Programs

Computer programs may run from just a few instructions to many millions of instructions, as in a program for a word processor. A typical modern computer can execute billions of instructions per second (gigahertz) and rarely make a mistake over many years of operation. Large computer programs consisting of several million instructions may take teams of programmers years to write, and due to the complexity of the task almost certainly contain errors.
Errors in computer programs are called bugs. Bugs 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 to hang become unresponsive to input such as mouse clicks or keystrokes, or to completely fail or crash. Otherwise benign bugs may sometimes may be harnessed for malicious intent by an unscrupulous user writing an exploit—code designed to take advantage of a bug and disrupt a program's proper execution. 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.

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 instructions) can be represented as lists of numbers and can themselves be manipulated inside the computer just as if they were 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. In some cases, a computer might store some or all of its program in memory that is kept separate from the data it operates on. This is called the Harvard architecture after the Havard 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 this technique was used with many early computers, it is extremely tedious to do so in practice, especially for complicated programs. Instead, each basic instruction can be given a short name that is indicative of its function and easy to remember—a mnemoic such as ADD, SUB, MULT or JUMP. 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. Machine languages and the assembly languages that represent them (collectively termed low level programming languages) tend to be unique to a particular type of computer.

Though considerably easier than in machine language, writing long programs in assembly language is often difficult and error prone. Therefore, most complicated 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 complier.Since high level languages are more abstract than assembly language, it is 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.

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.

antivires

Antivirus (or anti-virus) software is used to prevent, detect, and remove malware, including computer viruses, worms, and trojan horses. Such programs may also prevent and remove adware, spyware, and other forms of malware.
A variety of strategies are typically employed. Signature-based detection involves searching for known malicious patterns in executable code. However, it is possible for a user to be infected with new malware in which no signature exists yet. To counter such so called zero-day threats, heuristics can be used. One type of heuristic approach, generic signatures, can identify new viruses or variants of existing viruses for looking for known malicious code (or slight variations of such code) in files. Some antivirus software can also predict what a file will do if opened/run by emulating it in a sandbox and analyzing what it does to see if it performs any malicious actions. If it does, this could mean the file is malicious.
However, no matter how useful antivirus software is, it can sometimes have drawbacks. Antivirus software can degrade computer performance if it is not designed efficiently. Inexperienced users may have trouble understanding the prompts and decisions that antivirus software presents them with. An incorrect decision may lead to a security breach. If the antivirus software employs heuristic detection (of any kind), the success of it is going to depend on whether it achieves the right balance between false positives and false negatives. False positives can be as destructive as false negatives. In one case, a faulty virus signature issued by Symantec mistakenly removed essential operating system files, leaving thousands of PCs unable to boot. Finally, antivirus software generally runs at the highly trusted kernel level of the operating system, creating a potential avenue of attack.
A virus removal tool is software for removing specific viruses from infected computers. Unlike complete antivirus scanners, they are usually not intended to detect and remove an extensive list of viruses; rather they are designed to remove specific viruses, usually more effectively than normal antivirus software. Sometimes they are also designed to run in places that regular antivirus software can't. This is useful in the case of a severely infected computer. Examples of these tools include McAfee Stinger and the Microsoft Windows Malicious Software Removal Tool

vires

A virus (from the Latin virus meaning toxin or poison) is a small infectious agent that can only replicate inside the cells of another organism. Viruses are too small to be seen directly with a light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Since the initial discovery of tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and these minute structures are the most abundant type of biological entity. The study of viruses is known as virology, a sub-specialty of microbiology.
A computer virus is a computer program that can copy itself and infect a computer. The term "virus" is also commonly but erroneously used to refer to other types of malware, adware, and spyware programs that do not have the reproductive ability. A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive. Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.
The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware. Malware includes computer viruses, worms, trojans, most rootkits, spyware, dishonest adware, crimeware, and other malicious and unwanted software, including true viruses. Viruses are sometimes confused with computer worms and Trojan horses, which are technically different. A worm can exploit security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a program that appears harmless but has a hidden agenda. Worms and Trojans, like viruses, may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when they are executed. Some viruses and other malware have symptoms noticeable to the computer user, but many are surreptitious or go unnoticed.

