perating system
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An
operating system (
OS) is the
software on a computer that manages the way different
programs use its
hardware, and regulates the ways that a
user controls the computer.
[1][2] 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 multiple programs—from
cellular phones and
video game consoles to
supercomputers and
web servers. Some popular modern operating systems for personal computers include
Microsoft Windows,
Mac OS X, and
Linux[3] (see also:
list of operating systems,
comparison of operating systems).
[edit] Summary
Because early computers were often built for only a single task, like a calculator, operating systems did not exist in their modern and more complex forms until the 1960s.
[4] As computers evolved into being devices that could run different programs in succession, programmers began putting libraries of common programs (in the form of
computer code) onto the computer in order to avoid duplication and speed up the process. Eventually, computers began being built to automatically switch from one task to the next. The creation of
runtime libraries to manage processing and printing speed came next, which evolved into programs that could interpret different types of programming languages into machine code. When personal computers by companies such as
Apple Inc.,
Atari,
IBM and
Amiga became popular in the 1980s, vendors began adding features such as software scheduling and hardware maintenance.
An operating system can be divided into many different parts. One of the most important parts is the
kernel, which controls low-level processes that the average user usually cannot see: 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, and deciding how to interpret information received by networks. The
user interface is the part of the operating system that interacts with the computer user directly, allowing them to control and use programs. The user interface may be
graphical with icons and a desktop, or
textual, with a command line. Another similar feature is an
Application programming interface, which is a set of services and code libraries that let applications interact with one another, as well as the operating system itself. Depending on the operating system, many of these components may not be considered an actual part. For example, Windows considers its user interface to be part of the operating system, while many versions of Linux do not.
[edit] History
The Manchester Mark became more than just a complex calculator by letting programmers run different types of processes, one of the early ideas of an operating system.
OS/360 was put on all IBM mainframe computers beginning in 1964, including the computers that helped NASA put a man on the moon
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 on punched paper cards and tape. The program would be loaded into the machine, and the machine would be set to work until the program completed or crashed. Programs could generally be debugged via a front panel using toggle switches and panel lights. It is said that
Alan Turing was a master of this on the early Manchester Mark 1 machine, and he was already deriving the primitive conception of an operating system from the principles of the Universal Turing machine.
[citation needed]
Later machines came with libraries of
software, which would be linked to a user's program to 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 colored clothes-pegs to indicate job-priority.
[edit] Mainframes
Through the 1950s, many major features were pioneered in the field of operating systems, including
batch processing, input/output
interrupt,
buffering,
multitasking,
spooling, and
runtime libraries. These features were included or not included in application software at the option of application programmers, rather than in a separate operating system used by all applications. In 1959 the
SHARE Operating System was released as an integrated utility for the
IBM 704 and
IBM 709 mainframes.
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.
[citation needed] In the mid-'70s, the
MVS, the descendant of OS/360 offered the first
[citation needed] implementation of using
RAM as a transparent
cache for data.
OS/360 also pioneered the concept that the operating system keeps track of all of the system resources that are used, including program and data space allocation in main memory and file space in secondary storage, and
file locking during update. When the process is terminated for any reason, all of these resources are re-claimed by the operating system.
An alternative
CP-67 system started a whole line of operating systems focused on the concept of
virtual machines.
Control Data Corporation developed the
SCOPE operating system in the 1960s, for batch processing. In cooperation with the University of Minnesota, the
KRONOS and later the
NOS operating systems were developed during the 1970s, which supported simultaneous batch and timesharing use. Like many commercial timesharing systems, its interface was an extension of the Dartmouth BASIC operating systems, one of the pioneering efforts in timesharing and programming languages. In the late 1970s, Control Data and the University of Illinois developed the
PLATO operating system, which used plasma panel displays and long-distance time sharing networks. Plato was remarkably innovative for its time, featuring real-time chat, and multi-user graphical games.
Burroughs Corporation introduced the
B5000 in 1961 with the
MCP, (
Master Control Program) operating system. The
B5000 was a
stack machine designed to exclusively support high-level languages with no machine language or assembler, and indeed the
MCP was the first OS to be written exclusively in a high-level language –
ESPOL, a dialect of
ALGOL.
