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IBM PC compatible computers are those generally similar to the original IBM PC, XT
IBM Personal Computer XT
The IBM Personal Computer XT, often shortened to the IBM XT, PC XT, or simply XT, was IBM's successor to the original IBM PC. It was released as IBM Machine Type number 5160 on March 8, 1983, and came standard with a hard drive...

, and AT
IBM Personal Computer/AT
The IBM Personal Computer AT, more commonly known as the IBM AT and also sometimes called the PC AT or PC/AT, was IBM's second-generation PC, designed around the 6 MHz Intel 80286 microprocessor and released in 1984 as machine type 5170...

. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to legally reverse engineer
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

 the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 through clean room design
Clean room design
Clean room design is the method of copying a design by reverse engineering and then recreating it without infringing any of the copyrights and trade secrets associated with the original design. Clean room design is useful as a defense against copyright and trade secret infringement because it...

. Columbia Data Products
Columbia Data Products
Columbia Data Products introduced the MPC 1600 "Multi Personal Computer" in June 1982. It was an exact functional copy of the IBM PC model 5150 except for the BIOS which was clean roomed...

 built the first clone of the IBM personal computer through a clean room implementation of its BIOS.
Many early IBM PC compatibles used the same computer bus as the original PC and AT models. The IBM AT compatible bus was later named the Industry Standard Architecture
Industry Standard Architecture
Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...

 bus by manufacturers of compatible computers. The term "IBM PC compatible" is now a historical description only since IBM has withdrawn from personal computer sales.

Descendants of the IBM PC compatibles make up the majority of personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

s on the market today, although interoperability with the bus structure and peripherals of the original PC architecture may be limited or non-existent.

Origins


The origins of this platform came with the decision by IBM in 1980 to market a low-cost single-user computer as quickly as possible in response to Apple Computer's
Apple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...

 success in the burgeoning microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...

 market. On 12 August 1981, the first IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

 went on sale. There were three operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s (OS) available for it but the most popular and least expensive was PC DOS, a modified version of 86-DOS, to which Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 acquired full rights from Seattle Computer Products
Seattle Computer Products
Seattle Computer Products was a Seattle, Washington microcomputer hardware company which was one of the first manufacturers of computer systems based on the 16-bit Intel 8086 processor...

. In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

, for non-IBM platforms. The only proprietary component of the original PC architecture was the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 (Basic Input/Output System).

A number of computers of the time based on the 8086 and 8088 processors were manufactured during this period, but with different architecture to the PC, and which ran under their own versions of DOS and CP/M-86
CP/M-86
CP/M-86 was a version of the CP/M operating system that Digital Research made for the Intel 8086 and Intel 8088. The commands are those of CP/M-80. Executable files used the relocatable .CMD file format...

. However, software which addressed the hardware directly instead of making standard calls to MS-DOS was faster. This was particularly relevant to games. The IBM PC was sold in high enough volumes to justify writing software specifically for it, and this encouraged other manufacturers to produce machines which could use the same programs, expansion card
Expansion card
The expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard or backplane to add functionality to a computer system via the expansion bus.One edge of the expansion card holds the contacts that fit exactly into the slot...

s and peripherals as the PC. The 808x computer marketplace rapidly excluded all machines which were not functionally very similar to the PC. The 640 kB barrier on "conventional" system memory available to MS-DOS is a legacy of that period; other non-clone machines did not have this limit.

The original "clones" of the IBM Personal Computer were created without IBM's participation or approval. Columbia
Columbia Data Products
Columbia Data Products introduced the MPC 1600 "Multi Personal Computer" in June 1982. It was an exact functional copy of the IBM PC model 5150 except for the BIOS which was clean roomed...

 closely modeled the IBM PC and produced the first "compatible" PC (i.e., more or less compatible to the IBM PC standard) in June 1982 closely followed by Eagle Computer
Eagle Computer
Eagle Computer of Los Gatos, California was an early microcomputer manufacturing company. Spun off from Audio-Visual Laboratories , it first sold a line of popular CP/M computers which were highly praised in the computer magazines of the day. After the IBM PC was launched, Eagle produced the...

