A VFS allows the operating system toprovide programs with access to an unlimited number of devices with an infinite variety offile systems installed on them through the use of specific device drivers and file systemdrivers.

A connected storage device such as a hard drive is accessed through a device driver. Thedevice driver understands the specific language of the drive and is able to translate thatlanguage into a standard language used by the operating system to access all disk drives.On UNIX, this is the language of block devices.

When the kernel has an appropriate device driver in place, it can then access the contentsof the disk drive in raw format, which may contain one or more file systems. A file systemdriver is used to translate the commands used to access each specific file system into astandard set of commands that the operating system can use to talk to all file systems.Programs can then deal with these file systems on the basis of filenames, anddirectories/folders, contained within a hierarchical structure. They can create, delete, open,and close files, as well as gather various information about them, including access

permissions, size, free space, and creation and modification dates.

Various differences between file systems make supporting all file systems difficult. Allowedcharacters in file names, case sensitivity, and the presence of various kinds of file attributesmakes the implementation of a single interface for every file system a daunting task.Operating systems tend to recommend using (and so support natively) file systemsspecifically designed for them; for example, NTFS in Windows and ext3 and ReiserFS inGNU/Linux. However, in practice, third party drives are usually available to give support forthe most widely used file systems in most general-purpose operating systems (for example,NTFS is available in GNU/Linux through NTFS-3g, and ext2/3 and ReiserFS are available inWindows through FS-driver and rfstool).

Support for file systems is highly varied among modern operating systems although thereare several common file systems which almost all operating systems include support anddrivers for. Operating systems vary on file system support and on the disk formats theymay be installed on. Under Windows each file system is usually limited in application tocertain media, for example CDs must use ISO 9660 or UDF, and as of Windows Vista, NTFSis the only file system which the operating system can be installed on. It is possible toinstall GNU/Linux onto many types of file systems. Unlike other operating systems,GNU/Linux and UNIX allow any file system to be used regardless of the media it is stored in,whether it is a hard drive, a disc (CD,DVD...), an USB key, or even contained within a filelocated on another file system.

Device drivers

A device driver is a specific type of computer software developed to allow interaction withhardware devices. Typically this constitutes an interface for communicating with the device,through the specific computer bus or communications subsystem that the hardware isconnected to, providing commands to and/or receiving data from the device, and on theother end, the requisite interfaces to the operating system and software applications. It is a


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specialized hardware-dependent computer program which is also operating system specificthat enables another program, typically an operating system or applications softwarepackage or computer program running under the operating system kernel, to interacttransparently with a hardware device, and usually provides the requisite interrupt handlingnecessary for any necessary asynchronous time-dependent hardware interfacing needs.The key design goal of device drivers is abstraction. Every model of hardware (even withinthe same class of device) is different. Newer models also are released by manufacturersthat provide more reliable or better performance and these newer models are oftencontrolled differently. Computers and their operating systems cannot be expected to knowhow to control every device, both now and in the future. To solve this problem, operativesystems essentially dictate how every type of device should be controlled. The function ofthe device driver is then to translate these operative system mandated function calls intodevice specific calls. In theory a new device, which is controlled in a new manner, shouldfunction correctly if a suitable driver is available. This new driver will ensure that the deviceappears to operate as usual from the operating system's point of view.Under versions of Windows before Vista and versions of Linux before 2.6, all driverexecution was co-operative, meaning that if a driver entered an infinite loop it would freezethe system. More recent revisions of these opearting systems incorporate kernelpreemption, where the kernel interrupts the driver to give it tasks, and then separates itself

from the process until it receives a response from the device driver, or gives it more tasksto do.

Networking

Currently most operating systems support a variety of networking protocols, hardware, andapplications for using them. This means that computers running dissimilar operatingsystems can participate in a common network for sharing resources such as computing,files, printers, and scanners using either wired or wireless connections. Networks canessentially allow a computer's operating system to access the resources of a remotecomputer to support the same functions as it could if those resources were connecteddirectly to the local computer. This includes everything from simple communication, to usingnetworked file systems or even sharing another computer's graphics or sound hardware.Some network services allow the resources of a computer to be accessed transparently,such as SSH which allows networked users direct access to a computer's command lineinterface.

Client/server networking involves a program on a computer somewhere which connects viaa network to another computer, called a server. Servers offer (or host) various services toother network computers and users. These services are usually provided through ports ornumbered access points beyond the server's network address[disambiguation needed]. Eachport number is usually associated with a maximum of one running program, which isresponsible for handling requests to that port. A daemon, being a user program, can in turnaccess the local hardware resources of that computer by passing requests to the operatingsystem kernel.

Many operating systems support one or more vendor-specific or open networking protocolsas well, for example, SNA on IBM systems, DECnet on systems from Digital Equipment


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Corporation, and Microsoft-specific protocols (SMB) on Windows. Specific protocols forspecific tasks may also be supported such as NFS for file access. Protocols like ESound, oresd can be easily extended over the network to provide sound from local applications, on aremote system's sound hardware.

Security

A computer being secure depends on a number of technologies working properly. A modernoperating system provides access to a number of resources, which are available to softwarerunning on the system, and to external devices like networks via the kernel.The operating system must be capable of distinguishing between requests which should beallowed to be processed, and others which should not be processed. While some systemsmay simply distinguish between "privileged" and "non-privileged", systems commonly havea form of requester identity, such as a user name. To establish identity there may be aprocess of authentication. Often a username must be quoted, and each username may havea password. Other methods of authentication, such as magnetic cards or biometric data,might be used instead. In some cases, especially connections from the network, resourcesmay be accessed with no authentication at all (such as reading files over a network share).Also covered by the concept of requester identity is authorization; the particular services

and resources accessible by the requester once logged into a system are tied to either therequester's user account or to the variously configured groups of users to which therequester belongs.

