computer

A computer is a machine that manipulates data according to a set of instructions.
The Columbia Supercomputer, located at the NASA Ames Research Center.
Although mechanical examples of computers have existed through much of recorded
human history, the first electronic computers were developed in the mid-20th century
(1940–1945). These were the size of a large room, consuming as much power as
several hundred modern personal computers (PCs). Modern computers based on
integrated circuits are millions to billions of times more capable than the early
machines, and occupy a fraction of the space.Simple computers are small enough to
fit into a wristwatch, and can be powered by a watch battery. Personal computers in
their various forms are icons of the Information Age and are what most people think
of as "computers". The embedded computers found in many devices from MP3
players to fighter aircraft and from toys to industrial robots are however the most
numerous.



A computer is a machine that manipulates data according to a set of instructions.

The Columbia Supercomputer, located at the NASA Ames Research Center.
Although mechanical examples of computers have existed through much of recorded
human history, the first electronic computers were developed in the mid-20th century
(1940–1945). These were the size of a large room, consuming as much power as
several hundred modern personal computers (PCs). Modern computers based on
integrated circuits are millions to billions of times more capable than the early
machines, and occupy a fraction of the space.Simple computers are small enough to
fit into a wristwatch, and can be powered by a watch battery. Personal computers in
their various forms are icons of the Information Age and are what most people think
of as "computers". The embedded computers found in many devices from MP3
players to fighter aircraft and from toys to industrial robots are however the most
numerous.
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 certain minimum
capability is, in principle, capable of performing the same tasks that any other
computer can perform. Therefore computers ranging from a mobile phone to a
supercomputer are all able to perform the same computational tasks, given enough
time and storage capacity.
History of computing
The Jacquard loom, on display at the Museum of Science and Industry in Manchester,
England, was one of the first programmable devices.The first use of the word
"computer" was recorded in 1613, referring to a person who carried out calculations,
or computations, and the word continued to be used in that sense until the middle of


the 20th century. From the end of the 19th century onwards though, the word began to
take on its more familiar meaning, describing a machine that carries out computations.
The history of the modern computer begins with two separate technologies—
automated calculation and programmability—but no single device can be identified as
the earliest computer, partly because of the inconsistent application of that term.
Examples of early mechanical calculating devices include the abacus, the slide rule
and arguably the astrolabe and the Antikythera mechanism (which dates from about
150–100 BC). Hero of Alexandria (c. 10–70 AD) built a mechanical theater which
performed a play lasting 10 minutes and was operated by a complex system of ropes
and drums that might be considered to be a means of deciding which parts of the
mechanism performed which actions and when.This is the essence of
programmability.
The "castle clock", an astronomical clock invented by Al-Jazari in 1206, is considered
to be the earliest programmable analog computer.It displayed the zodiac, the solar and
lunar orbits, a crescent moon-shaped pointer travelling across a gateway causing
automatic doors to open every hour,and five robotic musicians who played music
when struck by levers operated by a camshaft attached to a water wheel. The length of
day and night could be re-programmed to compensate for the changing lengths of day
and night throughout the year.
The Renaissance saw a re-invigoration of European mathematics and engineering.
Wilhelm Schickard's 1623 device was the first of a number of mechanical calculators
constructed by European engineers, but none fit the modern definition of a computer,
because they could not be programmed.
In 1801, Joseph Marie Jacquard made an improvement to the textile loom by
introducing a series of punched paper cards as a template which allowed his loom to
weave intricate patterns automatically. The resulting Jacquard loom was an important
step in the development of computers because the use of punched cards to define
woven patterns can be viewed as an early, albeit limited, form of programmability.
It was the fusion of automatic calculation with programmability that produced the first
recognizable computers. In 1837, Charles Babbage was the first to conceptualize and
design a fully programmable mechanical computer, his analytical engine.Limited
finances and Babbage's inability to resist tinkering with the design meant that the
device was never completed.

