Computer
A computer is a machine for manipulating data according to a list of instructions.
A list of computer instructions designed to perform some task is known as a program. When programs are contained in storage that may be easily modified by the computer itself, the device is said to have a von Neumann architecture. The ability to execute stored programs makes computers extremely versatile. 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 with capability and complexity ranging from that of a personal digital assistant to a supercomputer are all able to perform the same computational tasks so long as time and storage capacity are not considerations. Hence the same computer design may be used for tasks ranging from processing company payrolls to controlling unmanned spaceflights.
Computers take numerous physical forms. Early electronic computers were the size of a large room, consuming as much power as several hundred modern personal computers. [1] On the other hand, an entire computer can now fit into a wrist watch. Today, large-scale computing facilities still exist for specialized scientific computation and for the transaction processing requirements of large organizations but society has come to recognise personal computers and their portable equivalent, the laptop computer as icons of the information age. These are what most people think of as a "computer". However, the most common form of computer in use today is the embedded computer. Embedded computers are usually relatively simple and physically small and are often used to control other devices. They may help control machines from fighter aircraft to industrial robots, digital cameras, childrens toys and even electric toothbrushes.
History of computing
The question of which was the earliest computer is a difficult one. The very definition of what a computer is has changed over the years and it is therefore impossible to definitively answer the question.
Originally, the term "computer" referred to a person who performed numerical calculations (a human computer), often with the aid of a mechanical calculating device or analog computer. Examples of early mechanical computing devices included the abacus and the Antikythera mechanism, an ancient Greek device for calculating the movements of planets which dates from about 87 BC. The end of the Middle Ages saw a reinvigoration of European mathematics and engineering, and Wilhelm Schickard's 1623 device was the first of a number of mechanical calculators constructed by European engineers.
However, none of those devices fit the modern definition of a computer as a programmable device. In 1801, Joseph Marie Jacquard improvement to existing loom designs that used a series of punched paper cards as a template to allow his loom to weave intricate patterns automatically. While the resulting Jacquard loom is not usually considered to be a computer, it was an important step because the use of punched cards to define woven patterns can be viewed as an early, albeit limited, form of programmability.
In 1837, Charles Babbage was the first to conceptualize and design a fully programmable mechanical computer that he called "The Analytical engine" (not to be confused with his Difference engine which was a non-programmable mechanical calculator). Due to limits of the technology of the time, finances, and an inability to resist tinkering with his design, the Analytical engine was never actually constructed. Hence, Babbage is generally credited with the first design of a programmable computer.
By the end of the 19th century a number of technologies that would later prove useful in the design of practical computers had begun to appear. The punched card and the vacuum tube had appeared and large-scale automated data processing using machines to read, sort and tabulate punched cards was performed by tabulating machines designed by Hermann Hollerith and manufactured by Computing Tabulating Recording Corporation (CTR) who later became IBM.
During the first half of the 20th century, many scientific computing needs were met by increasingly sophisticated special-purpose 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 of today's computers. Analog and mechanical computers and calculators became increasingly rare as they were largely superceded by the development of the programmable digital computer.
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A succession of steadily more powerful and flexible computing devices were constructed in the 1930s and 1940s, gradually adding the key features that are seen in modern computers. The use of digital electronics (largely invented by Claude Shannon in 1937) and more flexible programmability Template:Ref harvard were vitally important steps but defining one point along this road as "the first digital electronic computer" is exceedingly difficult. Notable achievements include the Atanasoff-Berry Computer (1937), a special-purpose machine that used valve-driven (vacuum tube) computation, binary numbers, and regenerative memory; the secret British Colossus computer (1944), which had limited programmability but demonstrated that a device using thousands of valves could be both made reliable and reprogrammed electronically; the Harvard Mark I, a large-scale electromechanical computer with limited programmability (1944); the decimal-based American ENIAC (1946)—which was the first general purpose electronic computer, but originally had an inflexible architecture that meant reprogramming it essentially required it to be rewired; and Konrad Zuse's Z machines, with the electromechanical Z3 (1941) being the first working machine featuring automatic binary arithmetic and feasible programmability.