In addition to the allow/disallow model of security, a system with a high level of security willalso offer auditing options. These would allow tracking of requests for access to resources(such as, "who has been reading this file?"). Internal security, or security from an alreadyrunning program is only possible if all possibly harmful requests must be carried outthrough interrupts to the operating system kernel. If programs can directly access hardwareand resources, they cannot be secured.External security involves a request from outside the computer, such as a login at aconnected console or some kind of network connection. External requests are often passedthrough device drivers to the operating system's kernel, where they can be passed ontoapplications, or carried out directly. Security of operating systems has long been a concernbecause of highly sensitive data held on computers, both of a commercial and militarynature. The United States Government Department of Defense (DoD) created the TrustedComputer System Evaluation Criteria (TCSEC) which is a standard that sets basicrequirements for assessing the effectiveness of security. This became of vital importance tooperating system makers, because the TCSEC was used to evaluate, classify and selectcomputer systems being considered for the processing, storage and retrieval of sensitive orclassified information.

Network services include offerings such as file sharing, print services, email, web sites, andfile transfer protocols (FTP), most of which can have compromised security. At the front lineof security are hardware devices known as firewalls or intrusion detection/prevention

systems. At the operating system level, there are a number of software firewalls available,as well as intrusion detection/prevention systems. Most modern operating systems include asoftware firewall, which is enabled by default. A software firewall can be configured to allow


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or deny network traffic to or from a service or application running on the operating system.Therefore, one can install and be running an insecure service, such as Telnet or FTP, andnot have to be threatened by a security breach because the firewall would deny all traffictrying to connect to the service on that port.

An alternative strategy, and the only sandbox strategy available in systems that do notmeet the Popek and Goldberg virtualization requirements, is the operating system notrunning user programs as native code, but instead either emulates a processor or providesa host for a p-code based system such as Java.Internal security is especially relevant for multi-user systems; it allows each user of thesystem to have private files that the other users cannot tamper with or read. Internalsecurity is also vital if auditing is to be of any use, since a program can potentially bypassthe operating system, inclusive of bypassing auditing.

Real-time operating systems

A real-time operating system (RTOS) is a multitasking operating system intended forapplications with fixed deadlines (real-time computing). Such applications include somesmall embedded systems, automobile engine controllers, industrial robots, spacecraft,

industrial control, and some large-scale computing systems.An early example of a large-scale real-time operating system was Transaction ProcessingFacility developed by American Airlines and IBM for the Sabre Airline Reservations System.Embedded systems that have fixed deadlines use a real-time operating system such asVxWorks, PikeOS, eCos, QNX, MontaVista Linux and RTLinux. Windows CE is a real-timeoperating system that shares similar APIs to desktop Windows but shares none of desktopWindows' codebase[citation needed].Some embedded systems use operating systems such as Symbian OS, Palm OS, BSD, andGNU/Linux, although such operating systems do not support real-time computing.Hobby development

Operating system development is one of the more involved and technical options for thecomputing hobbyist. A hobby operating system is classified as one that has been writtenfrom scratch (not based on another system) and has few developers who work in theirspare time. Development usually begins with an existing operating system. The hobbyist istheir own developer, or they interact in a relatively small and unstructured group ofindividuals who are all similarly situated with the same code base. Examples of a hobbyoperating system include Syllable and ReactOS.

Diversity of operating systems and portability

Application software is generally written for use on a specific operating system, andsometimes even for specific hardware. When porting the application to run on another OS,the functionality required by that application may be implemented differently by that OS(the names of functions, meaning of arguments, etc.) requiring the application to be

adapted, changed, or otherwise maintained.


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This cost in supporting operating systems diversity can be avoided by instead writingapplications against software platforms like Java, or Qt for web browsers. Theseabstractions have already borne the cost of adaptation to specific operating systems andtheir system libraries.Another approach is for operating system vendors to adopt standards. For example, POSIXand OS abstraction layers provide commonalities that reduce porting costs.

Proprietary

Acorn ComputersAcorn MOSARX projectArthur OSRISC OSAmiga Inc.AmigaOSAmigaOS 1.0-3.9 (68k)AmigaOS 4 (PowerPC)Amiga Unix (aka Amix)

Apollo ComputerAEGISDomain/OS One of the first network-based systems. Ran on Apollo/Domain hardware. Laterbought by Hewlett-Packard.vikek OSApple Inc.For Apple IIApple DOSUCSD PascalProDOSGS/OSFor Apple IIISOS (Sophisticated Operating System)For Apple LisaLisa OSFor Apple NewtonNewton OSClassic Mac OSSystem Software 1System Software 2System Software 3System Software 4System Software 5System 6System 7 (codenamed "Big Bang")

Mac OS 8Mac OS 9Unix-like operating systemsFor Macintosh ComputersFor 68k CPUsA/UXFor PowerPC CPUsMkLinuxFor PowerPC and Intel CPUs


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Mac OS XMac OS X v10.0 (aka "Cheetah")Mac OS X v10.1 (aka "Puma")Mac OS X v10.2 (aka "Jaguar")Mac OS X v10.3 (aka "Panther")Mac OS X v10.4 (aka "Tiger")Mac OS X v10.5 (aka "Leopard")Mac OS X v10.6 (aka "Snow Leopard")Mac OS X ServerFor iPhone/iPod Touch/iPadiOSAtariAtari DOS (for 8-bit computers)Atari TOSAtari MultiTOSBAE SystemsXTS-400Be Inc.BeOS