Web Hosting

A web hosting service is a type of Internet hosting service that allows individuals and organizations to make their own website accessible via the World Wide Web. Web hosts are companies that provide space on a server they own or lease for use by their clients as well as providing Internet connectivity, typically in a data center. Web hosts can also provide data center space and connectivity to the Internet for servers they do not own to be located in their data center, called colocation.
Types of hosting A typical server "rack," commonly seen in colocation centres.Internet hosting services can run Web servers; see Internet hosting services.
Hosting services limited to the Web:
Many large companies who are not internet service providers also need a computer permanently connected to the web so they can send email, files, etc. to other sites. They may also use the computer as a website host so they can provide details of their goods and services to anyone interested. Additionally these people may decide to place online orders.
Free web hosting service: Free web hosting is offered by different companies with limited services, sometimes advertisement-supported web hosting, and is often limited when compared to paid hosting. Shared web hosting service: one's website is placed on the same server as many other sites, ranging from a few to hundreds or thousands. Typically, all domains may share a common pool of server resources, such as RAM and the CPU. The features available with this type of service can be quite extensive. A shared website may be hosted with a reseller. Reseller web hosting: allows clients to become web hosts themselves. Resellers could function, for individual domains, under any combination of these listed types of hosting, depending on who they are affiliated with as a provider. Resellers' accounts may vary tremendously in size: they may have their own virtual dedicated server to a collocated server. Many resellers provide a nearly identical service to their provider's shared hosting plan and provide the technical support themselves. Virtual Dedicated Server: also known as a Virtual Private Server (VPS for short) divides server resources into virtual servers, where resources can be allocated in a way that does not directly reflect the underlying hardware. VPS will often be allocated resources based on a one server to many VPSs relationship, however virtualisation may be done for a number of reasons, including the ability to move a VPS container between servers. The users may have root access to their own virtual space. This is also known as a virtual private server or VPS. Customers are sometimes responsible for patching and maintaining the server. Dedicated hosting service: the user gets his or her own Web server and gains full control over it (root access for Linux/administrator access for Windows); however, the user typically does not own the server. Another type of Dedicated hosting is Self-Managed or Unmanaged. This is usually the least expensive for Dedicated plans. The user has full administrative access to the box, which means the client is responsible for the security and maintenance of his own dedicated box. Managed hosting service: the user gets his or her own Web server but is not allowed full control over it (root access for Linux/administrator access for Windows); however, they are allowed to manage their data via FTP or other remote management tools. The user is disallowed full control so that the provider can guarantee quality of service by not allowing the user to modify the server or potentially create configuration problems. The user typically does not own the server. The server is leased to the client. Colocation web hosting service: similar to the dedicated web hosting service, but the user owns the colo server; the hosting company provides physical space that the server takes up and takes care of the server. This is the most powerful and expensive type of the web hosting service. In most cases, the colocation provider may provide little to no support directly for their client's machine, providing only the electrical, Internet access, and storage facilities for the server. In most cases for colo, the client would have his own administrator visit the data center on site to do any hardware upgrades or changes. Cloud hosting: is a new type of hosting platform that allows customers powerful, scalable and reliable hosting based on clustered load-balanced servers and utility billing. Removing single-point of failures and allowing customers to pay for only what they use versus what they could use. Clustered hosting: having multiple servers hosting the same content for better resource utilization. Clustered Servers are a perfect solution for high-availability dedicated hosting, or creating a scalable web hosting solution. A cluster may separate web serving from database hosting capability. Grid hosting: this form of distributed hosting is when a server cluster acts like a grid and is composed of multiple nodes. Home server: usually a single machine placed in a private residence can be used to host one or more web sites from a usually consumer-grade broadband connection. These can be purpose-built machines or more commonly old PCs. Some ISPs actively attempt to block home servers by disallowing incoming requests to TCP port 80 of the user's connection and by refusing to provide static IP addresses. A common way to attain a reliable DNS hostname is by creating an account with a dynamic DNS service. A dynamic DNS service will automatically change the IP address that a URL points to when the IP address changes.