MCP also introduced many other ground-breaking innovations, such as being the first commercial implementation of
virtual memory. During development of the
AS400,
IBM made an approach to Burroughs to licence MCP to run on the AS400 hardware. This proposal was declined by Burroughs management to protect its existing hardware production.
MCP is still in use today in the
Unisys ClearPath/MCP line of computers.
UNIVAC, the first commercial computer manufacturer, produced a series of EXEC operating systems. Like all early main-frame systems, this was a batch-oriented system that managed magnetic drums, disks, card readers and line printers. In the 1970s, UNIVAC produced the Real-Time Basic (RTB) system to support large-scale time sharing, also patterned after the Dartmouth BASIC system.
General Electric and MIT developed General Electric Comprehensive Operating Supervisor (GECOS), which introduced the concept of ringed security privilege levels. After acquisition by Honeywell it was renamed to
General Comprehensive Operating System (GCOS).
Digital Equipment Corporation developed many operating systems for its various computer lines, including
TOPS-10 and
TOPS-20 time sharing systems for the 36-bit PDP-10 class systems. Prior to the widespread use of UNIX, TOPS-10 was a particularly popular system in universities, and in the early
ARPANET community.
In the late 1960s through the late 1970s, several hardware capabilities evolved that allowed similar or ported software to run on more than one system. Early systems had utilized microprogramming 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's after the 360/40 (except the 360/165 and 360/168) were microprogrammed implementations. But soon other means of achieving application compatibility were proven to be more significant.
The enormous investment in software for these systems made since 1960s caused most of the original computer manufacturers to continue to develop compatible operating systems along with the hardware. The notable supported mainframe operating systems include:
[edit] Microcomputers
PC-DOS was an early OS for personal computers that featured a command line interface.
Mac OS by
Apple Computers became the first widespread OS to feature a graphical user interface. Many of its features such as windows and icons would later become commonplace in GUIs.
The first
microcomputers did not have the capacity or need for the elaborate operating systems that had been developed for mainframes and minis; minimalistic operating systems were developed, often loaded from
ROM and known as
Monitors. One notable early disk-based operating system was
CP/M, which was supported on many early microcomputers and was closely imitated in
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. In the 80's Apple Computer Inc. (now
Apple Inc.) abandoned its popular
Apple II series of microcomputers to introduce the
Apple Macintosh computer with an innovative
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 operating systems like those of earlier
minicomputers and
mainframes. Microsoft responded to this progress by hiring
Dave Cutler, who had developed the
VMS operating system for
Digital Equipment Corporation. He would lead the development of the
Windows NT operating system, which continues to serve as the basis for Microsoft's operating systems line.
Steve Jobs, a co-founder of
Apple Inc., started
NeXT Computer Inc., which developed the
Unix-like NEXTSTEP 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 of a complete
free software replacement to the proprietary
UNIX operating system. While the project 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 volunteers over the Internet, released the first version of the
Linux kernel. It was soon merged with the 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
Free Software Foundation remain opposed to, preferring the name "GNU/Linux" instead. The Berkeley Software Distribution, known as
BSD, is the UNIX derivative distributed by the University 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 as
FreeBSD,
NetBSD and
OpenBSD.
putang ina mo chocks
[edit] Examples of operating systems
[edit] Microsoft Windows
Windows XP operating system for personal computers.
Microsoft Windows is a family of
proprietary operating systems most commonly used on personal computers. It is the most common family of operating systems for the personal computer, with about 90% of the market share.
[5][6][7] Currently, the most widely used version of the Windows family is
Windows XP[citation needed], released on October 25, 2001. The newest version is
Windows 7 for personal computers and
Windows Server 2008 R2 for servers.
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 versions sometimes supported other architectures.
Windows is also used on servers, supporting applications such as
web servers and
database servers. In recent years, Microsoft has spent significant marketing and research & development money to demonstrate that Windows is capable of running any enterprise application, which has resulted in consistent price/performance records (see the
TPC) and significant acceptance in the enterprise market. However, its usage in servers is not as widespread as personal computers, and here Windows actively competes against Linux and BSD for market share, while still capturing a steady majority by some accounts.
