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This section applies to most common RAM machine —based computers. In most cases, computer instructions are simple: add one number to another, move some data from one location to another, send a message to some external device, etc.
These instructions are read from the computer's memory and are generally carried out executed in the order they were given. However, there are usually specialized instructions to tell the computer to jump ahead or backwards to some other place in the program and to carry on executing from there.
These are called "jump" instructions or branches. Furthermore, jump instructions may be made to happen conditionally so that different sequences of instructions may be used depending on the result of some previous calculation or some external event. Many computers directly support subroutines by providing a type of jump that "remembers" the location it jumped from and another instruction to return to the instruction following that jump instruction.
Program execution might be likened to reading a book. While a person will normally read each word and line in sequence, they may at times jump back to an earlier place in the text or skip sections that are not of interest. Similarly, a computer may sometimes go back and repeat the instructions in some section of the program over and over again until some internal condition is met.
This is called the flow of control within the program and it is what allows the computer to perform tasks repeatedly without human intervention. Comparatively, a person using a pocket calculator can perform a basic arithmetic operation such as adding two numbers with just a few button presses. But to add together all of the numbers from 1 to 1, would take thousands of button presses and a lot of time, with a near certainty of making a mistake.
On the other hand, a computer may be programmed to do this with just a few simple instructions. The following example is written in the MIPS assembly language :. Once told to run this program, the computer will perform the repetitive addition task without further human intervention.
It will almost never make a mistake and a modern PC can complete the task in a fraction of a second. In most computers, individual instructions are stored as machine code with each instruction being given a unique number its operation code or opcode for short.
The command to add two numbers together would have one opcode; the command to multiply them would have a different opcode, and so on. The simplest computers are able to perform any of a handful of different instructions; the more complex computers have several hundred to choose from, each with a unique numerical code.
Since the computer's memory is able to store numbers, it can also store the instruction codes. This leads to the important fact that entire programs which are just lists of these instructions can be represented as lists of numbers and can themselves be manipulated inside the computer in the same way as numeric data. The fundamental concept of storing programs in the computer's memory alongside the data they operate on is the crux of the von Neumann, or stored program, architecture.
This is called the Harvard architecture after the Harvard Mark I computer. Modern von Neumann computers display some traits of the Harvard architecture in their designs, such as in CPU caches. While it is possible to write computer programs as long lists of numbers machine language and while this technique was used with many early computers, [h] it is extremely tedious and potentially error-prone to do so in practice, especially for complicated programs.
These mnemonics are collectively known as a computer's assembly language. Converting programs written in assembly language into something the computer can actually understand machine language is usually done by a computer program called an assembler.
Programming languages provide various ways of specifying programs for computers to run. Unlike natural languages , programming languages are designed to permit no ambiguity and to be concise. They are purely written languages and are often difficult to read aloud. They are generally either translated into machine code by a compiler or an assembler before being run, or translated directly at run time by an interpreter. Sometimes programs are executed by a hybrid method of the two techniques.
Machine languages and the assembly languages that represent them collectively termed low-level programming languages are generally unique to the particular architecture of a computer's central processing unit CPU. Although considerably easier than in machine language, writing long programs in assembly language is often difficult and is also error prone.
Therefore, most practical programs are written in more abstract high-level programming languages that are able to express the needs of the programmer more conveniently and thereby help reduce programmer error. High level languages are usually "compiled" into machine language or sometimes into assembly language and then into machine language using another computer program called a compiler.
It is therefore often possible to use different compilers to translate the same high level language program into the machine language of many different types of computer. This is part of the means by which software like video games may be made available for different computer architectures such as personal computers and various video game consoles. Program design of small programs is relatively simple and involves the analysis of the problem, collection of inputs, using the programming constructs within languages, devising or using established procedures and algorithms, providing data for output devices and solutions to the problem as applicable.
As problems become larger and more complex, features such as subprograms, modules, formal documentation, and new paradigms such as object-oriented programming are encountered. Large programs involving thousands of line of code and more require formal software methodologies. The task of developing large software systems presents a significant intellectual challenge.
Producing software with an acceptably high reliability within a predictable schedule and budget has historically been difficult; the academic and professional discipline of software engineering concentrates specifically on this challenge.