BeIABeOS r5.1d0magnussoft ZETA (based on BeOS r5.1d0 source code, developed by yellowTAB)Blue Eyed OSCosmoeBurroughs CorporationBurroughs MCPControl Data CorporationCOS (Chippewa Operating System)SIPROS (for Simultaneous Processing Operating System)SCOPE (Supervisory Control Of Program Execution)MACE (Mansfield and Cahlander Executive)KRONOS (Kronographic OS)NOS (Network Operating System)NOS/BE NOS Batch EnvironmentConvergent TechnologiesConvergent Technologies Operating System (later acquired by Unisys)Data GeneralRDOS Real-time Disk Operating System, with variants: RTOS and DOS (not related to IBMPC DOS).AOS for 16-bit Data General Eclipse computers and AOS/VS for 32-bit (MV series) Eclipses,MP/AOS for microNOVA-based computersDG/UXDataPointCTOS Z-80 based, Cassette Tape Operating System for early desktop systems. Capable of

up to 8 simultaneous users. Replaced by DataPoint DOS.DOS Intel 808x/80x86-based, Disk Operating Systems for desktop systems. Capable of upto 32 users per node. Supported a sophisticated network of nodes that were often purpose-built. The name DOS was used in these products login screens before it was popularized byIBM, Microsoft and others.Digital Research IncControl Program/Monitor (CP/M)CP/M-80 CP/M for Intel 8080/8085 and Zilog Z80MP/M-80 Multi-user version of CP/M-80


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CP/M-86 CP/M for Intel 8088/86MP/M-86 Multi-user version of CP/M-86CP/M-68k CP/M for Motorola 68000CP/M-8000 CP/M for Zilog Z8000DR-DOS (Digital Research's [later Novell, Caldera, ...] DOS variant, based on CP/Mdescendants)Concurrent DOS (Digital Research's first multiuser DOS variant)Multiuser DOS (Digital Research's [later CCI's. Real's/...] multiuser DOS variant)Digital/Tandem Computers/Compaq/HPOS/8ITS (for the PDP-6 and PDP-10)Multi-Programming Executive (from HP)TOPS-10 (for the PDP-10)WAITS (for the PDP-6 and PDP-10)TENEX (from BBN, for the PDP-10)TOPS-20 (for the PDP-10)RSTS/E (multi-user time-sharing OS for PDP-11s)RSX-11 (multiuser, multitasking OS for PDP-11s)RT-11 (single user OS for PDP-11)

VMS (originally by DEC, now by HP) for the VAX mini-computer range, Alpha and IntelItanium 2; later renamed OpenVMS)Domain/OS (originally Aegis, from Apollo Computer who were bought by HP)RTE HP's Real Time Executive (ran on the HP 1000)TSB HP's Time Share Basic (yes, it was an operating system, ran on the HP 2000 series)Digital UNIX (derived from OSF/1, became HP's Tru64 UNIX)HP-UXUltrixNonStop Kernel (Originally from Tandem Computers for their line of fault-tolerant platforms;originally called Guardian). It supports concurrent execution of:GuardianOSS (POSIX-compliant Open System Services)ENEA ABOSE Flexible, small footprint, high-performance RTOS for control processorsOSEck Small footprint, scalable, high-performance RTOS for DSPsOSE Small footprint, scalable, high-performance RTOS for microcontrollersFujitsuTowns OSGoogleGoogle Chrome OS is an upcoming open source operating system designed to workexclusively with web applications. Announced on July 7, 2009, Chrome OS is set to have apublicly available stable release during the second half of 2010.Gould CSD (Computer System Division)UTX-32, Unix based OSGreen Hills Software

INTEGRITY Reliable Operating systemINTEGRITY-178B A DO-178B certified version of INTEGRITY.-velOSity A lightweight microkernel.Hewlett-PackardHP Real-Time Environment; ran on HP1000 series computers.HP Multi-Programming Executive; (MPE, MPE/XL, and MPE/iX) runs on HP 3000 and HPe3000 mini-computers.HP-UX; runs on HP9000 and Itanium servers - from small to mainframe-class computers.Honeywell


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OLERT-E; Online Executive for Real Time. Ran on Honeywell DDP-516 computers.GCOSMulticsHeartOSDEOSIntel CorporationiRMX; real-time operating system originally created to support the Intel 8080 and 8086processor families in embedded applications.ISIS-II; "Intel Systems Implementation Supervisor" was THE environment for developmentof software within the Intel microprocessor family in the early 1980'ies on their IntellecMicrocomputer Development System and clones. ISIS-II worked with 8 inch floppy disksand had an editor, cross-assemblers, a linker, an object locator, debugger, compilers forPLM (PL/I for microprocessors of the 8080/86 family), a BASIC interpreter, etc. and allowedfile management through a console.IBMOS/360 and successors on IBM mainframesOS/360 (First official OS targeted for the System/360 architecture, saw customerinstallations of the following variations:)PCP (Primary Control Program, a kernel and a ground breaking automatic space allocating