Several developers of ENIAC, recognizing its flaws, came up with a far more flexible and elegant design, which came to be known as the stored program architecture or von Neumann architecture. This design was first formally described by John von Neumann in the paper "First Draft of a Report on the EDVAC", published in 1945. A number of projects to develop computers based on the stored program architecture commenced around this time; the first of these being completed in Great Britain. The first to be demonstrated working was the Manchester Small-Scale Experimental Machine (SSEM) or "Baby". However, the EDSAC, completed a year after SSEM, was perhaps the first practical implementation of the stored program design. Shortly thereafter, the machine originally described by von Neumann's paper—EDVAC—was completed but didn't see full-time use for an additional two years.
Nearly all modern computers implement some form of the stored program architecture, making it the single trait by which the modern word computer is defined. By this standard, many earlier devices would no longer be called "computers" by today's definition, but are usually referred to as such in their historical context.
Vacuum tube driven computer designs were in use throughout the 1950s, but were eventually replaced in the 1960s by transistor-based computers, which were smaller, faster, cheaper, and much more reliable - which allowed them to be produced on a commercial scale. By the 1970s, the adoption of integrated circuit technology and the subsequent creation of the microprocessor caused another major improvement in size, speed, cost and reliability. By the 1980s, computers were sufficiently small and cheap that it became cost-effective to use them in domestic appliances such as washing machines and it became easily possible for individuals to own a personal computer. Fuelled largely by the widespread growth of the Internet in the 1990s, ownership of personal computers is becoming as common as the television or the telephone and almost all modern electronic devices contain a computer of some kind.
Stored program architecture
The defining feature of modern computers - and that which distinguishes them from all other machines - is that they can be programmed. That is to say that a list of instructions (the program) can be given to the computer and it will store them and carry them out at some time in the future.
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 specialised 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.
It is a convenient metaphor to imagine the computer working its way through the list of instructions in the program, just as a person might read a book—sometimes going back to an earlier place in the text—sometimes skipping sections that are not of interest but generally reading one line at a time. 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.
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 1,000 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. For example:
sum = 0 ; for ( i = 1 ; i <= 1000 ; i++ ) sum += i ; print ( sum ) ;
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 about a millionth of a second.
However, computers cannot "think" for themselves in the sense that they only solve problems in exactly the way they are programed to. An intelligent human faced with the above addition task might soon realise that instead of adding up all of those numbers one can simply use the equation
and arrive at the correct answer (500,500) with little work. Some modern computers are able to make some decisions that speed up the execution of some programs by "guessing" about the outcomes of certain jump instructions and re-arranging the order of instructions slightly without changing their meaning (branch prediction, speculative execution, and out-of-order execution). However, computers cannot determine a more efficient way to perform the task given to them because they do not have an overall understanding of what the task, or the "big picture", is. In other words, a computer programmed to add up the numbers one by one as in the example above would do exactly that without regard to efficiency or alternative solutions.
Programs
In practical terms, a computer program might include anywhere from a dozen instructions (such as the simple traffic light example) to many millions of instructions for something like a word processor or a web browser. A typical modern computer can execute billions of instructions every second and nearly never make a mistake over years of operation.
Large computer programs may take teams of computer programmers years to write and the probability of the entire program having been written completely in the manner intended is unlikely. Errors in computer programs are called bugs. Sometimes bugs are benign and do not affect the usefulness of the program, in other cases they might cause the program to completely fail (or crash), in yet other cases there may be subtle problems. In the second traffic light example (above), there is a bug; if the traffic signal is showing red when someone switches the 'flash red' switch, it will cycle through green once more before starting to flash red. It is worthwhile to note that bugs are usually not the fault of the computer. Since computers merely execute the instructions they are given, bugs nearly always are the result of programmer error or an oversight made in the program's design. [2]
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 number and so on. The simplest computers are able to perform any of a handful of different instructions, the more complex computers have several thousand 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.
While the technologies used in computers have changed dramatically since the first electronic, general-purpose computers of the 1940s, most still use the von Neumann architecture. The design made the universal computer a practical reality.