. Compaq Computer Corp.
Compaq
Compaq Computer Corporation is a personal computer company founded in 1982. Once the largest supplier of personal computing systems in the world, Compaq existed as an independent corporation until 2002, when it was acquired for US$25 billion by Hewlett-Packard....

 announced its first IBM PC compatible a few months later in November 1982—the Compaq Portable
Compaq Portable
The Compaq Portable was the first product in the Compaq portable series to be commercially available under the Compaq Computer Corporation brand . It was the first IBM PC compatible portable computer...

. The Compaq was the first sewing machine-sized portable computer
Portable computer
A portable computer is a computer that is designed to be moved from one place to another and includes a display and keyboard. Portable computers, by their nature, are generally microcomputers. Portable computers, because of their size, are also commonly known as 'Lunchbox' or 'Luggable' computers...

 that was essentially 100% PC-compatible. The company could not directly copy the BIOS as a result of the court decision in Apple v. Franklin, but it could reverse-engineer
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

 the IBM BIOS and then write its own BIOS using clean room design
Clean room design
Clean room design is the method of copying a design by reverse engineering and then recreating it without infringing any of the copyrights and trade secrets associated with the original design. Clean room design is useful as a defense against copyright and trade secret infringement because it...

.

Compatibility issues




At the same time, many manufacturers such as Xerox
Xerox
Xerox Corporation is an American multinational document management corporation that produced and sells a range of color and black-and-white printers, multifunction systems, photo copiers, digital production printing presses, and related consulting services and supplies...

, HP, Digital
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...

, Sanyo
Sanyo
is a major electronics company and member of the Fortune 500 whose headquarters is located in Moriguchi, Osaka prefecture, Japan. Sanyo targets the middle of the market and has over 230 Subsidiaries and Affiliates....

, Texas Instruments
Texas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...

, Tulip
Tulip Computers
Tulip Computers NV was a Dutch computer manufacturer that manufactured PC clones. It was founded in 1979, and listed on the Amsterdam Stock Exchange in 1984....

, Wang
Wang Laboratories
Wang Laboratories was a computer company founded in 1951 by Dr. An Wang and Dr. G. Y. Chu. The company was successively headquartered in Cambridge , Tewksbury , and finally in Lowell, Massachusetts . At its peak in the 1980s, Wang Laboratories had annual revenues of $3 billion and employed over...

 and Olivetti
Olivetti
Olivetti S.p.A. is an Italian manufacturer of computers, printers and other business machines.- Founding :The company was founded as a typewriter manufacturer in 1908 in Ivrea, near Turin, by Camillo Olivetti. The firm was mainly developed by his son Adriano Olivetti...

 introduced personal computers that were MS DOS compatible, but not completely software- or hardware-compatible with the IBM PC.

Microsoft's intention, and that of the industry from 1981 to as late as the mid-1980s, was that application writers would write to the APIs
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

 in MS-DOS or the firmware BIOS, and that this would form what would now be called a hardware abstraction layer. Each computer would have its own OEM
Original Equipment Manufacturer
An original equipment manufacturer, or OEM, manufactures products or components that are purchased by a company and retailed under that purchasing company's brand name. OEM refers to the company that originally manufactured the product. When referring to automotive parts, OEM designates a...

 version of MS-DOS, customized to its hardware. Any software written for MS-DOS would run on any MS-DOS computer, despite variations in hardware design. A similar trend was seen with the MSX
MSX
MSX was the name of a standardized home computer architecture in the 1980s conceived by Kazuhiko Nishi, then Vice-president at Microsoft Japan and Director at ASCII Corporation...

 home computer series.

This expectation seemed reasonable in the computer marketplace of the time. Until then Microsoft was primarily focused on computer languages such as BASIC
BASIC
BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....

. The established small system operating software was CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...

 from Digital Research
Digital Research
Digital Research, Inc. was the company created by Dr. Gary Kildall to market and develop his CP/M operating system and related products. It was the first large software company in the microcomputer world...

 which was in use both at the hobbyist level and at the more professional end of those using microcomputers. To achieve such widespread use, and thus make the product economically viable, the OS had to operate across a range of machines from different vendors that had widely varying hardware. Those customers who needed other applications beyond the starter pack could reasonably expect publishers to offer their products for a variety of computers, on suitable media for each.