file system)MFT (original Multi-programming with a Fixed number of Tasks, replaced by MFT II.MFT II (Multi-Programming with a Fixed number of Tasks, had up to 15 fixed sizeapplication partitions, plus partitions for system tasks, initially defined at boot time butredefinable by operator command).MVT (Multi-Programming Variable Tasks, had up to 15 application regions defineddynamically, plus additional regions for system tasks)OS/VS (The official port of OS/360 targeted for the System/370 virtual memoryarchitecture. "OS/370" is not correct name for OS/VS1 and OS/VS2, but rather refers toOS/VS2 MVS and MVS/SP Version 1. Customer installations in the following variations:)SVS (Single Virtual Storage (both VS1 & VS2 began as SVS systems))OS/VS1 (Operating System/Virtual Storage 1, Virtual-memory version of MFT II)OS/VS2 (Operating System/Virtual Storage 2, Virtual-memory version of OS/MVT) butwithout multiprocessing support.OS/VS2 R2 (called Multiple Virtual Storage, MVS, eliminated most need for VS1)MVS/SE (MVS System Extensions)MVS/SP (MVS System Product)MVS/XA (MVS/SP V2. MVS supported eXtended Architecture, 31bit addressing)MVS/ESA (MVS supported Enterprise System Architecture, horizontal addressing extensions:data only address spaces called Dataspaces); a Unix environment was available startingwith MVS/ESA V4R3.OS/390 (Upgrade from MVS, with an additional Unix environment.)z/OS (OS/390 supported z/Architecture, 64bit addressing.)DOS/360 and successors on IBM mainframesBOS/360 (Early interim version of DOS/360, briefly available at a few Alpha & Beta System

360 sites)TOS/360 (Similar to BOS above and more fleeting, able to boot and run from 2x00 seriestape drives)DOS/360 (Disk Operating System (DOS). First commonly available OS for System/360 dueto problems in the OS/360 Project. Multi-programming system with up to 3 partitions.)DOS/360/RJE (DOS/360 with a control program extension that provided for the monitoringof remote job entry hardware (card reader & printer) connected by dedicated phone lines.)DOS/VS (First DOS offered on System/370 systems, provided virtual storage.)


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DOS/VSE (upgrade of DOS/VS. Still had fixed size processing partitions, but up to 14partitions.) Also referred to as VSE.VSE/SP Program product replacing DOS/VSE and VSE/AF.VSE/ESA (DOS/VSE extended virtual memory support to 32 bit addresses (ExtendedSystem Architecture)).z/VSE (Latest version of the four decades old DOS lineage. Now supports 64 bit addresses,multiprocessing, multiprogramming, SNA, TCP/IP, and some virtual machine features insupport of Linux workloads. (All DOS ref. IBM website))CP/CMS and successors on IBM mainframes (Control Program / Cambridge Monitor System,virtual machine operating system, VM line)CP-40/CMS (for System/360 Model 40)CP-67/CMS (for System/360 Model 67)VM/370 Virtual Machine / Conversational Monitor System, VM (operating system) forSystem/370 with virtual memory.VM/XA VM (operating system) eXtended Architecture for System/370 with extended virtualmemory.VM/ESA Virtual Machine /Extended System Architecture, added 32 bit addressing to VMseries.z/VM z/Architecture version of the VM OS (64 bit addressing).

TPF Line on IBM mainframes (real-time operating system, largely used by airlines)ACP (Airline Control Program)TPF (Transaction Processing Facility)z/TPF (z/Architecture extension)Unix-like on IBM mainframesUTSAIX/370AIX/ESALinux (Linux on System z, z/Linux)OpenSolaris (OpenSolaris for IBM System z)Others on IBM mainframesIBSYS (tape based operating system for IBM 7090 and IBM 7094)CTSS (The Compatible Time-Sharing System developed at MIT's Computation Center)RTOS/360 (Real Time Operating System, run on 5 NASA custom System/360/75s. A mashup by the Federal Systems Division of the MFT system management, PCP basic kernel andfile system, with MVT task management and FSD custom real time kernel extensions anderror management. The pinnacle of OS/360 development.)MTS (Michigan Terminal System for IBM System/360)TSS/360 (Time Sharing System for IBM System/360)MUSIC/SP (developed by McGill University for IBM System/370)IJMON (A bootable serial I/O monitor for loading programs for IBM 1400 and IBM 1800.)IBM Series/1EDX (Event Driven Executive)RPS (Realtime Programming System)IBM 8100

DPCX (Distributed Processing Control eXecutive)DPPX (Distributed Processing Programming Executive)IBM System/3DMS (Disk Management System)IBM System/34, IBM System/36SSP (System Support Program)IBM System/38CPF (Control Program Facility)IBM System/88


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Stratus VOS (developed by Stratus, and used for IBM System/88, Original equipmentmanufacturer from Stratus.)AS/400, iSeries, System i, Power Systems i EditionOS/400 (descendant of System/38 CPF, include System/36 SSP environment.)i5/OS (extends OS/400 with significant interoperability features.)IBM i (extends i5/OS.)UNIX on IBM POWERAIX (Advanced Interactive eXecutive, a System V Unix version)AOS (a BSD Unix version), not related to Data General AOSIBM PC and successors on x86 architecturePC DOS / IBM DOSPC DOS 1.x, 2.x, 3.x (developed jointly with Microsoft)IBM DOS 4.x, 5.0 (developed jointly with Microsoft)PC DOS 6.x, 7, 2000OS/2OS/2 1.x (developed jointly with Microsoft)OS/2 2.xOS/2 Warp 3OS/2 Warp 4

eComStation (Warp 4.5/Workspace on Demand, rebundled by Serenity SystemsInternational)OthersIBM Workplace OS (Microkernel based operating system, developed and canceled in 1990s)K42 (open-source research operating system on PowerPC or x86 based cache-coherentmultiprocessor systems)Dynix (developed by Sequent, and used for IBM NUMA-Q too.)International Computers LimitedJ and MultiJob for the System 4 series mainframesGEORGE 2/3/4 GEneral ORGanisational Environment, used by ICL 1900 series mainframesExecutive, used on the 290x range of minicomputersTME, used on the ME29 minicomputerICL VME, including early variants VME/B VME/K, appearing on the ICL 2900 Series andSeries 39 mainframesLynuxWorks (originally Lynx Real-time Systems)LynxOSMicrium Inc.MicroC/OS-II (Small pre-emptive priority based multi-tasking kernel)MicrosoftXenix (licensed version of Unix; licensed to SCO in 1987)MSX-DOS (developed by MS Japan for the MSX 8-bit computer)MS-DOS (developed jointly with IBM, versions 1.06.22)Windows 1.0 (Windows 1 - Based on Visi-On) April 30, 1985Windows 2.0 (Windows 2) May 25, 1988Windows 3.0 (Windows 3 - Is the first version of Windows to make substantial commercial