In the late 1880s, Herman Hollerith invented the recording of data on a machine
readable medium. Prior uses of machine readable media, above, had been for control,
not data. "After some initial trials with paper tape, he settled on punched cards ..." To
process these punched cards he invented the tabulator, and the keypunch machines.
These three inventions were the foundation of the modern information processing
industry. Large-scale automated data processing of punched cards was performed for
the 1890 United States Census by Hollerith's company, which later became the core
of IBM. By the end of the 19th century a number of technologies that would later
prove useful in the realization of practical computers had begun to appear: the
punched card, Boolean algebra, the vacuum tube (thermionic valve) and the
teleprinter.
During the first half of the 20th century, many scientific computing needs were met
by increasingly sophisticated analog computers, which used a direct mechanical or
electrical model of the problem as a basis for computation. However, these were not
programmable and generally lacked the versatility and accuracy of modern digital
computers.
Alan Turing is widely regarded to be the father of modern computer science. In 1936
Turing provided an influential formalisation of the concept of the algorithm and
computation with the Turing machine. Of his role in the modern computer, Time
magazine in naming Turing one of the 100 most influential people of the 20th
century, states: "The fact remains that everyone who taps at a keyboard, opening a
spreadsheet or a word-processing program, is working on an incarnation of a Turing
machine".
The inventor of the program-controlled computer was Konrad Zuse, who built the
first working computer in 1941 and later in 1955 the first computer based on magnetic
storage.
George Stibitz is internationally recognized as a father of the modern digital
computer. While working at Bell Labs in November 1937, Stibitz invented and built a
relay-based calculator he dubbed the "Model K" (for "kitchen table", on which he had
assembled it), which was the first to use binary circuits to perform an arithmetic
operation. Later models added greater sophistication including complex arithmetic
and programmability.
CPU design and Control unit

Die of an Intel 80486DX2 microprocessor (actual size: 12×6.75 mm) in its packaging.
Diagram showing how a particular MIPS architecture instruction would be decoded
by the control system.The control unit (often called a control system or central
controller) manages the computer's various components; it reads and interprets
(decodes) the program instructions, transforming them into a series of control signals
which activate other parts of the computer.Control systems in advanced computers
may change the order of some instructions so as 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:
1.Read the code for the next instruction from the cell indicated by the program
counter.
2.Decode the numerical code for the instruction into a set of commands or signals for
each of the other systems.
3.Increment the program counter so it points to the next instruction.
4.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.
5.Provide the necessary data to an ALU or register.
6.If the instruction requires an ALU or specialized hardware to complete, instruct the
hardware to perform the requested operation.


7.Write the result from the ALU back to a memory location or to a register or perhaps
an output device.
8.Jump back to step.
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).
It is noticeable that 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 that runs a microcode program that causes all of these events to
happen.
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
adding and subtracting or might include multiplying or dividing, trigonometry
functions (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. An 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 both for creating complicated conditional statements and processing boolean
logic.


Superscalar computers may contain multiple ALUs so that they can process several
instructions at the same time.Graphics processors and computers with SIMD and
MIMD features often provide ALUs that can perform arithmetic on vectors and
matrices.

Memory and Computer data storage

Magnetic core memory was the computer memory of choice throughout the 1960s,
until it was replaced by semiconductor 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 (2^8 = 256); either from 0 to 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 contexts. 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: random-access memory or
RAM and 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,
so 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 BIOS that 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 distinction 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 programmer's part.
Input/output (I/O)
Hard disk drives are common storage devices used with computers.I/O is the means
by which a computer exchanges information with the outside world. Devices that
provide input or output to the computer are called peripherals.On a typical personal
computer, peripherals include input devices like the keyboard and mouse, and output
devices such as the display and printer. Hard disk drives, floppy disk drives and
optical disc drives serve as both input and output devices. Computer networking is
another form of I/O.


Often, I/O devices are complex computers in their own right with their own CPU and
memory. A graphics processing unit might contain fifty or more tiny computers that
perform the calculations necessary to display 3D graphics[citation needed]. Modern
desktop computers contain many smaller computers that assist the main CPU in
performing I/O.
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