Microsoft's competing OS was initially targeted to run on a similar varied spectrum of hardware, although all based on the 8086 processor. Thus, MS-DOS was for many years sold only as an OEM
OEM
OEM means the original manufacturer of a component for a product, which may be resold by another company.OEM may also refer to:-Computing:* OEM font, or OEM-US, the original character set of the IBM PC, circa 1981...

 product. There was no Microsoft-branded MS-DOS: MS-DOS could not be purchased directly from Microsoft, and each OEM release was packaged with the trade dress
Trade dress
Trade dress is a legal term of art that generally refers to characteristics of the visual appearance of a product or its packaging that signify the source of the product to consumers...

 of the given PC vendor. Bugs were to be reported to the OEM, not to Microsoft. However, as "compatibles" became widespread, it soon became clear that the OEM versions of MS-DOS were virtually identical, except perhaps for the provision of a few utility programs.

MS-DOS provided adequate support for character-oriented applications such as those that could have been implemented on a text-only terminal
Computer terminal
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...

. Had the bulk of commercially important software fallen within these bounds, low-level hardware compatibility might not have mattered. However, in order to provide maximum performance and leverage hardware features (or work around hardware bugs), PC applications very quickly evolved beyond the simple terminal applications that MS-DOS supported directly. Spreadsheet
Spreadsheet
A spreadsheet is a computer application that simulates a paper accounting worksheet. It displays multiple cells usually in a two-dimensional matrix or grid consisting of rows and columns. Each cell contains alphanumeric text, numeric values or formulas...

s, WYSIWYG
WYSIWYG
WYSIWYG is an acronym for What You See Is What You Get. The term is used in computing to describe a system in which content displayed onscreen during editing appears in a form closely corresponding to its appearance when printed or displayed as a finished product...

 word processors, presentation software and remote communication software
Communication software
Communication software is used to provide remote access to systems and exchange files and messages in text, audio and/or video formats between different computers or user IDs...

 established new markets that exploited the PC's strengths, but required capabilities beyond what MS-DOS provided. Thus, from very early in the development of the MS-DOS software environment, many significant commercial software products were written directly to the hardware, for a variety of reasons:
  • MS-DOS itself did not provide any way to position the text cursor (except to advance it after printing each letter). While the BIOS video interface routines were adequate for rudimentary output, they were inefficient; they did not have "string" output (only output by individual character) and they inserted delay periods to compensate for CGA hardware "snow" (a display artifact of CGA cards produced when writing directly to screen memory)-- an especially bad artifact since they were called via IRQs, thus making multitasking very difficult. A program that wrote directly to video memory could achieve output rates 5 to 20 times faster than making standard calls to the BIOS and MS-DOS. Turbo Pascal
    Turbo Pascal
    Turbo Pascal is a software development system that includes a compiler and an integrated development environment for the Pascal programming language running on CP/M, CP/M-86, and DOS, developed by Borland under Philippe Kahn's leadership...

     used this technique from its earliest incarnations.
  • Graphics
    Computer graphics
    Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....

     capability was not taken seriously in the original IBM design brief; graphics were considered only from the perspective of generating static business graphics such as charts and graphs. MS-DOS did not have an API for graphics, and the BIOS only included the most rudimentary of graphics functions (such as changing screen modes and plotting single points). To make a BIOS call for every point drawn or modified also increased overhead considerably, making the BIOS interface notoriously slow. Because of this, line-drawing
    Line drawing algorithm
    A line drawing algorithm is a graphical algorithm for approximating a line segment on discrete graphical media. On discrete media, such as pixel-based displays and printers, line drawing requires such an approximation ....

    , arc-drawing, and blitting
    Bit blit
    Bit BLIT is a computer graphics operation in which several bitmaps are combined into one using a raster operator....

     had to be performed by the application to achieve acceptable speed, which was usually done by bypassing the BIOS and accessing video memory directly.
  • Games, even early ones, mostly required a true graphics mode. They also performed any machine-dependent trick the programmers could think of in order to gain speed. Though initially the major market for the PC was for business applications, games capability became an important factor in driving PC purchases as prices fell. The availability and quality of games could mean the difference between the purchase of a PC compatible and a different though somewhat interoperable platform like the Amiga
    Amiga
    The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...

