Computer (disambiguation) and Computer system (disambiguation)

 " A computer is a device that can be instructed to carry out an arbitrary set of-arithmetic or logical operations automatically. 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" in the 2000s is synonymous with a personal electronic computer, 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." Any device which processes information qualifies as a computer An amazing machine!We are living in the computer age today and most of our day to day activities can not be accomplished without using computers. Sometimes knowingly and sometimes unknowingly we use computers. The computer has become an indispensableand multipurpose tool. We are breathing in the computer age and gradually computer has become such a desire necessity of life that it is difficult to imagine life without it.

GENERATION OF COMPUTER
As the time passed, the device of more suitable and reliable machine needed which could perform our work more quickly. During this time, in the year 1946, the first successful electronic computer called ENIAC was developed and it was the starting point of the current generation of computer

FIRST GENERATION

 
ENIAC was the world first successful electronic computer which was developed by the two scientists namely J. P. Eckert and J. W. Mauchly. It was the beginning of first generation computer. The full form of ENIAC is “Electronic Numeric Integrated And Calculator” ENIAC was a very huge and big computer and its weight was 30 tonnes. It could store only limited or small amount of information. Initially, in the first generation computer, the concept of vacuum tubes was used. A vacuum tube was such an electronic component which had very less work efficiency and so it could not work properly and it required a large cooling system.

SECOND GENERATION
As the development moved further, the second generation computers knocked the door. In this generation, transistors were used as the electronic component instead of vacuum tubes.A transistor is much smaller in the size than that of a vacuum tube. As the size of electrons components decreased from vacuum tube of the transistor, the size of the computer also decreased and it became much smaller than that of the earlier computer.

THIRD GENERATION
The third generation computers were invented in the year 1964. In this generation of computer, IC (Integrated circuits) was used as the electronic component for computers. The development of IC gave birth to a new field of microelectronics. The main advantage of IC is not only its small size but its superior performance and reliability than the previous circuits. It was first developed by T.S Kilby. This generation of the computer has huge storage capacity and higher calculating speed.

FOURTH GENERATION

This is the generation where we are working today. The computers which we see around us belong to the fourth generation computers. ‘Micro processor’ is the main concept behind this generation of computer. A microprocessor is a single chip (L.S.I circuit), which is used in a computer for any arithmetical or logical functions to be performed in any program. The honor of developing microprocessor goes to Ted Hoff of U.S.A. He developed first microprocessor, the Intel 4004, as he was working for Intel Corporation, U.S.A with the use of the microprocessor in the fourth generation computers, the size of computer become very fast and efficient. It is evident that the next generation of computer i.e. fifth generation will be developed soon. In that generation, the computer will possess artificial intelligence and it would be able to take self-decisions like a human being.

Components A general purpose computer has four main-components: the arithmetic logic unit (ALU), the control unit, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by buses, often made of groups of wires. Inside each of these parts are thousands to trillions of small electrical circuits which can be turned off or on by means of an electronic switch. Each circuit represents a bit (binary digit) of information so that when the circuit is on it represents a "1", and when off it represents a "0" (in positive logic-representation). The circuits are arranged in logic gates so that one or more of the circuits may control the state of one or more of the other circuits.
Control unit The control unit (often called a control system or central controller) manages the computer's various components; it reads and interprets (decodes) the program instruction, transforming them into control signals that activate other parts of the computer. Control systems in advanced computers may change the order of execution of some instructions to improve performance. A key component common to all CPUs is the program counter, a special memory cell (a register) that keeps track of which location in memory the next instruction is to be read from.The control system's function is as follows—note that this is a simplified description, and some of these steps may be performed concurrently or in a different order depending on the type of CPU: Read the code for the next instruction from the cell indicated by the program counter. Decode the numerical code for the instruction into a set of commands or signals for each of the other systems. Increment the program counter so it points to the next instruction. Read whatever data the instruction requires from cells in memory(or perhaps from an input device). The location of this required data is typically stored within the instruction code. Provide the necessary data to an ALU or register. If the instruction requires an ALU or specialized hardware to complete, instruct the hardware to perform the requested operation. Write the result from the ALU back to a memory location or to a register or perhaps an output device. Jump back to step (1). Since the program counter is(conceptually) just another set of memory cells, it can be changed by calculations done in the ALU. Adding 100 to the program counter would cause the next instruction to be read from a place 100 locations further down the program. Instructions that modify the program counter are often known as "jumps" and allow for loops (instructions that are repeated by the computer) and often conditional instruction execution (both examples of control flow). The sequence of operations that the control unit goes through to process an instruction is in itself like a short computer program, and indeed, in some more complex CPU designs, there is another yet smaller computer called a microsequencer, which runs a microcode program that causes all of these events to happen.

Central processing unit (CPU)The control unit, ALU, and registers are collectively known as a central processing unit (CPU). Early CPUs were composed of many separate components but since the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor.