[8][9]
[edit] Mac OS X
The standard user interface of 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
Macintosh computers. Mac OS X is the successor to the original
Mac OS, which had been Apple's primary operating system since 1984. Unlike its predecessor, Mac OS X is a
UNIX operating system built on technology that had been developed at
NeXT through the second half of the 1980s 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
Mac OS X v10.6, which was first made available on August 28, 2009. Releases of Mac OS X are named 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 counterpart but usually runs on Apple's line of Macintosh
server hardware. Mac OS X Server includes work group management and administration software tools that provide simplified access to key
network services, including a
mail transfer agent, a
Samba server, an
LDAP server, a
domain name server, and others.
[edit] Unix and Unix-like operating systems
Evolution of
Unix systems
Ken Thompson wrote
B, mainly based on
BCPL, which he used to write Unix, based on his experience 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 every modern operating system (see
History). The
Unix-like family is a diverse group of operating systems, with several major sub-categories including
System V,
BSD, and
GNU/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-like systems run on a wide variety of machine architectures. They are used heavily for
servers 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's hardware. Others, such as
Solaris, can run on multiple types of hardware, including
x86 servers and PCs. Apple's
Mac OS X, a
hybrid kernel-based BSD variant derived from
NeXTSTEP,
Mach, and
FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS.
Unix interoperability was sought by establishing the
POSIX standard. The POSIX standard can be applied to any operating system, although it was originally created for various Unix variants.
[edit] BSD and its descendants
The first server for the World Wide Web ran on NeXTSTEP, based on BSD.
A subgroup of the Unix family is the
Berkeley Software Distribution family, which includes
FreeBSD,
NetBSD, and
OpenBSD. These operating systems are most commonly found on
webservers, 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 commonly used by computers to connect, send and receive data over a network were widely implemented and refined in BSD. The
world wide web was also first demonstrated on a number 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 Unix system. Over time, students and staff in the computer science department there began adding new programs to make things easier, such as text editors. When Berkely received new
VAX computers in 1978 with Unix installed, the school's undergraduates modified Unix even more in order to take advantage of the computer's hardware possibilities. The
Defense Advanced Research Projects Agency of the US
Department of Defense took interest, and decided to fund the project. Many schools, corporations, and government organizations took notice and started to use Berkeley's version of Unix instead of the official one distributed by AT&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 of these computers was used by
Tim Berners-Lee as the first webserver to create the World Wide Web.
Developers like
Keith Bostic encouraged the project to replace any non-free code that originated with Bell Labs. Once this was done, however, AT&T sued. Eventually, after two years of legal disputes, the BSD project came out ahead and spawned a number of free derivatives, such as
FreeBSD and
NetBSD. However, the two year wait had set the stage for two projects that would ultimately eclipse both BSD and Unix: GNU and Linux.
[edit] Plan 9
Ken Thompson,
Dennis Ritchie and
Douglas McIlroy at
Bell Labs designed and developed the C programming language to build the operating system Unix. Programmers at Bell Labs went on to develop Plan 9 and
Inferno, which were engineered for modern distributed environments. Plan 9 was designed from the start to be a networked operating system, and had graphics built-in, unlike Unix, which added these features to the design later. It is currently released under the
Lucent Public License. Inferno was sold to
Vita Nuova Holdings and has been released under a GPL/MIT license.
[edit] Linux and GNU
Main articles:
GNU and
Linux 
Ubuntu, a common desktop distribution of Linux
Linux is a generic name for a member in a family of operating systems that can be found on anything 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 silent giant in the world of cell phones, servers, and embedded systems. Linux has superseded Unix in most places, and is used on the 10 most powerful supercomputers in the world.
The GNU project is a mass collaboration of programmers who seek to create a completely free 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 most Linux variants. For this reason, Linux is often called
GNU/Linux. Thousands of pieces of software for virtually every operating system are licensed under the
GNU General Public License. Meanwhile, the Linux kernel began as a side project of
Linus Torvalds, a university student from Finland. In 1991, Torvalds began work on it, and posted information about his project on a newsgroup for computer students and programmers. He received a wave of support and volunteers who ended up creating a full-fledged kernel. Programmers from GNU took notice, and members of both projects worked to integrate the finished GNU parts into the linux kernel in order to create a full-fledged operating system.