    .
  • Communications software directly accessed the UART chip, because the MS-DOS API and the BIOS did not provide full support for the chip's capabilities and was far too slow to keep up with hardware which could transfer data at 19200 bit/s.
  • Even for standard business applications, speed of execution was a significant competitive advantage. This was shown dramatically by Lotus 1-2-3
    Lotus 1-2-3
    Lotus 1-2-3 is a spreadsheet program from Lotus Software . It was the IBM PC's first "killer application"; its huge popularity in the mid-1980s contributed significantly to the success of the IBM PC in the corporate environment.-Beginnings:...

    's competitive knockout of rival Context MBA
    Context MBA
    Context MBA was the first integrated software application for personal computers, providing five functions in one program: spreadsheet, database, charting, word processing, and communication software...

     in the then-popular genre of integrated software
    Integrated software
    Integrated software is software for personal computers that combines the most commonly used functions of many productivity software programs into one application....

    . Context MBA, now almost forgotten, preceded Lotus to market and included more functions; it was written in standard Pascal, making it highly portable but too slow to be truly usable on a PC. 1-2-3 was written in x86 assembly language and performed some machine-dependent tricks. It was so much faster that Context MBA was dead as soon as Lotus arrived.
  • Disk copy-protection schemes, in common use at the time, worked by reading nonstandard data patterns on the diskette to verify originality. These patterns were difficult or impossible to detect using standard DOS or BIOS calls, so direct access to the disk controller hardware was necessary for the protection to work.

At first, few clones other than Compaq's offered full compatibility. Reviewers and users developed suites of programs to test compatibility; the ability to run Lotus 1-2-3 or Microsoft Flight Simulator
Microsoft Flight Simulator
Microsoft Flight Simulator is a series of flight simulator programs for the Microsoft Windows operating system, although it was marketed as a video game. It is one of the longest-running, best-known and most comprehensive home flight simulator series...

 became a standard stress test
Stress testing (software)
In software testing, stress testing refers to tests that determine the robustness of software by testing beyond the limits of normal operation. Stress testing is particularly important for "mission critical" software, but is used for all types of software...

. Vendors gradually learned not only how to emulate the IBM BIOS but also where to use identical hardware chips to perform key functions within the system. Although Compaq chose to keep its BIOS proprietary, eventually the Phoenix BIOS and similar commercially-available products permitted computer makers to build essentially 100%-compatible clones without having to reverse-engineer the IBM PC BIOS themselves.

Over time, IBM damaged its own market by itself failing to appreciate the importance of "IBM compatibility", introducing products such as the IBM PC Convertible (which was outsold by the heavier but much earlier and more compatible Compaq Portable
Compaq Portable
The Compaq Portable was the first product in the Compaq portable series to be commercially available under the Compaq Computer Corporation brand . It was the first IBM PC compatible portable computer...

) and the PCjr (which had significant incompatibilities with the original PC and was soon discontinued).

By the mid to late 1980s buyers began to regard PCs as commodities
Commodity
In economics, a commodity is the generic term for any marketable item produced to satisfy wants or needs. Economic commodities comprise goods and services....

, and doubted that the security blanket of the IBM brand warranted the higher price. Meanwhile, MS-DOS-compatible (but not hardware-compatible) systems did not succeed in the marketplace. Being unable to run off-the-shelf software packages for the IBM PC and true compatibles made for poor sales and the eventual extinction of this category of systems. Also, because of hardware incompatibility with the IBM PC design, the 80186
Intel 80186
The 80188 is a version with an 8-bit external data bus, instead of 16-bit. This makes it less expensive to connect to peripherals. The 80188 is otherwise very similar to the 80186. It has a throughput of 1 million instructions per second....

 processor released only a year after the IBM PC was never popular in general-purpose personal computers.