computer vires and antivires

Development in a computer hardware and software there is developed many kind of vires are developed to create many problem in your computer and to defence your computer fromvires,many antivires are also developed.Due to many cause vires enter to your computer like USB port where you use pendrive and all,due to internet,many viresed CD, DVD and many more.Vires is a one of the computer program which can copy itself and infect your computer.A vires can spread from one computer to another computer,and one drive to computer and vice versa.To solve vireswe can have many antivires software.To help avoid vires,you must keep your computer latest updates and antivires tools and you must be alert when you having internet ,downloads and openattachments.

Global Area Network

A global area network (GAN) specification is in development by several groups,and there is no common defition.In general,however,a GAN is a model for supporting mobile communications across an arbitrary number of wireless LANs ,satellite coverage areas etc.The key challenge inmobile communications is "handing off" the user communications from one local coverage area to the next.In IEEE project 802,this involves a succession of terrestrial WIRELESS local area networks(WLAN).

Wide Area Network

A wide area network (WAN) is a computer network that covers a board area.Less formally ,a WAN is a network that uses routers and public communications links.Contrast with personal area networks (PANs) ,local area networks (LANs) metropolitan area networks (MANs),which are usually limited to a room,building,campus or specific metropolitan area (e.g.,acity) respectively.The largest and most well-known example of a WAN is the internet.A WAN is a datacommunications network that coversa relatively broard geographic area(i.e.one city to another and one country to another country) and that often uses transmission facilities provided by common carries, such as telephone companies.WAN technologies generally function at the lower three layersof the lower three layers of the OSI reference model ,the physical layer, the data link layer and thenetwork layer.

Local area network

A local area network(LAN) is a computer network covering a small physical area,like a home office,or small group of buildings,such as a school or an airport.Current wired LANs are most likely to be based on Ethernet technology provide a way to create a wired LAN using existing home wires(coaxial cables,phone lines and power lines).For example ,a libary may have a wired or wireless LAN for users to interconnect local devices (e.g.,printers and servers) and to connect to the internet.On a wired LAN,PCs in the libary are typically connected by category 5 (Cat5) cable,running the IEEE 802.3 protocol through a system of interconnected devices and eventuallyconnect to the internet.The cables to the serves are typically on CAT 5e enhanced cable,which willsupport IEEE 802.3 at 1 Gbit/s.A wireless LAN may exit using a different IEEE protocol,802.11b,802.11g or possibly 802.11n. The staff computers can get to the color printer,checkout records andthe academic network and the internet. All user computer can the internet and the card catalog.

Personal and Metropolitan area network

A personal area network(PAN) is a computer network used for communication among computer device close to one person.Some examples of devices that are used ina a PAN pan are personal
computers, printers, fax machines,PDAs, scanners and ecen video game consoles.Such a PAN may
include wired and wireless connections between devices.The reach of a PAN is typically at least about 20-30 feet (approximately6-9 meters).

A metropolitan area network (MAN) is a network that connects two or more local area networks or campus area networks together but dose not extend beyond the boundaries of the immediate town\city.Routers,switches and hubs are connected to create a metropolitan and network.

Networking

A computer network allows computers to communicate with many other and to share resources
and information.Development of the network began in 1969,based on designs begun in the 1960s.
Computer networking can network with similar and non similar device like computer with computer and computer with other like router.You can network the internet from a server to allcomputer by net sharing and many more.So networking is biggest achievment for all .You can haveit for official purpose,organization etc.Ther are many kind of netwoking;
1Personal area network
2Local area network
3Metropolitan area network
4Wide area network
5Global area network