Errors in computer programs are called " bugs ". They may be benign and not affect the usefulness of the program, or have only subtle effects. But in some cases, they may cause the program or the entire system to " hang ", becoming unresponsive to input such as mouse clicks or keystrokes, to completely fail, or to crash.
Bugs are usually not the fault of the computer. Since computers merely execute the instructions they are given, bugs are nearly always the result of programmer error or an oversight made in the program's design. Computers have been used to coordinate information between multiple locations since the s. The U. In time, the network spread beyond academic and military institutions and became known as the Internet. The emergence of networking involved a redefinition of the nature and boundaries of the computer.
Computer operating systems and applications were modified to include the ability to define and access the resources of other computers on the network, such as peripheral devices, stored information, and the like, as extensions of the resources of an individual computer. Initially these facilities were available primarily to people working in high-tech environments, but in the s the spread of applications like e-mail and the World Wide Web , combined with the development of cheap, fast networking technologies like Ethernet and ADSL saw computer networking become almost ubiquitous.
In fact, the number of computers that are networked is growing phenomenally. A very large proportion of personal computers regularly connect to the Internet to communicate and receive information. A computer does not need to be electronic , nor even have a processor , nor RAM , nor even a hard disk. While popular usage of the word "computer" is synonymous with a personal electronic computer, [l] the modern definition of a computer is literally: " A device that computes , especially a programmable [usually] electronic machine that performs high-speed mathematical or logical operations or that assembles, stores, correlates, or otherwise processes information.
There is active research to make computers out of many promising new types of technology, such as optical computers , DNA computers , neural computers , and quantum computers. Most computers are universal, and are able to calculate any computable function , and are limited only by their memory capacity and operating speed. However different designs of computers can give very different performance for particular problems; for example quantum computers can potentially break some modern encryption algorithms by quantum factoring very quickly.
There are many types of computer architectures :. Of all these abstract machines , a quantum computer holds the most promise for revolutionizing computing. The ability to store and execute lists of instructions called programs makes computers extremely versatile, distinguishing them from calculators. The Church—Turing thesis is a mathematical statement of this versatility: any computer with a minimum capability being Turing-complete is, in principle, capable of performing the same tasks that any other computer can perform.
Therefore, any type of computer netbook , supercomputer , cellular automaton , etc. A computer will solve problems in exactly the way it is programmed to, without regard to efficiency, alternative solutions, possible shortcuts, or possible errors in the code. Computer programs that learn and adapt are part of the emerging field of artificial intelligence and machine learning. Artificial intelligence based products generally fall into two major categories: rule-based systems and pattern recognition systems.
Rule-based systems attempt to represent the rules used by human experts and tend to be expensive to develop. Pattern-based systems use data about a problem to generate conclusions.
Examples of pattern-based systems include voice recognition , font recognition, translation and the emerging field of on-line marketing. As the use of computers has spread throughout society, there are an increasing number of careers involving computers.
The need for computers to work well together and to be able to exchange information has spawned the need for many standards organizations, clubs and societies of both a formal and informal nature.
From Wikipedia, the free encyclopedia. Automatic general-purpose device for performing arithmetic or logical operations. For other uses, see Computer disambiguation. Computers and computing devices from different eras.
Main articles: History of computing and History of computing hardware. For a chronological guide, see Timeline of computing. Main article: Analog computer. Main article: Stored-program computer. Main articles: Transistor and History of the transistor.
Main articles: Integrated circuit and Invention of the integrated circuit. Further information: Planar process and Microprocessor.
See also: Classes of computers. Main articles: Computer hardware , Personal computer hardware , Central processing unit , and Microprocessor. Main article: History of computing hardware. Main articles: CPU design and Control unit. Main articles: Central processing unit and Microprocessor.
Main article: Arithmetic logic unit. Main articles: Computer memory and Computer data storage. Main article: Computer multitasking. Main article: Multiprocessing. Main article: Software. Main articles: Computer program and Computer programming. Main article: Programming language. Main article: Low-level programming language. Main article: High-level programming language.
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Main article: Software bug. Main articles: Computer networking and Internet. Main article: Human computer. See also: Harvard Computers.