The declining influence of IBM


After 1987 IBM PC compatibles dominated both the home and business
Business
A business is an organization engaged in the trade of goods, services, or both to consumers. Businesses are predominant in capitalist economies, where most of them are privately owned and administered to earn profit to increase the wealth of their owners. Businesses may also be not-for-profit...

 markets of commodity computers, with other notable alternative architectures being used in niche markets, like the Macintosh
Macintosh
The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...

 computers offered by Apple Inc. and used mainly in desktop publishing
Desktop publishing
Desktop publishing is the creation of documents using page layout software on a personal computer.The term has been used for publishing at all levels, from small-circulation documents such as local newsletters to books, magazines and newspapers...

 at the time, the aging 8-bit Commodore 64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

 which was selling for $150 by this time and became the world's best-selling computer, the 16-bit Commodore Amiga line used in television and video production and the 16-bit Atari ST
Atari ST
The Atari ST is a home/personal computer that was released by Atari Corporation in 1985 and commercially available from that summer into the early 1990s. The "ST" officially stands for "Sixteen/Thirty-two", which referred to the Motorola 68000's 16-bit external bus and 32-bit internals...

 used in the music industry. However, IBM itself lost the leadership role in the market for IBM PC compatibles by 1990. A few events in retrospect are key turning points:
  • The 1982 introduction of the Compaq Portable
    Compaq Portable
    The Compaq Portable was the first product in the Compaq portable series to be commercially available under the Compaq Computer Corporation brand . It was the first IBM PC compatible portable computer...

    , the first 100% IBM PC compatible computer, providing portability unavailable from IBM at the time. The compatibility and performance of the Portable legitimized the PC clone in the eyes of many.
  • The availability by 1986 of sub-$1000 PC XT compatibles, including early offerings from Dell Computer, reducing demand for IBM's models. It was possible to buy two of these "generic" systems for less than the cost of one IBM-branded PC AT, and many companies did just that.
  • Compaq beating IBM to market in 1986 with the first 80386-based PC.
  • IBM's 1987 introduction of the incompatible and proprietary MicroChannel Architecture (MCA) computer bus
    Computer bus
    In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...

    , in its PS/2
    IBM Personal System/2
    The Personal System/2 or PS/2 was IBM's third generation of personal computers. The PS/2 line, released to the public in 1987, was created by IBM in an attempt to recapture control of the PC market by introducing an advanced proprietary architecture...

     line.
  • The 1988 introduction by the "Gang of Nine" companies of a rival bus, Extended Industry Standard Architecture
    Extended Industry Standard Architecture
    The Extended Industry Standard Architecture is a bus standard for IBM PC compatible computers...

    , aimed at competing with, rather than copying, MCA.
  • The duelling Expanded memory
    Expanded memory
    In DOS memory management, expanded memory is a system of bank switching introduced April 24, 1985 that provided additional memory to DOS programs beyond the limit of conventional memory. Expanded memory uses parts of the address space normally dedicated to communication with peripherals for program...

     and Extended memory
    Extended memory
    In DOS memory management, extended memory refers to memory above the first megabyte of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems...

     standards of the late 1980s, both developed with no input from IBM.


As the market evolved, however, despite the failure of MCA, IBM derived a considerable income stream from license fees from companies who paid for licenses to use IBM patents that were in the PC design—to the extent that IBM's focus changed from discouraging PC clones to maximizing its revenue from license sales. IBM finally relinquished its role as a PC manufacturer in April 2005, when it sold its PC division to Lenovo for $1.75 billion.

As of October 2007, Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...

 and Dell hold the largest shares of the PC market in North America. They are also successful overseas, with Acer
Acer (company)
Acer Incorporated is a multinational information technology and electronics corporation headquartered in Xizhi, New Taipei City, Taiwan. Acer's products include desktop and laptop PCs, tablet computers, servers, storage devices, displays, smartphones and peripherals...

, Lenovo, and Toshiba
Toshiba
is a multinational electronics and electrical equipment corporation headquartered in Tokyo, Japan. It is a diversified manufacturer and marketer of electrical products, spanning information & communications equipment and systems, Internet-based solutions and services, electronic components and...

 also notable. Worldwide, a huge number of PCs are "white box
White box (computer hardware)
In computer hardware, a white box is a personal computer or server without a registered brand name. For instance, the term is applied to systems assembled by small system integrators and to homebuilt computer systems assembled by end users from parts purchased separately at retail. In this latter...

" systems assembled by a myriad of local systems builders. Despite advances in computer technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

, all current IBM PC compatibles remain very much compatible with the original IBM PC computers, although most of the components implement the compatibility in special backward compatibility
Backward compatibility
In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...

 modes used only during a system boot
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...

.