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 Harvard Mark I computer. Often, modern 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,[3] it becomes extremely tedious to do so in practice, especially for complicated programs. Instead, each basic instruction can be given a short name that is both indicative of its function and easy to remember—a mnemonic 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. This means that an ARM architecture computer (such as may be found in a PDA or a hand-held videogame) cannot understand the machine language of an Intel Pentium or the AMD Athlon 64 computer that might be in a PC.[4]
Writing long programs in assembly language is often difficult and error prone, so most complicated programs are written in more abstract High-level programming languages that are able to express the needs of the computer 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.[5] 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.
Example
Suppose a computer were being employed to drive a traffic light. A simple stored program might say:
- Turn off all of the lights
- Turn on the red light, wait for sixty seconds
- Turn off the red light, turn on the green light, wait for sixty seconds
- Turn off the green light, turn on the amber light, wait for two seconds
- Turn off the amber light
- Jump to instruction number (2).
With this set of instructions, the computer would cycle the light continually through red, green, amber and back to red again until told to stop running the program.
However, suppose there is a simple on/off switch connected to the computer that is intended be used to make the light flash red while some maintenance operation is being performed. The program might then instruct the computer to:
- Turn off all of the lights
- Turn on the red light, wait for sixty seconds
- Turn off the red light, turn on the green light, wait for sixty seconds
- Turn off the green light, turn on the amber light, wait for two seconds
- Turn off the amber light
- If the maintainance switch is NOT turned on then jump to instruction number (2)
- Turn on the red light, wait for one second
- Turn off the red light, wait for one second
- Jump to instruction number (6)
In this manner, the computer is either running the instructions from number (2) to (6) over and over or it's running the instructions from (6) down to (9) over and over, depending on the position of the switch.
How computers work
A general purpose computer has 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 bundles of wires (called buses).
The control unit, ALU, registers, and basic I/O (and often other hardware closely linked with these) are collectively known as a central processing unit (CPU). While early CPUs were comprised of many discrete components, since the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor.
Control system
The control system (often called a control unit or central control) directs the other core components of a computer. It reads and interprets (decodes) instructions in the program one by one. Usually the control system will decode each instruction and turn it into a series of control signals that operate the various other portions of the computer.[6] Control systems in advanced computers may also resequence instructions in order to improve performance. A key component common in all control systems is the program counter, a special memory cell (usually a register) that keeps track of which location or locations in memory the next instruction is to be read from.[7]
A simplified sequence describing the control system's function follows. Some of these steps may be performed concurrently or in a slightly different order, depending on the computer's particular design.
- 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 register or perhaps an output device.
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).
Arithmetic/logic unit (ALU)
The ALU is capable of performing two classes of basic operations: Arithmetic and logic.
Arithmetic operations include adding, subtracting, multiplying or dividing two numbers. The set of arithmetic operations may be very limited. Indeed, some designs do not directly support multiplication and division operations. Others include more complicated operations such as trigonometry functions (sine, cosine, etc) and square roots. However, any computer that is capable of performing the simplest operations can be programmed to break down the more complex operations into simple steps that it can perform. Therefore, all computers can be programmed to perform all of the arithmetic operations given sufficient instructions and storage. An ALU may also compare numbers and return boolean truth values (true or false) in response to operations that instruct the computer to determine the relationship between two numbers. For instance, "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 for certain kinds of data processing that are easily accomplished with boolean logic.
Often, modern computers will contain multiple ALUs in order to facilitate techniques like superscalar design. Sometimes they will also include specialized structures similar in function to ALUs that perform more specialized tasks. SIMD units perform many vector arithmetic operations useful for certain types of multimedia and scientific programs. Floating point units specifically designed for floating point arithmetic are also commonly seen.[8]
Memory
A computer's memory may be viewed as a list of cells. Each cell has a numbered "address" and can store a single number. The computer may 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. Characters, integers, even computer instructions may be placed into memory with equal ease. Since stored program computers do not usually differentiate between different types of information, it is up to the software to give significance to what the computer sees as nothing but a series of numbers.