[edit] Google Chrome OS
Chrome is an operating system based on the Linux kernel and designed by
Google. It is currently in development, and is targeted towards use in specific types of
netbooks. Chrome targets computer users that spend most of their time on the internet—it is technically only a web browser with no other applications, and relies on
internet applications used in the web browser to accomplish tasks such as word processing and media viewing.
Older operating systems which are still used in niche markets include
OS/2 from IBM and Microsoft;
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 minority platforms for enthusiast communities and specialist applications.
OpenVMS formerly from
DEC, is still under active development by
Hewlett-Packard. Yet other operating systems are used almost exclusively in academia, for operating systems education or to do research on operating system concepts. A typical example of a system that fulfills both roles is
MINIX, while for example
Singularity is used purely for research.
[edit] Components
The components of an operating system all exist in order to make the different parts of a computer work together. All software—from financial databases to film editors—needs to go through the operating system in order to use any of the hardware, whether it be as simple as a mouse or keyboard or complex as an internet connection.
[edit] The user interface
An example of the command line. Each command is typed out after the 'prompt', and then its output appears below, working its way down the screen. The current command prompt is at 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 used to navigate the computer.
Main article:
User InterfaceEvery computer that receives some sort of human input needs a user interface, which allows a person to interact with the computer. While devices like keyboards, mice and touchscreens make up the hardware end of this task, the user interface makes up the software for it. The two most common forms of a user interface have historically been the
Command-line interface, where computer commands are typed out line-by-line, and the
Graphical user interface, where a visual environment (most commonly with windows, buttons, and icons) is present.
[edit] Graphical user interfaces
Most of the modern computer systems support
graphical user interfaces (GUI), and often include them. In some computer systems, such as the original implementations of
Microsoft Windows and the
Mac OS, the GUI is integrated into the
kernel.
While technically a graphical user interface is not an operating system service, incorporating support for one into the operating system kernel can allow the GUI to be more responsive by reducing the number of
context switches required for the GUI to perform its output functions. Other operating systems are
modular, separating the graphics subsystem from the kernel and the Operating System. In the 1980s UNIX, VMS and many others had operating 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 graphics subsystem that is mostly in user-space, however versions between
Windows NT 4.0 and
Windows Server 2003's graphics drawing routines exist mostly in kernel space.
Windows 9x had very little distinction between the interface and the kernel.
Many computer 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-like (BSD, GNU/Linux, Solaris) systems. A number of
Windows shell replacements have been released for Microsoft Windows, which offer alternatives to the included
Windows shell, but the shell itself cannot be separated from Windows.
Numerous Unix-based GUIs have existed over time, most derived from X11. Competition among the various vendors of Unix (HP, IBM, Sun) led to much fragmentation, though an effort to standardize in the 1990s to
COSE and
CDE failed for the most part due to various reasons, eventually eclipsed by the widespread adoption of GNOME and KDE. Prior to
free software-based toolkits and desktop environments, Motif was the prevalent toolkit/desktop combination (and was the basis upon which CDE was developed).
Graphical user interfaces evolve over time. For example, Windows has modified its user interface almost every time a new major version of Windows is released, and the Mac OS GUI changed dramatically with the introduction of Mac OS X in 1999.
[10]
[edit] 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 most basic level of control over all of the computer's hardware devices. It manages memory access 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 all times, and it organizes the data for long-term
non-volatile storage with
file systems on such media as disks, tapes, flash memory, etc.
[edit] Program execution
The operating system acts as an interface between an application and the hardware. The user interacts with the hardware from "the other side". The operating system is a set of services which simplifies development of applications. Executing a program involves the creation of a process by the operating system. The
kernel creates a process by assigning memory and other resources, establishing a priority for the process (in multi-tasking systems), loading program code into memory, and executing the program. The program then interacts with the user and/or other devices and performs its intended function.
[edit] Interrupts
Interrupts are central to operating systems, as they provide an efficient way for the operating system to interact with and react to its environment. The alternative—having the operating system "watch" the various sources of input for events (polling) that require action—can 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 directly supported by most modern CPUs. Interrupts provide a computer with a way of automatically saving local register contexts, and running specific code in response to events. Even very basic computers support hardware interrupts, and allow the programmer to specify code which may be run when that event takes place.