Expandability


One of the strengths of the PC compatible platform is its modular hardware design. End-users could readily upgrade peripherals and to some degree, processor and memory without modifying the computer's motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...

 or replacing the whole computer, as was the case with many of the microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...

s of the time. However, as processor speed and memory width increased, the limits of the original XT/AT bus design were soon reached, particularly when driving graphics video cards. IBM did introduce an upgraded bus in the IBM PS/2 computer that overcame many of the technical limits of the XT/AT bus, but this was rarely used as the basis for IBM compatible computers since it required licence payments to IBM both for the PS/2 bus and any prior AT-bus designs produced by the company seeking a license. This was unpopular with hardware manufacturers and several competing bus standards were developed by consortiums, with more agreeable license terms. Various attempts to standardize the interfaces were made, but in practice, many of these attempts were either flawed or ignored. Even so, there were many expansion options, and despite the confusion of its users, the PC compatible platform advanced much faster than other competing platforms of the time, even if only because of its market dominance.

"IBM PC compatible" becomes "Wintel"


In the 1990s, IBM's influence on PC architecture became increasingly irrelevant. An IBM-brand PC became the exception not the rule. Instead of focusing on staying compatible with the IBM PC, vendors began to focus on compatibility with the evolution of Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

. In 1993, a version of Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...

 was released that could run on processors other than x86
X86 architecture
The term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...

. It did require that applications be recompiled—a step most developers didn't take—still, its hardware independence was taken advantage of by SGI
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...

 x86 workstations - thanks to NT's HAL, they could run NT (and its vast application library).

No mass-market personal computer hardware vendor dared to be incompatible with the latest version of Windows, and Microsoft's annual WinHEC conferences provided a setting in which Microsoft could lobby for and —in some cases— dictate the pace and direction of the hardware
Hardware
Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

 side of the PC industry. Microsoft and Intel had become so important to the ongoing development of the PC hardware that industry writers began using the portmanteau word
Portmanteau word
A portmanteau or portmanteau word is a blend of two words or morphemes into one new word. A portmanteau word typically combines both sounds and meanings, as in smog, coined by blending smoke and fog. More generally, it may refer to any term or phrase that combines two or more meanings...

 Wintel
Wintel
Wintel is a portmanteau of Windows and Intel, referring to personal computers using Intel x86 compatible processors running Microsoft Windows...

 to refer to the combined hardware-software platform. This terminology itself is becoming a misnomer, as Intel has lost absolute control over the direction of x86 hardware development with AMD's AMD64, and non-Windows operating systems like Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

 and Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 have established a presence on the x86 architecture.

Design limitations and more compatibility issues


Although the IBM PC was designed for expandability, the designers could not anticipate the hardware developments of the '80s, nor the size of the industry they would engender. To make things worse, IBM's choice of the Intel 8088
Intel 8088
The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...

 for the CPU introduced several limitations which were hurdles for developing software for the PC compatible platform. For example, the 8088 processor only had a 20-bit memory addressing space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...

. To expand PCs beyond one megabyte, Lotus, Intel, and Microsoft jointly created expanded memory
Expanded memory
In DOS memory management, expanded memory is a system of bank switching introduced April 24, 1985 that provided additional memory to DOS programs beyond the limit of conventional memory. Expanded memory uses parts of the address space normally dedicated to communication with peripherals for program...

 (EMS), a bank-switching scheme to allow more memory provided by add-in hardware, and seen through a set of four 16-Kilobyte
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...

 "windows" inside the 20-bit addressing. Later, Intel CPUs had larger address spaces and could directly address 16- MiB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...

s (80286) or more, leading Microsoft to develop extended memory
Extended memory
In DOS memory management, extended memory refers to memory above the first megabyte of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems...

 (XMS) which did not require additional hardware.

"Expanded" and "extended" memory have incompatible interfaces, so anyone writing software that used more than one megabyte had to support both systems for the greatest compatibility until MS-DOS began including EMM386, which simulated EMS memory using XMS memory. A protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...

 OS can also be written for the 80286, but DOS application compatibility was harder than expected, not only because most DOS applications accessed the hardware directly, bypassing BIOS routines intended to ensure compatibility, but also that most BIOS requests were made via interrupt vectors that were marked as "reserved" by Intel.