impact) August 29, 1990Windows 3.1x (Windows 3.1) March 18, 1992Windows 3.2 (Chinese-only release) November 19, 1992Windows for Workgroups 3.11 January 21, 1993Windows 95 (Windows 4) August 24, 1995Windows Chicago May 18, 1995Windows Utopia June 19, 1997Windows Plus! November 13, 1997Windows 98 (Windows 4.1) June 25, 1998


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Windows Memphis July 24, 1997Windows Millennium Edition (Windows Me - Windows 4.9) September 14, 2000OS/2 (developed jointly with IBM) October 24, 1996Windows NT 3.1 (OS/2 3.0 - Windows 3.1) March 31, 1994Windows NT 3.5 (Windows 3.5) December 29, 1994Windows NT 3.51 (Windows 3.51) February 8, 1995Windows 95 64-Bit Edition March 20, 1997Windows NT 4.0 (Windows 4) February 21, 1996Windows 2000 (Windows NT 5.0 - Windows 5) February 29, 2000Windows XP October 22, 2001Windows Whistler December 28, 2000Windows V-smile July 13, 2006Windows Neptune November 10, 1999Windows Odyssey April 5, 2001Windows Trident September 11, 2001Windows Server 2003 October 28, 2003Windows Fundamentals for Legacy PCs March 24, 2005Windows Vista January 26, 2006Windows Home Server April 19, 2007

Windows Server 2008 September 29, 2008Windows 7 October 26, 2009Windows Preinstallation Environment (WinPE) April 13, 2009Windows CE (OS for handhelds, embedded devices, and real-time applications that is similarto other versions of Windows) May 30, 2002Windows CE 3.0 December 25, 2002Windows CE 5.0 February 27, 2003Windows CE 6.0 May 5, 2004Windows Mobile (based on Windows CE, but for a smaller form factor) April 14, 2005Singularity - A research operating system written mostly in managed code (C#) November30, 2009Midori - A managed code operating system July 21, 2010Xbox and Xbox 360 OS custom operating systems May 12, 2005 and August 10, 2006MontaVista SoftwareMontaVista LinuxMontaVista Professional EditionMontaVista Carrier Grade EditionMontaVista MobilinuxNCR CorporationTMX - Transaction Management eXecutiveNovellNetWare network operating system providing high-performance network services. Has beensuperseded by Open Enterprise Server line, which can be based on NetWare or Linux toprovide the same set of services.Open Enterprise Server, the successor to NetWare.

OpenSUSE, SUSE operating system (formerly SuSe and s.u.s.e)Quadros SystemsRTXC Quadros RTOS proprietary C-based RTOS used in embedded systemsQANTELBEST - Business Executive System for TimesharingRCATSOS, first OS supporting virtual addressing of the main storage and support for bothtimeshare and batch interfaceRoweBots


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Unison RTOS Ultra Tiny Embedded Linux Compatible RTOSDSPnano RTOS Ultra Tiny Embedded Linux Compatible RTOSUnison/Reliant V3 pSOS derivative RTOSSCO / The SCO GroupXenix, Unix System III based distribution for the Intel 8086/8088 architectureXenix 286, Unix System V Release 2 based distribution for the Intel 80286 architectureXenix 386, Unix System V Release 2 based distribution for the Intel 80386 architectureSCO Unix, SCO UNIX System V/386 was the first volume commercial product licensed byAT&T to use the UNIX System trademark (1989). Derived from AT&T System V Release 3.2with an infusion of Xenix device drivers and utilities plus most of the SVR4 features

SCO Open Desktop, the first 32-bit graphical user interface for UNIX Systems running onIntel processor-based computers. Based on SCO UnixSCO OpenServer 5, AT&T UNIX System V Release 3 basedSCO OpenServer 6, SVR5 (UnixWare 7) based kernel with SCO OpenServer 5 applicationand binary compatibility, system administration, and user environmentsUnixWareUnixWare 2.x, based on AT&T System V Release 4.2MPUnixWare 7, UnixWare 2 kernel plus parts of 3.2v5 (UnixWare 2 + OpenServer 5 =

UnixWare 7). Referred to by SCO as SVR5ScosScos 1Scos 1.2Scos 1.5Scos 2.0Scos 2.5Scos 3Scos 4Scos 5Scos 6 (Latest Stable Version)SDS (Scientific Data Systems)CP Control Program. SDS later acquired by Xerox, then Honeywell.SEL (Systems Engineering Laboratories)Real Time Monitor (RTM)MPX-32SYSGOPikeOS is a certified real time operating system for safety and security critical embeddedsystemsTmaxSoftTmax WindowTRON ProjectTRON (open real-time operating system kernel)Unicoi SystemsFusion RTOS highly prolific, license free Real-time operating system.