When an interrupt is received, the computer's hardware automatically suspends whatever program is currently running, saves its status, and runs computer code previously associated with the interrupt; this is analogous to placing a bookmark in a book in response to a phone call. In modern operating systems, interrupts are handled by the operating system's
kernel. Interrupts may come from either the computer's hardware or from the running program.
When a hardware device triggers an interrupt, the operating system's kernel decides how to deal with this event, generally by running some processing code. The amount of code being run depends on the priority of the interrupt (for example: a person usually responds to a smoke detector alarm before answering the phone). The processing of hardware interrupts is a task that is usually delegated to software called
device driver, which may be either part of the operating system's kernel, part of another program, or both. Device drivers may then relay information to a running program by various means.
A program may also trigger an interrupt to the operating system. If a program wishes to access hardware for example, it may interrupt the operating system's kernel, which causes control to be passed back to the kernel. The kernel will then process the request. If a program wishes additional resources (or wishes to shed resources) such as memory, it will trigger an interrupt to get the kernel's attention.
[edit] Protected mode, supervisor mode, and virtual modes
Privilege rings for the
x86 available in
protected mode. Operating systems determine which processes run in each mode.
Modern CPUs support multiple modes of operation.
CPUs with this capability use at least two modes:
protected mode and
supervisor mode. The supervisor mode is used by the operating 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 like graphics cards. Protected mode, in contrast, is used for almost everything else. Applications operate within protected mode, and can only use hardware by communicating with the kernel, which controls everything in supervisor mode.
CPUs might have other modes similar to protected mode as well, such as the virtual modes in order to emulate older processor types, 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 first few programs to run on the computer, being the
BIOS,
bootloader and the operating system have unlimited access to hardware - and this is required because, by definition, initializing a protected environment can only be done outside of one. However, when the operating system 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 can maintain exclusive control over things like access to hardware and memory.
The term "protected mode resource" generally refers to one or more CPU registers, which contain information that the running program isn't allowed to alter. Attempts to alter these resources generally causes a switch to supervisor mode, where the operating system can deal with the illegal operation the program was attempting (for example, by killing the program).
[edit] Memory management
Among other things, a multiprogramming operating system
kernel must be responsible for managing all system memory which is currently in use by programs. This ensures that a program does not interfere with memory already used by another program. Since programs time share, each program must have independent access to memory.
Cooperative memory management, used by many early operating systems assumes that all programs make voluntary use of the
kernel's memory manager, and do not exceed their allocated 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 affected or overwritten. Malicious programs, or viruses may purposefully alter another program's memory or may affect the operation of the operating system itself. With cooperative memory management it takes only one misbehaved program to crash the system.
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't exist in all computers.
In both segmentation and paging, certain
protected mode registers specify to the CPU what memory address it should allow a running program to access. Attempts to access other addresses 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, and since it is both difficult to assign a meaningful result to such an operation, and because it is usually a sign of a misbehaving program, the
kernel will generally resort to terminating the offending program, and will report the error.
Windows 3.1-Me had some level of memory protection, but programs could easily circumvent the need to use it. A
general protection fault would be produced indicating a segmentation violation had occurred, however the system would often crash anyway.
[edit] Virtual memory
Many operating systems can "trick" programs into using memory scattered around the hard disk and RAM as if it is one continuous chunk of memory called virtual memory.
Main article:
Virtual memoryThe use of virtual memory addressing (such as paging or segmentation) means that the kernel can choose what memory each program may use at any given time, allowing the operating 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 it would if the program were to exceed its allocated memory. (See section on memory management.) 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 the program which triggered it, granting it access to the memory requested. This gives the kernel discretionary power over where a particular application's memory is stored, or even whether or not it has actually been allocated yet.
In modern operating systems, memory which is accessed less frequently can be temporarily stored on disk or other media to make that space available for use by other programs. This is called
swapping, as an area of memory can be used by multiple programs, and what that memory area contains can be swapped or exchanged on demand.
[edit] Multitasking
Multitasking refers to the running of multiple independent computer programs on the same computer; giving the appearance that it is performing the tasks at the same time. Since most computers can do at most one or two things at one time, this is generally done via time-sharing, which means that each program uses a share of the computer's time to execute.
