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Generally, a computer is any electronic data-processing device that performs tasks, such as mathematical calculations or electronic communication, under the control of a set of instructions called a program. Programs usually reside within the computer's main memory and are retrieved and processed by the computer's electronics, and the program results are stored or routed to output devices, such as video display monitors or printers. It communicates with other electronic devices to receive data, store and manipulate them (using mathematical and logical calculations specified in a sequence of instructions called a program), and transmit the results; e.g accept a sequence of numbers typed in at a keyboard, and plot a graph of them on a visual display unit, or monitor.

Computers are now familiar at work, home, and school, as desktop personal computers (PCs). They also exist as very small devices called microprocessors, to control electronic equipment and machinery, e.g. car engines; and as much larger devices such as supercomputers, used to model and predict weather, earthquakes, nuclear explosions, etc.

The heart of today's computers are integrated circuits (ICs), sometimes called microchips, or simply chips. These tiny silicon wafers can contain millions of microscopic electronic components and are designed for many specific operations: some control an entire computer (CPU, or central processing unit, chips); some perform millions of mathematical operations per second (math coprocessors); others can store more than 16 million characters of information at one time (memory chips).

A computer is a general-purpose machine that processes data according to a specific set of instructions. The instructions the computer uses are either stored permanently (in read-only memory, or 'ROM') or temporarily (in random-access memory, or 'RAM'). The computer and the equipment attached to it ('peripherals') are called hardware. The instructions the computer receives are called software. A set of instructions grouped together to perform a certain task is called a program.

Abstractly, the essential elements of a computer comprise:-

• Input and output devices;
• A central processing unit (CPU);
• The memory stores programs and the initial, intermediate, and final computational data.

Almost all personal computers, workstations, minicomputers, and mainframes are based on the von Neumann design principle of stored programs. John Von Neumann (1945) proposed that the program to control the computer should be stored in the memory of the computer. Changing the program in memory allowed the computer to perform a completely different computation. Thus computers became general-purpose problem solving machines.

• A program is stored in memory; the CPU fetches instructions and data from there via the bus.
• It decodes, and executes the stored instructions of the program sequentially, inputting data as needed, and outputting the results via the bus.
• This procedure repeats until the computer is turned off.

Von Neumann's architecture describes a computer with four main sections: the Arithmetic and Logic Unit (ALU), the control circuitry, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by a bundle of wires, a "bus."

Central Processing Unit (CPU)

Carries out computation and has overall control of the computer. Each type of CPU has its own machine language. The machine language is the set of instructions it can obey. The CPU consists of a Control Unit and an Arithmetic/Logic Unit (ALU). The control unit controls the operation of the peripheral devices and the transfer of information between the units that make up the computer. The Arithmetic/Logic Unit performs calculation.

Memory

The internal representation of information in the computer and memory is in terms of the Binary system using only the basic symbols 0 and 1.

Main Memory

Stores programs and data while computer is running. Programs to be executed by the computer are placed in main memory and the CPU fetches each instruction in turn from memory and executes it. Main memory is fast and limited in capacity. The CPU can only directly access information in main memory. Main memory cannot retain information when the computer is switched off. Main memory consists of a series of locations called bytes, each byte being eight bits. Each byte has an address associated with it by which it can be accessed. The address is a number.

External Memory

Holds information too large for storage in main memory. Information on external memory can only be accessed by the CPU if it is first transferred to main memory. External memory is slow and virtually unlimited in capacity. It retains information when the computer is switched off; used to keep a permanent copy of programs and data.

Input and Output (IO)

IO devices allow the computer to communicate with the outside world. The computer receives input through the keyboard or another input device. This input is transferred into memory (RAM), and then processed by calculating, comparing, or copying it. The computer outputs the results of the processing, usually on the screen, onto a disk, or over a communications channel. There's an incredibly broad range of I/O devices, from the familiar keyboards, monitors and floppy disk drives, to the more unusual such as webcams. Some typical IO devices:

• storage devices such as floppy disk drive, CD-ROM drive;
• input devices such as the keyboard, mouse, joystick, optical scanner, light pen, touch panel, or microphone;
• output devices include the monitor (display unit), liquid crystal display, printer, and sound card and one or two speakers;
• communications devices such as modems and network units (e.g. LAN, the Local Area Network for connecting local computers).

Input/output devices each have their own controller. The CPU communicates with input/output devices via this controller. Associated with the controller is a memory area called a Buffer. Data buffering allows for the gross mismatch in transfer rates between CPU and main memory and the transfer rates of most peripheral devices. The data buffer is a block of memory which is used to act as an intermediate staging post for data as it flows to and from peripheral devices to main memory (and hence to the CPU). All output is sent by the CPU to the buffer for re-transmission by the device controller to the device. Similarly input from devices is sent to the buffer from where the CPU can transfer it at high speed to main memory.

The System Bus

All communication between the major components of the computer is via the System Bus. The bus is merely a cable which is capable of carrying signals representing data from one place to another. The bus used in a particular individual computer may be specific to that computer or may (increasingly) be an industry-standard bus. If it is an industry standard bus then it should be easy to upgrade the computer by buying a component from an independent manufacturer which can plug directly into the system bus. Most modern Personal Computers use the PCI bus; older machines used the ISE bus. The system bus carries data and control signals. Possible transfers of information via the system bus are:

• data transmitted from main memory to the CPU.
• input data from (say) the keyboard travels from the device to main memory.
• information from External Memory is transmitted to Main Memory.

The speed of the system bus is very important since if it is too slow then the CPU may have its speed restricted by having to wait for data.

A modern electronic digital computer typically contains in a desktop or tower case the following parts:

• Motherboard which holds a central processing unit (CPU), read-only memory (ROM) and random-access-memory (RAM), main memory and other parts, and has slots for expansion cards
• Power supply - a case that holds a transformer, voltage control and fan
• Storage controllers, of IDE, SCSI or other type, that control hard disk , floppy disk, CD-ROM and other drives; the controllers sit directly on the motherboard (on-board) or on expansion cards
• Graphics controller that produces the output for the monitor
• Hard disk, floppy disk and other drives for mass storage
• Interface controllers (parallel, serial, USB, Firewire) to connect the computer to external peripheral devices such as printers or scanners