Arithmetic logic unit (ALU) The ALU is capable of performing two classes of operations: arithmetic and logic. The set of arithmetic operations that a particular ALU supports may be limited to addition and subtraction or might include multiplication, division, trigonometry functions such as sine, cosine, etc., and square roots. Some can only operate on whole numbers (integers) whilst others use floating point to represent real numbers, albeit with limited precision. However, any computer that is capable of performing just the simplest operations can be programmed to break down the more complex operations into simple steps that it can perform. Therefore, any computer can be programmed to perform any arithmetic operation—although it will take more time to do so if its ALU does not directly support the operation. ALU may also compare numbers and return boolean truth values (true or false)depending on whether one is equal to, greater than or less than the other("is 64 greater than 65?"). Logic operations involve Boolean logic: AND, OR, XOR, and NOT. These can be useful for creating complicated conditional statements and processing boolean logic. Superscalar computers may contain multiple ALUs, allowing them to process several instructions simultaneously.Graphics processors and computers with SIMD and MIMDfeatures often contain ALUs that can perform arithmetic on vectors and matrices.
Memory A computer's memory can be viewed as a list of cells into which numbers can be placed or read. Each cell has a numbered "address" and can store a single number. The computer can be instructed to "put the number 123 into the cell numbered 1357" or to "add the number that is in cell 1357 to the number that is in cell 2468 and put the answer into cell 1595." The information stored in memory may represent practically anything. Letters, numbers, even computer instructions can be placed into memory with equal ease.Since the CPU does not differentiate between different types of information, it is the software's responsibility to give significance to what the memory sees as nothing but a series of numbers. In almost all modern computers, each memory cell is set up to store binary numbers in groups of eight bits (called a byte).Each byte is able to represent 256 different numbers (28 = 256); either from 0to 255 or −128 to +127. To store larger numbers, several consecutive bytes may be used (typically, two, four or eight). When negative numbers are required, they are usually stored in two's complement notation. Other arrangements are possible but are usually not seen outside of specialized applications or historical context. A computer can store any kind of information in memory if it can be represented numerically. Modern computers have billions or even trillions of bytes of memory. The CPU contains a special set of memory cells called registers that can be read and written to much more rapidly than the main memory area. There are typically between two and one hundred registers depending on the type of CPU. Registers are used for the most frequently needed data items to avoid having to access main memory every time data is needed. As data is constantly being worked on, reducing the need to access main memory(which is often slow compared to the ALU and control units) greatly increases the computer's speed.

Computer main memory comes in two principal varieties:

1. random-access memory or RAM
2. read-only memory or ROM

RAM can be read and written to anytime the CPU commands it, but ROM is pre-loaded with data and software that never changes, therefore the CPU can only read from it. ROM is typically used to store the computer's initial start-up instructions. In general, the contents of RAM are erased when the power to the computer is turned off, but ROM retains its data indefinitely. In a PC, the ROM contains a specialized program called the BIOSthat orchestrates loading the computer's operating system from the hard disk drive into RAM whenever the computer is turned on or reset. In embedded computers, which frequently do not have disk drives, all of the required software may be stored in ROM. Software stored in ROM is often called firmware because it is notionally more like hardware than software. Flash memory blurs the distinctions between ROM and RAM, as it retains its data when turned off but is also rewritable. It is typically much slower than conventional ROM and RAM, however, so its use is restricted to applications where high speed is unnecessary. In more sophisticated computers there may be one or more RAM cache memories, which are slower than registers but faster than main memory. Generally, computers with this sort of cache are designed to move frequently needed data into the cache automatically, often without the need for any intervention on the programmers part.

Multitasking

While a computer may be viewed as running a onegigantic program stored in its main memory, in some systems it is necessary to give the appearance of running several programs simultaneously. This is achieved by multitasking i.e. having the computer switch rapidly between running each program in turn. One means by which this is done is with a special signal called an interrupt, which can periodically cause the computer to stop executing instructions where it was and do something else instead. By remembering where it was executing prior to the interrupt, the computer can return to that task later. If several programs are running "at the same time".then the interrupt generator might be causing several hundred interrupts per second, causing a program switch each time. Since modern computers typically execute instructions several orders of magnitude faster than human perception, it may appear that many programs are running at the same time even though only one is ever executing in any given instant. This method of multitasking is sometimes termed "time-sharing" since each program is allocated a"slice" of time in turn. Before the era of cheap computers, the principal use for multitasking was to allow many people to share the same computer. Seemingly, multitasking would cause a computer that is switching between several programs to run more slowly, in direct proportion to the number of programs it is running, but most programs spend much of their time waiting for slow input/output devices to complete their tasks. If a program is waiting for the user to click on the mouse or press a key on the keyboard, then it will not take a "time slice" until the event it is waiting for has occurred. This frees up time for other programs to execute so that many programs may be run simultaneously without unacceptable speed loss.

Multiprocessing
Some computers are designed to distribute their work across several CPUs in a multiprocessing configuration, a technique once employed only in large and powerful machines such as supercomputers, mainframe-computers, and servers. Multiprocessor and multi-core (multiple CPUs on a single integrated circuit) personal and laptop computers are now widely available and are being increasingly used in lower-end markets as a result. Supercomputers in particular often have highly unique architectures that differ significantly from the basic stored-program architecture and from general purpose computers.They often feature thousands of CPUs, customized high-speed interconnects, and specialized computing hardware. Such designs tend to be used only for specialized tasks due to the large scale of program organization required to successfully utilize most of the available resources at once. Supercomputers usually see usage in large-scale simulation, graphics rendering, and cryptography applications, as well as with other so-called "embarrassingly parallel" tasks.