In almost all modern computers, each memory cell is set up to store binary numbers in octets of bits. Each octet is able to represent 256 different numbers; usually either 0 to 255 or -128 to 127. An octet of bits is called a byte. To store larger numbers than a single byte allows, 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 seen in specialized applications or historical contexts. However, a modern computer may store any kind of information in memory as long as it can be somehow represented in numerical form.
Many computers—nearly all modern computers—include a special set of memory locations called registers that can be read and written to much more rapidly than the main memory area. There are typically several registers; ranging in quantity from a two or three to more than a hundred (though a more typical value is twenty or thirty). Registers are used for the most frequently needed data items to avoid having to access main memory every time data is needed. Since computers constantly work with data, reducing the need to access main memory (which is often slow compared to the ALU and control units) greatly increases the computer's speed.
In more sophisticated computers there may be one or more memory 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)
I/O is the means by which a computer receives information from the outside world, and reports information back to that world. On a typical personal computer, inputs include devices like the keyboard and mouse, and output devices include computer monitors, printers and the like.[9] However, a great variety of devices can be connected to a computer and serve as I/O. The computer that controls a modern automobile might read the position of the pedals and steering wheel, the output of the engine oxygen sensor, the fuel tank level sensor, temperature readings inside the engine and sensors that monitor the speed of each wheel. The auto computer's output devices may include the various lights and gauges that the driver uses, the spark ignition circuits, fuel injection and other engine controls. Practically any device that can be made to interface with a digital system may be used as I/O.
Some devices, such as hard disks, serve as both inputs and outputs. Others, such as a graphics processing unit, are complex computers in their own right with their own I/O devices. Full computers may therefore also be used as I/O devices. A typical personal computer may contain several small computers that assist the main computer in I/O or are connected for communication purposes. Indeed, various forms of computer networking may easily be considered nothing more than a long-range extension of I/O.
Multitasking
While a computer may be viewed as running one gigantic 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 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 may return to that task later. If several programs are running "at the same time", then the interrupt generator may be causing several hundred interrupts a second, causing a program switch each time. Since modern computers typically execute instructions several orders of magnitude faster than human perception, many programs may seem to be 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 principle 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 considering. However, 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 run so that many programs may be run at the same time without unacceptable speed loss.
Multiprocessing
Some computers may divide their work between one or more separate CPUs, creating a multiprocessing configuration. Traditionally, this technique was utilized only in large and powerful computers such as supercomputers, mainframe computers and servers. However, multiprocessor and multi-core (multiple CPUs on a single integrated circuit) personal and laptop computers have become widely available and are beginning to see increased usage 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 useful only for specialized tasks due to the large scale of program organization required to sucessfully utilize most of a 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.
Networking and the Internet
Computers have been used to coordinate information in multiple locations since the 1950s, with the US military's SAGE system the first large-scale example of such a system, which led to a number of special-purpose commercial systems like Sabre.
In the 1970s, computer engineers at research institutions throughout the US began to link their computers together using telecommunications technology. This effort was funded by ARPA (now DARPA), and the computer network that it produced was called the ARPANET. The technologies that made the Arpanet possible spread and evolved. 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. In the phrase of John Gage and Bill Joy (of Sun Microsystems), "the network is 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 1990s 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 ubiquitous almost everywhere. 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.[10] "Wireless" networking, often utilizing mobile phone networks, has meant networking is becoming increasingly ubiquitous even in mobile computing environments.
Further topics
Hardware
The term hardware covers all of those parts of a computer that are tangible objects. Circuits, displays, power supplies, cables, keyboards, printers and mice are all hardware.
| Peripheral device (Input/output) | Input | Mouse, Keyboard, Joystick, Image scanner |
| Output | Monitor, Printer | |
| Both | Floppy disk drive, Hard disk, Optical disc drive, Teleprinter | |
| Computer busses | Short range | RS-232, SCSI, PCI, USB |
| Long range (Computer networking) | Ethernet, ATM, FDDI |
Software
Software refers to parts of the computer that have no material form; programs, data, protocols, etc are all software. When software is stored in hardware that cannot easily be modified (such as BIOS ROM in an IBM PC compatible), it is sometimes termed firmware to indicate that it falls into an area of uncertainty between hardware and software.