An operating system
kernel contains a piece of software called a
scheduler which determines how much time each program will spend executing, and in which order execution control should be passed to programs. Control is passed to a process by the kernel, which allows the program access to the
CPU and memory. Later, control is returned to the kernel through some mechanism, so that another program may be allowed to use the CPU. This so-called passing of control between the kernel and applications is called a
context switch.
An early model which governed the allocation of time to programs was called
cooperative multitasking. In this model, when control is passed to a program by the kernel, it may execute for as long as it wants before explicitly returning control to the kernel. This means that a malicious or malfunctioning program may not only prevent any other programs from using 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 drivers and 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 are given regular time on the CPU. This implies that all programs must be limited in how much time 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 has elapsed. (See above sections on Interrupts and Dual Mode Operation.)
On many single user operating systems cooperative multitasking is perfectly adequate, as home computers generally run a small number of well tested programs.
Windows NT was the first version of
Microsoft Windows which enforced preemptive multitasking, but it didn't reach the home user market until
Windows XP, (since
Windows NT was targeted at professionals.)
[edit] Disk access and file systems
Filesystems allow users and programs to organize and sort files on a computer, often through the use of directories (or "folders")
Access to data stored on disks is a central feature of all operating systems. Computers store data on
disks using
files, which are structured in specific ways in order to allow for faster access, higher reliability, and to make better use out of the drive's available space. The specific way in which files are stored on a disk is called a
file system, and enables files to have names and attributes. It also allows them to be stored in a hierarchy of directories or folders arranged in a
directory tree.
Early operating systems generally supported a single type of disk drive and only one kind of file system. Early file systems were limited in their capacity, speed, and in the kinds of file names and directory structures they could use. These limitations often reflected limitations in the operating systems they were designed for, making it very difficult for an operating system 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 known as a
virtual file system or VFS. An operating system like UNIX supports a wide array of storage devices, regardless of their design or
file systems to be accessed through a common
application programming interface (API). This makes it unnecessary for programs to have any knowledge about the device they are accessing. A VFS allows the operating system to provide programs with access to an unlimited number of devices with an infinite variety of file systems installed on them through the use of specific
device drivers and file system drivers.
A connected
storage device such as a
hard drive is accessed through a
device driver. The device driver understands the specific language of the drive and is able to translate that language 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 contents of the disk drive in raw format, which may contain one or more file systems. A file system driver is used to translate the commands used to access each specific file system into a standard 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, and directories/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. Allowed characters in file names,
case sensitivity, and the presence of various kinds of
file attributes makes the implementation of a single interface for every file system a daunting task. Operating systems tend to recommend using (and so support natively) file systems specifically designed for them; for example,
NTFS in Windows and
ext3 and
ReiserFS in GNU/Linux. However, in practice, third party drives are usually available to give support for the 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 in Windows through
FS-driver and
rfstool).
Support for file systems is highly varied among modern operating systems although there are several common file systems which almost all operating systems include support and drivers for. Operating systems vary on file system support and on the disk formats they may be installed on. Under Windows each file system is usually limited in application to certain media, for example CDs must use ISO 9660 or UDF, and as of
Windows Vista,
NTFS is the only file system which the operating system can be installed on. It is possible to install 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 file located on another file system.
[edit] Device drivers
Main article:
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, operative systems essentially dictate how every type of device should be controlled. The function of the device driver is then to translate these operative system 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 system's point of view.
Under versions of Windows before Vista and versions of Linux before 2.6, all driver execution was co-operative, meaning that if a driver entered an infinite loop it would freeze the system. More recent revisions of these opearting systems incorporate kernel preemption, 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 tasks to do.
[edit] Networking
Currently most operating systems support a variety of networking protocols, hardware, and applications for using them. This means that computers running dissimilar operating systems can participate in a common
network for sharing resources such as
computing, files, printers, and scanners using either wired or wireless connections. Networks can essentially allow a computer's operating system to access the resources of a remote computer to support the same functions as it could if those resources were connected directly to the local computer. This includes everything from simple communication, to using networked 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 line interface.
Client/server networking involves a program on a computer somewhere which connects via a network to another computer, called a server. Servers offer (or host) various services to other network computers and users. These services are usually provided through ports or numbered access points beyond the server's
network address[disambiguation needed]. Each port number is usually associated with a maximum of one running program, which is responsible for handling requests to that port. A daemon, being a user program, can in turn access the local hardware resources of that computer by passing requests to the operating system kernel.