DSPOS was the original project which would become the royalty free Fusion RTOS.UnisysUnisys OS 2200 operating systemUNIVAC (later Unisys)EXEC IEXEC IIEXEC 8 Ran on 1100 series.VS/9, successor to RCA TSOSWavecom


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Open AT OSWang Laboratories2200T Wang BASIC based system for the multi-user, 2200T systems. Products included asystem called Personal Computer before the term was made more popular with IBMproducts.2200VP/MVP Wang BASIC based system for the higher performance, 2200VP/MVP multi-user systems. Contained sophisticated micro-code programming for high performanceoperation.WPS Wang Word Processing System. Micro-code based system. Very clever and productivesystem developed by Harold Kaplow while at Wang. Eventually phased out by the PC andWord Perfect.OIS Wang Office Information System. Successor to the WPS. Combined the WPS andVP/MVP systems. Harold Kaplow was its principal architect. Eventually phased out by the2200VS.2200VS IBM assembler instruction set microcode emulation. Supported the Wang 2200VShigh-performance, multi-user systems. Designed to be a COBOL developers dream machine.Included some of the OIS operating system code. Eventually phased out by the UNIXoperating system.Wind River Systems

VxWorks Small footprint, scalable, high-performance RTOOtherLisp-basedSymbolics Genera written in a systems dialect of the Lisp programming language calledZetaLisp and Symbolics Common Lisp. Genera was ported to a virtual machine for the DECAlpha line of computers.Texas Instruments' Explorer Lisp machine workstations also had systems code written inLisp Machine Lisp.The Xerox 1100 series of Lisp machines ran an operating system written in Interlisp thatwas also ported to virtual machine called "Medley."Lisp Machines, Inc. also known as LMI, also ran an operating system based on MIT's LispMachine Lisp.Non-standard language-basedThe Mesa programming language was used to implement the Pilot operating system, used inXerox Star workstations.PERQ Operating System (POS) was written in PERQ Pascal.Other proprietary non-Unix-likeEOS; developed by ETA Systems for use in their ETA-10 line of supercomputersEMBOS; developed by Elxsi for use on their mini-supercomputersGCOS is a proprietary Operating System originally developed by General ElectricPC-MOS/386; DOS-like, but multiuser/multitaskingSINTRAN III; an operating system used with Norsk Data computers.THEOSTRS-DOS; A floppy-disk-oriented OS supplied by Tandy/Radio Shack for their Z80-basedline of personal computers.

NewDos/80; A third-party OS for Tandy's TRS-80 personal computers.TX990/TXDS, DX10 and DNOS; proprietary operating systems for TI-990 minicomputersMAI Basic Four; An OS implementing Business Basic from MAI Systems.Michigan Terminal System; Developed by a group of American universities for IBM 360series mainframesMUSIC/SP; an operating system developed for the S/370, running normally under VMSkyOS; commercial desktop OS for PCsTSX-32; a 32-bit operating system for x86 platform.OS ES; an operating system for ES EVM


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Prolog-Dispatcher; used to control Soviet Buran space ship.Other proprietary Unix-like and POSIX-compliantAegis (Apollo Computer)Amiga Unix (Amiga ports of Unix System V release 3.2 with Amiga A2500UX and SVR4 withAmiga A3000UX. Started in 1989, last version was in 1992)CLIX (Intergraph's System V implementation)Coherent (Unix-like OS from Mark Williams Co. for PC class computers)DC/OSx (DataCenter/OSx was an operating system for MIPS based systems developed byPyramid Technology)DG/UX (Data General Corp)DNIX from DIABDSPnano RTOS (POSIX nanokernel, DSP Optimized, Open Source)Idris workalike from WhitesmithsINTERACTIVE UNIX (a port of the UNIX System V operating system for Intel x86 byINTERACTIVE Systems Corporation)IRIX from SGIMeikOSNeXTSTEP (developed by NeXT; a Unix-based OS based on the Mach microkernel)OS-9 Unix-like RTOS. (OS from Microware for Motorola 6809 based microcomputers)

OS9/68K Unix-like RTOS. (OS from Microware for Motorola 680x0 based microcomputers;based on OS-9)OS-9000 Unix-like RTOS. (OS from Microware for Intel x86 based microcomputers; basedon OS-9, written in C)OSF/1 (developed into a commercial offering by Digital Equipment Corporation)OpenStepQNX (POSIX, microkernel OS; usually a real time embedded OS)Rhapsody (an early form of Mac OS X)RISC/os (a port by MIPS of 4.3BSD to the RISC MIPS architecture)RMXSCO UNIX (from SCO, bought by Caldera who renamed themselves SCO Group)SINIX (a port by SNI of Unix to the RISC MIPS architecture)Solaris (Sun's System V-based replacement for SunOS)SunOS (BSD-based Unix system used on early Sun hardware)SUPER-UX (a port of System V Release 4.2MP with features adopted from BSD and Linux forNEC SX architecture supercomputers)System V (a release of AT&T Unix, 'SVR4' was the 4th minor release)System V/AT, 386 (The first version of AT&T System V UNIX on the IBM 286 and 386 PCs,ported and sold by Microport)Trusted Solaris (Solaris with kernel and other enhancements to support multilevel security)UniFLEX (Unix-like OS from TSC for DMA-capable, extended addresses, Motorola 6809based computers; e.g. SWTPC, GIMIX, )

Unicos (the version of Unix designed for Cray Supercomputers, mainly geared to vectorcalculations)Unison RTOS (Multicore RTOS with DSP Optimization)

Non-proprietaryUnix-likeResearch Unix-like and other POSIX-compliantMinix (study OS developed by Andrew S. Tanenbaum in the Netherlands)Plan 9 (distributed OS developed at Bell Labs, based on original Unix design principles yetfunctionally different and going much further)Inferno (distributed OS derived from Plan 9, originally from Bell Labs)Plan B (distributed OS derived from Plan 9 and Off++ microkernel)Unix (OS developed at Bell Labs ca 1970 initially by Ken Thompson)