Programming Languages
These are the languages used by Computer programmers to describe their algorithms. They are generally either translated into machine language by a compiler or an assembler - or they may be executed directly by an interpreter - or perhaps even some hybrid of the two techniques. There are thousands of different programming languages - some intended to be general purpose, others are useful only for some highly specialised application.
Professions and organizations
As the use of computers has spread throughout society, there are an increasing number of careers involving computers. Following the theme of hardware, software and firmware, the brains of people who work in the industry are sometimes known irreverently as wetware or meatware.
| Hardware-related | Electrical engineering, Electronics engineering, Computer engineering |
| Software-related | Human-computer interaction, Information technology, Software engineering, Systems analyst, Web design |
The need for computers to work well together and to be able to exchange information has spawned the need for many standards organisations, clubs and societies of both a formal and informal nature.
| Standards groups | ANSI, IEC, IEEE, IETF, ISO, W3C |
| Professional Societies | ACM, SIGGRAPH, IET |
| Enthusiasts Clubs | .... |
| Free/Open source software groups | Free Software Foundation (FSF), Mozilla Foundation, Apache Software Foundation |
See also
- Computability theory
- Computer science
- Computing
- Computers in fiction
- Computer music
- Computer security and Computer insecurity
- List of computer term etymologies
Notes
- ^ In 1961, ENIAC consumed an estimated 174 kW. By comparison, a typical personal computer may use around 400 W; over four hundred times less. Template:Ref harvard
- ^ It is not universally true that bugs are solely due to programmer oversight. 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 1990s 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.
- ^ Even some later computers were commonly programmed directly in machine code. Some minicomputers like the DEC PDP-8 could be programmed directly from a panel of switches. 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
- ^ However, there is sometimes some form of machine language compatibility between different computers. An x86-64 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 80486. This contrasts with very early commercial computers, which were often one-of-a-kind and totally incompatible with other computers.
- ^ High level languages are also often interpreted rather than compiled. Interpreted languages are translated into machine code on the fly by another program called an interpreter.
- ^ The control unit's rule in interpreting instructions has varied somewhat in the past. While the control unit is solely responsible for instruction interpretation in most modern computers, this is not always the case. Many computers include some instructions that may only be partially interpreted by the control system and partially interpreted by another device. This is especially the case with specialized computing hardware that may be partially self-contained. For example, EDVAC, the first modern stored program computer to be designed, used a central control unit that only interpreted four instructions. All of the arithmetic-related instructions were passed on to its arithmetic unit and further decoded there.
- ^ Instructions often occupy more than one memory address, so program counters usually increase by the number of memory locations required to store one instruction.
- ^ Though specialized hardware structures are often included, they are rarely necessary—at least in principle—for the computer to perform a given task. Floating point and vector math can be performed with programs using a series of simpler operations. However, specialized hardware can perform the task many times faster, and is therefore often needed if a computer is expected to complete a related task in a short time.
- ^ Although computer keyboards and monitors are the most readily recognizable forms of I/O, they are almost never directly connected to the CPU. Rather, they communicate with the CPU with the aid of several I/O busses and interfaces.
- ^ "North America Internet Usage Stats". Internet World Stats. April 3, 2006. Retrieved 2006-04-05.
- ^ Most major 64-bit instruction set architectures are extensions of earlier designs. All of the architectures mentioned here existed in 32-bit forms before their 64-bit incarnations were introduced.
References
- a Kempf, Karl (1961). "Historical Monograph: Electronic Computers Within the Ordnance Corps". Aberdeen Proving Ground (United States Army).
{{cite journal}}: Cite journal requires|journal=(help) - a Phillips, Tony (2000). "The Antikythera Mechanism I". American Mathematical Society. Retrieved 2006-04-05.
- a Shannon, Claude Elwood (1940). "A symbolic analysis of relay and switching circuits". Massachusetts Institute of Technology.
{{cite journal}}: Cite journal requires|journal=(help)