Many operating systems support one or more vendor-specific or open networking protocols as well, for example,
SNA on
IBM systems,
DECnet on systems from
Digital Equipment Corporation, and Microsoft-specific protocols (
SMB) on Windows. Specific protocols for specific tasks may also be supported such as
NFS for file access. Protocols like
ESound, or esd can be easily extended over the network to provide sound from local applications, on a remote system's sound hardware.
[edit] Security
A computer being secure depends on a number of technologies working properly. A modern operating system provides access to a number of resources, which are available to software running on the system, and to external devices like networks via the kernel.
The operating system must be capable of distinguishing between requests which should be allowed to be processed, and others which should not be processed. While some systems may simply distinguish between "privileged" and "non-privileged", systems commonly have a form of requester
identity, such as a user name. To establish identity there may be a process of
authentication. Often a username must be quoted, and each username may have a password. Other methods of authentication, such as magnetic cards or biometric data, might be used instead. In some cases, especially connections from the network, resources may be accessed with no authentication at all (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 the requester's user account or to the variously configured groups of users to which the requester belongs.
In addition to the allow/disallow model of security, a system with a high level of security will also offer auditing options. These would allow tracking of requests for access to resources (such as, "who has been reading this file?"). Internal security, or security from an already running program is only possible if all possibly harmful requests must be carried out through interrupts to the operating system kernel. If programs can directly access hardware and resources, they cannot be secured.
External security involves a request from outside the computer, such as a login at a connected console or some kind of network connection. External requests are often passed through device drivers to the operating system's kernel, where they can be passed onto applications, or carried out directly. Security of operating systems has long been a concern because of highly sensitive data held on computers, both of a commercial and military nature. The United States
Government Department of Defense (DoD) created the
Trusted Computer System Evaluation Criteria (TCSEC) which is a standard that sets basic requirements for assessing the effectiveness of security. This became of vital importance to operating system makers, because the TCSEC was used to evaluate, classify and select computer systems being considered for the processing, storage and retrieval of sensitive or
classified information.
Network services include offerings such as file sharing, print services, email, web sites, and
file transfer protocols (FTP), most of which can have compromised security. At the front line of 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 a software firewall, which is enabled by default. A software firewall can be configured to allow 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, 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.
An alternative strategy, and the only
sandbox strategy available in systems that do not meet the
Popek and Goldberg virtualization requirements, is the operating system not running user programs as native code, but instead either
emulates a processor or provides a host for a
p-code based system such as Java.
Internal security is especially relevant for multi-user systems; it allows each user of the system to have private files that the other users cannot tamper with or read. Internal security is also vital if auditing is to be of any use, since a program can potentially bypass the operating system, inclusive of bypassing auditing.
[edit] Real-time operating systems
A
real-time operating system (RTOS) is a multitasking operating system intended for applications with fixed deadlines (
real-time computing). Such applications include some small
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 Processing Facility 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 as
VxWorks,
PikeOS,
eCos,
QNX,
MontaVista Linux and
RTLinux.
Windows CE is a
real-time operating system that shares similar APIs to desktop Windows but shares none of desktop Windows' codebase
[citation needed].
Some embedded systems use operating systems such as
Symbian OS,
Palm OS,
BSD, and
GNU/Linux, although such operating systems do not support real-time computing.
[edit] Hobby development
Operating system development is one of the more involved and technical options for the
computing hobbyist. A hobby operating system is classified as one that has been written from scratch (not based on another system) and has few
developers who work in their spare time.
[11] Development usually begins with an existing operating system. The hobbyist is their own developer, or they interact in a relatively small and unstructured group of individuals who are all similarly situated with the same code base. Examples of a hobby operating system include
Syllable and
ReactOS.
[edit] Diversity of operating systems and portability
Application software is generally written for use on a specific operating system, and sometimes 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.
This cost in supporting operating systems diversity can be avoided by instead writing applications against
software platforms like
Java, or
Qt for web browsers. These abstractions have already borne the cost of adaptation to specific operating systems and their
system libraries.
Another approach is for operating system vendors to adopt standards. For example,
POSIX and
OS abstraction layers provide commonalities that reduce porting costs.
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