Glossary of computers Computability theory Computer security Glossary of computer hardware terms History of computer science List of computer term etymologies List of fictional computers List of pioneers in computer science Pulse computation TOP list of most powerful computers Unconventional computing.
The containers thus served as something of a bill of lading or an accounts book. In order to avoid breaking open the containers, first, clay impressions of the tokens were placed on the outside of the containers, for the count; the shapes of the impressions were abstracted into stylized marks; finally, the abstract marks were systematically used as numerals; these numerals were finally formalized as numbers.
Eventually the marks on the outside of the containers were all that were needed to convey the count, and the clay containers evolved into clay tablets with marks for the count. Schmandt-Besserat estimates it took years. All of the architectures listed in this table, except for Alpha, existed in bit forms before their bit incarnations were introduced. Although the control unit is solely responsible for instruction interpretation in most modern computers, this is not always the case.
Some computers have instructions that are partially interpreted by the control unit with further interpretation performed by another device. For example, EDVAC , one of the earliest stored-program computers, used a central control unit that interpreted only four instructions. All of the arithmetic-related instructions were passed on to its arithmetic unit and further decoded there.
These so-called computer clusters can often provide supercomputer performance at a much lower cost than customized designs. While custom architectures are still used for most of the most powerful supercomputers, there has been a proliferation of cluster computers in recent years. However, this method was usually used only as part of the booting process.
Most modern computers boot entirely automatically by reading a boot program from some non-volatile memory.
An x compatible microprocessor like the AMD Athlon 64 is able to run most of the same programs that an Intel Core 2 microprocessor can, as well as programs designed for earlier microprocessors like the Intel Pentiums and Intel This contrasts with very early commercial computers, which were often one-of-a-kind and totally incompatible with other computers.
Interpreted languages are translated into machine code on the fly, while running, by another program called an interpreter. Computer hardware may fail or may itself have a fundamental problem that produces unexpected results in certain situations. For instance, the Pentium FDIV bug caused some Intel microprocessors in the early s to produce inaccurate results for certain floating point division operations. This was caused by a flaw in the microprocessor design and resulted in a partial recall of the affected devices.
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Archived from the original PDF on 9 December Retrieved 31 July Nanoelectronics: Materials, Devices, Applications, 2 Volumes. Archived from the original on 3 March The registry contains important configuration information for the operating system, for installed applications as well as individual settings for each user and application. If a user wants to edit the registry manually, Microsoft recommends that a backup of the registry be performed before the change.
After this, the user might need to manually remove any reference to the uninstalled program in the registry. This is usually done by using RegEdit.
Windows Registry can be edited manually using programs such as RegEdit. The registry editor for the 3. REG files also known as Registration entries are text-based human-readable files for exporting and importing portions of the registry using an INI -based syntax.
On Windows 9x and NT 4. REG files are compatible with Windows and later. The Registry Editor on Windows on these systems also supports exporting. Data is stored in. REG files using the following syntax: [26]. Data from. REG files can also be used to remove registry data. To remove a key and all subkeys, values and data , the key name must be preceded by a minus sign "-".
Windows group policies can change registry keys for a number of machines or individual users based on policies. When a policy first takes effect for a machine or for an individual user of a machine, the registry settings specified as part of the policy are applied to the machine or user settings.
Windows will also look for updated policies and apply them periodically, typically every 90 minutes. Whether a machine or user is within the scope of a policy or not is defined by a set of rules which can filter on the location of the machine or user account in organizational directory, specific users or machine accounts or security groups.
More advanced rules can be set up using Windows Management Instrumentation expressions. Such rules can filter on properties such as computer vendor name, CPU architecture, installed software, or networks connected to. For instance, the administrator can create a policy with one set of registry settings for machines in the accounting department and policy with another lock-down set of registry settings for kiosk terminals in the visitors area.
When a machine is moved from one scope to another e. When a policy is changed it is automatically re-applied to all machines currently in its scope. The policy is edited through a number of administrative templates which provides a user interface for picking and changing settings.
The set of administrative templates is extensible and software packages which support such remote administration can register their own templates. The registry can be manipulated in a number of ways from the command line. The Reg. It is also possible to use Reg.