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Xinu, (Study OS developed by Douglas E. Comer in the USA)Free/Open source Unix-likeBSD (Berkeley Software Distribution, a variant of Unix for DEC VAX hardware)FreeBSD (one of the outgrowths of UC Regents' abandonment of CSRG's 'BSD Unix')NetBSD (one of the outgrowths of UC Regents' abandonment of CSRG's 'BSD Unix')OpenBSD forked from NetBSDGNUnix (concept unix-like operating system for ATMEL microcontrollers)Linux (GNU Free/Open Source Operating System Software combined with the Linux kernel)See also: List of Linux distributionsDarwinOpenSolaris, contains original Unix (SVR4) codeAuroraUX, fork of OpenSolarisRTEMS (Real-Time Executive for Multiprocessor Systems)SSS-PC, developed at Tokyo UniversitySyllable DesktopVSTaFMI/OS, successor of VSTaOther Unix-like

TUNIS (University of Toronto)Non-Unix-likeResearch non-Unix-likeAmoeba (research OS by Andrew S. Tanenbaum)Cambridge CAP computer operating system demonstrated the use of security capabilities,both in hardware and software, also a useful fileserver. Written in ALGOL 68C.CroquetHouse Haskell User's Operating System and Environment, research OS written in Haskelland C.ILIOS Research OS designed for routingEROS microkernel, capability-basedCapROS microkernel EROS successor.Coyotos microkernel EROS successor, goal: be first formally verified OS.L4 Second generation microkernelMach (from OS kernel research at Carnegie Mellon University; see NeXTSTEP)MONADS, capability-based OS designed to support the MONADS hardware projectsSPEEDOS (Secure Persistent Execution Environment for Distributed Object Systems) buildson MONADS ideasNemesis Cambridge University research OS - detailed quality of service abilities.Spring (research OS from Sun Microsystems)V from Stanford, early 1980sFreeNOS, a microkernel educational operating systemFree/Open source non-Unix-likeFullPliant (programming language-based)FreeDOS (open source DOS variant)

FreeVMS (open source VMS variant)Haiku (open source inspired by BeOS, under development)Kinetic (written in Haskell)MonaOS (written in C++)ReactOS (Windows NT-compatible OS, in early devOZONE (object-oriented)MustiOS (32-bits operating system)Disk Operating Systems


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86-DOS (developed at Seattle Computer Products by Tim Paterson for the new Intel 808xCPUs; licensed to Microsoft, became PC DOS/MS-DOS. Also known by its working titleQDOS.)PC DOS (IBM's DOS variant, developed jointly with Microsoft, versions 1.0 7, 2000)MS-DOS (Microsoft's DOS variant for OEM, developed jointly with IBM, versions 1.x 6.Microsoft's now abandoned DOS variant)FreeDOS (open source DOS variant)ProDOS (operating system for the Apple II series computers)PTS-DOS (DOS variant by Russian company Phystechsoft)RDOS by Leif Ekblad (not to be confused with Data General Corporation's "Real-time DiskOperating System" for Data General Nova and Data General Eclipse minicomputers).TurboDOS (Software 2000, Inc.) for Z80 and Intel 8086 processor-based systemsMulti-tasking user interfaces and environments for DOSDESQview+ QEMM 386 multi-tasking user interface for DOSDESQView/X (X-windowing GUI for DOS)NetworkCambridge RingCSIRONET by (CSIRO)CTOS (Convergent Technologies, later acquired by Unisys)

Data ONTAP by NetAppSAN-OS by Cisco (now NX-OS)Enterprise OS by McDATAExtremeWare by Extreme NetworksExtremeXOS by Extreme NetworksFabric OS by BrocadeJUNOS (Juniper Networks)NetWare (networking OS by Novell)NOS (developed by CDC for use in their Cyber line of supercomputers)Novell Open Enterprise Server (Open Source networking OS by Novell. Can incorporateeither SUSE Linux or Novell NetWare as its kernel).OliOSPlan 9 (distributed OS developed at Bell Labs, based on Unix design principles but notfunctionally identical)Inferno (distributed OS derived from Plan 9, originally from Bell Labs)Plan B (distributed OS derived from Plan 9 and Off++ microkernel)TurboDOS (Software 2000, Inc.)XPATH OS by BrocadeWeb operating systemsChrome OSG.ho.steyeOSDesktopTwoYouOSBrowser OS

Glide OSLucid DesktopGeneric/commodity and otherBLIS/COBOLBluebottle also known as AOS (a concurrent and active object update to the Oberonoperating system)BS1000 by Siemens AGBS2000 by Siemens AG, now BS2000/OSD from Fujitsu-Siemens Computers (formerlySiemens Nixdorf Informationssysteme)


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BS3000 by Siemens AG (functionally similar to OS-IV and MSP from Fujitsu)FLEX9 (by TSC for Motorola 6809 based machines; successor to FLEX, which was forMotorola 6800 CPUs)FutureOS (for Amstrad/Schneider CPC6128 and CPCPlus machines)GEM (windowing GUI for CP/M, DOS, and Atari TOS)GEOS (popular windowing GUI for PC, Commodore, Apple computers)JavaOSJNode JNode.org's OS written 99% in Java (native compiled), provides own JVM and JITcompiler. Based on GNU ClasspathJX Java operating system that focuses on a flexible and robust operating systemarchitecture developed as an open source system by the University of Erlangen.KERNAL (default OS on Commodore 64)MERLIN for the Corvus ConceptMorphOS (Amiga compatible)MSP by Fujitsu (successor to OS-IV), now MSP/EX, also known as Extended SystemArchitecture (EXA), for 31-bit modenSystem by Luis Mateu at DCC, Universidad de ChileNetWare (networking OS by Novell)Oberon (operating system) (developed at ETH-Zrich by Niklaus Wirth et al.) for the Ceres