Here is a sample to display the value of the registry value Version:. Registry permissions can be manipulated through the command line using RegIni. Windows PowerShell comes with a registry provider which presents the registry as a location type similar to the file system. The same commands used to manipulate files and directories in the file system can be used to manipulate keys and values of the registry.
Also like the file system, PowerShell uses the concept of a current location which defines the context on which commands by default operate.
The Get-ChildItem also available through the aliases ls , dir or gci retrieves the child keys of the current location. By using the Set-Location or the alias cd command the user can change the current location to another key of the registry. Through PowerShell scripts files, an administrator can prepare scripts which, when executed, make changes to the registry.
Such scripts can be distributed to administrators who can execute them on individual machines. The PowerShell Registry provider supports transactions, i. An atomic transaction ensures that either all of the changes are committed to the database, or if the script fails, none of the changes are committed to the database.
Many programming languages offer built-in runtime library functions or classes that wrap the underlying Windows APIs and thereby enable programs to store settings in the registry e. Registry in VB.
Shell object. Similarly, scripting languages such as Perl with WinTieRegistry , Python with winreg , TCL which comes bundled with the registry package , [32] Windows Powershell and Windows Scripting Host also enable registry editing from scripts. The offreg. It is also possible to edit the registry hives of an offline system from Windows PE or Linux in the latter case using open source tools. Prior to the introduction of registration-free COM , developers were encouraged to add initialization code to in-process and out-of-process binaries to perform the registry configuration required for that object to work.
For in-process binaries such as. DLL and. OCX files, the modules typically exported a function called DllInstall [34] that could be called by installation programs or invoked manually with utilities like Regsvr Windows exposes APIs that allows user-mode applications to register to receive a notification event if a particular registry key is changed. Windows also supports remote access to the registry of another computer via the RegConnectRegistry function [40] if the Remote Registry service is running, correctly configured and its network traffic is not firewalled.
Each key in the registry of Windows NT versions can have an associated security descriptor. The security descriptor contains an access control list ACL that describes which user groups or individual users are granted or denied access permissions. As with other securable objects in the operating system, individual access control entries ACE on the security descriptor can be explicit or inherited from a parent object.
Windows Resource Protection is a feature of Windows Vista and later versions of Windows that uses security to deny Administrators and the system WRITE access to some sensitive keys to protect the integrity of the system from malware and accidental modification.
Special ACEs on the security descriptor can also implement mandatory integrity control for the registry key and subkeys. Outside security, registry keys cannot be deleted or edited due to other causes. Registry keys containing NUL characters cannot be deleted with standard registry editors and require a special utility for deletion, such as RegDelNull. Different editions of Windows have supported a number of different methods to back up and restore the registry over the years, some of which are now deprecated:.
Windows and later versions of Windows use Group Policy to enforce registry settings through a registry-specific client extension in the Group Policy processing engine. The policy file allows administrators to prevent non-administrator users from changing registry settings like, for instance, the security level of Internet Explorer and the desktop background wallpaper.
The policy file is primarily used in a business with a large number of computers where the business needs to be protected from rogue or careless users. The default extension for the policy file is. The policy file filters the settings it enforces by user and by group a "group" is a defined set of users. To do that the policy file merges into the registry, preventing users from circumventing it by simply changing back the settings.
The policy file is usually distributed through a LAN, but can be placed on the local computer. The policy file is created by a free tool by Microsoft that goes by the filename poledit. The editor requires administrative permissions to be run on systems that uses permissions. The editor can also directly change the current registry settings of the local computer and if the remote registry service is installed and started on another computer it can also change the registry on that computer.
The policy editor loads the settings it can change from. ADM files, of which one is included, that contains the settings the Windows shell provides. ADM file is plain text and supports easy localisation by allowing all the strings to be stored in one place. Non-compliant bit applications can also be redirected in this manner, even though the feature was originally intended for bit applications.
Similarly, application virtualization redirects all of an application's invalid registry operations to a location such as a file. Used together with file virtualization, this allows applications to run on a machine without being installed on it. Low integrity processes may also use registry virtualization. For example, Internet Explorer 7 or 8 running in "Protected Mode" on Windows Vista and above will automatically redirect registry writes by ActiveX controls to a sandboxed location in order to frustrate some classes of security exploits.
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