and Chameleon workstation projects.OSD/XC by Fujitsu-Siemens (BS2000 ported to an emulation on a Sun SPARC platform)OS-IV by Fujitsu (based on early versions of IBM's MVS)Pick (often licensed and renamed)PRIMOS by Prime Computer (sometimes spelled PR1MOS and PR1ME)Sinclair QDOS (multitasking for the Sinclair QL computer)SSB-DOS (by TSC for Smoke Signal Broadcasting; a variant of FLEX in most respects)SymbOS (GUI based multitasking operating system for Z80 computers)Symobi (GUI based modern micro-kernel OS for x86, ARM and PowerPC processors,developed by Miray Software; used and developed further at Technical University of Munich)TraOS, kin to Darwin? Seems active late 2009TripOS, 1978TurboDOS (Software 2000, Inc.)UCSD p-System (portable complete programming environment/operating system/virtualmachine developed by a long running student project at the Univ Calif/San Diego; directedby Prof Ken Bowles; written in Pascal)UMIX, made for the ICFP Programming Contest 2006.ScaraOS, a 32 bit mutiboot OS kernel for IA32VOS by Stratus Technologies with strong influence from MulticsVOS by Hitachi for its IBM-compatible mainframes, based on IBM's MVSVM2000 by Siemens AGVisiOn (first GUI for early PC machines; not commercially successful)VPS/VM (IBM based, main operating system at Boston University for over 10 years.)aceos under GPLMiraculix Russian OS, under unknown license.

For Elektronika BKANDOSAO-DOSBASISCSI-DOSDOSB10DX-DOSFA-DOSHC-DOS


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KMONMicroDOSMK-DOSNORDNORTON-BKRAMONPascalDOSRT-11ROM embeddedRT-11SJOS BK-11 (RT-11 version)Turbo-DOSBKUNIXOS/A WASPHobbyAROS (AROS Research Operating System, formerly known as Amiga Research OperatingSystem)AtheOS (branched to become Syllable Desktop)Syllable Desktop (a modern, independently originated OS; see AtheOS)

DexOS, (Games console OS, for x86, written in FASM)DSPnano RTOS FREEEROS (Extremely Reliable Operating System)FAMOS (Foremost Advanced Memory Operating System)HelenOS, based on a preemptible microkernel designKolibriOS (a fork of MenuetOS)LoseThos, the stated goal is "programming as entertainment" - oriented toward videogamesLSE/OSMenuetOS (extremely compact OS with GUI, written entirely in FASM assembly language)Mbius (an open-source operating system for the IA-32 platform (Intel i386 andcompatibles) )MikeOSNewOSUnison RTOS FREEVisopsysTajOSeSTORMDreamOS Implemented in SchemeEmbeddedPersonal digital assistants (PDAs)iPhone OS (a subset of Mac OS X)Inferno (distributed OS originally from Bell Labs)Palm OS from Palm, Inc; now spun off as PalmSourceSymbian OS

Windows CE, from MicrosoftPocket PC from Microsoft, a variant of Windows CE.Windows Mobile from Microsoft, a variant of Windows CE.Embedded LinuxOpenZaurusngstrm distributionFamiliar LinuxwebOS from Palm, Inc.Maemo based on Debian deployed on Nokia's Nokia 770, N800 and N810 Internet Tablets.


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MS-DOS on Poqet PCNewton OS on Apple Newton MessagepadVT-OS for the Vtech HelioMagic CapNetBSDPlan 9 from Bell LabsDigital media playersDSPnano RTOSipodlinuxPixo OSRockBoxiPhone OS (a subset of Mac OS X)iriver clix OSRobotsRobotic Operating SystemDave's Robotic Operating SystemSmartphonesBlackBerry OSEmbedded Linux

Access Linux PlatformAndroidbadaOpenmoko LinuxOPhoneMeeGo (from merger of Maemo & Moblin)MobilinuxMotoMagxQt ExtendedLiMo PlatformwebOSiPhone OS (a subset of Mac OS X)Palm OSSymbian platform (successor to Symbian OS)Windows Mobile (recently changed to Windows Phone)RoutersAlliedWare by Allied Telesis (aka Allied Telesyn)AirOS by Ubiquiti NetworksCatOS by Cisco SystemsCisco IOS (originally Internetwork Operating System) by Cisco SystemsCyROS by Cyclades CorporationDD-WRT by NewMedia-NETInferno (distributed OS originally from Bell Labs)IOS-XR by Cisco SystemsIronWare by Foundry Networks

JunOS by Juniper NetworksRouterOS by MikrotikROX by RuggedcomScreenOS by Juniper Networks, originally from NetscreenTimos by Alcatel-LucentUnison Operating System by RoweBotsFTOS by Force10 NetworksRTOS by Force10 NetworksOther embedded


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FreeBSDuClinuxLOCUSMINIXfreeRTOSpolyBSD (embedded NetBSD)ROM-DOSTinyOSTasker

Windows EmbeddedWindows CEWindows Embedded StandardWindows Embedded EnterpriseWindows Embedded POSReadyCapability-basedLEGO MindstormsbrickOSleJOSChyanOS

SOOSOther capability-basedKeyKOS nanokernelEROS microkernelCapROS EROS successorCoyotos EROS successor, goal: be first formally verified OSMONADS, designed to support the MONADS hardware projects.SPEEDOS builds on MONADS ideas

neteffect-operatingsystems

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