Video card
Video card
A video card, Graphics Card, or Graphics adapter is an expansion card which generates output images to a display. Most video cards offer various functions such as accelerated rendering of 3D scenes and 2D graphics, MPEG-2/MPEG-4 decoding, TV output, or the ability to connect multiple monitors...

s suffered from their own incompatibilities. Once video cards advanced to SVGA the standard for accessing them was no longer clear. At the time, PC programming used a memory model
Memory model (computing)
In computing, a memory model describes the interactions of threads through memory and specifies the assumptions the compiler is allowed to make when generating code for segmented memory or paged memory platforms.-History and significance:...

 that had 64 KB memory segments. The most common VGA graphics mode's screen memory fit into a single memory segment. SVGA modes required more memory, so accessing the full screen memory was tricky. Each manufacturer developed their own ways of accessing the screen memory, even going so far as not to number the modes consistently. An attempt at creating a standard called VBE
VESA BIOS Extensions
VESA BIOS Extensions is a VESA standard, currently at version 3, that defines the interface that can be used by software to access compliant video boards at high resolutions and bit depths...

 was made, but not all manufacturers adhered to it.

When the 386 arrived, again a protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...

 OS could be written for it. This time, DOS compatibility was much easier because of virtual 8086 mode
Virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running a protected mode operating system.VM86 mode uses a segmentation scheme identical to that of real mode In...

. Unfortunately programs could not switch directly between them, so eventually, some new memory-model APIs were developed, VCPI and DPMI, the latter becoming the most popular.

Because of the wide number of third-party adapters and no standard for them, programming the PC could be difficult. Professional developers would run a large test-suite of various known-to-be-popular hardware combinations.

Meanwhile, consumers were overwhelmed by the competing, incompatible standards and many different combinations of hardware on offer. To give them some idea of what sort of PC they would need to run their software, the Multimedia PC
Multimedia PC
The Multimedia PC, or MPC, was a recommended configuration for a PC with a CD-ROM drive. The standard was set and named by the "Multimedia PC Marketing Council", which was a working group of the Software Publishers Association . The MPMC comprised companies including Microsoft, Creative Labs,...

 (MPC) standard was set in 1990. A PC that met the minimum MPC standard could be marketed with the MPC logo, giving consumers an easy-to-understand specification to look for. Software that could run on the most minimal MPC-compliant PC would be guaranteed to run on any MPC. The MPC level 2 and MPC level 3 standards were later set, but the term "MPC compliant" never caught on. After MPC level 3 in 1996, no further MPC standards were established.

Challenges to Wintel domination


By the late 1990s, the success of Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 had driven nearly all other rival commercial operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s into near-extinction, and had ensured that the “IBM PC compatible” computer was the dominant computing platform. This meant that if a developer made their software only for the Wintel
Wintel
Wintel is a portmanteau of Windows and Intel, referring to personal computers using Intel x86 compatible processors running Microsoft Windows...

 platform, they would still be able to reach the vast majority of computer users. By the late 1980s, the only major competitor to Windows with more than a few percentage points of market share
Market share
Market share is the percentage of a market accounted for by a specific entity. In a survey of nearly 200 senior marketing managers, 67 percent responded that they found the "dollar market share" metric very useful, while 61% found "unit market share" very useful.Marketers need to be able to...

 was Apple Inc.'s Macintosh
Macintosh
The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...

. The Mac started out billed as "the computer for the rest of us" but the Mac's high prices and closed architecture meant the DOS/Windows/Intel onslaught quickly drove the Macintosh into an education and desktop publishing
Desktop publishing
Desktop publishing is the creation of documents using page layout software on a personal computer.The term has been used for publishing at all levels, from small-circulation documents such as local newsletters to books, magazines and newspapers...

 niche, from which it has only recently begun to emerge. By the mid 1990s the Mac's market share had dwindled to around 5% and introducing a new rival operating system had become too risky a commercial venture. Experience had shown that even if an operating system was technically superior to Windows, it would be a failure in the marketplace (BeOS
BeOS
BeOS is an operating system for personal computers which began development by Be Inc. in 1991. It was first written to run on BeBox hardware. BeOS was optimized for digital media work and was written to take advantage of modern hardware facilities such as symmetric multiprocessing by utilizing...

 and OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

 for example). In 1989 Steve Jobs
Steve Jobs
Steven Paul Jobs was an American businessman and inventor widely recognized as a charismatic pioneer of the personal computer revolution. He was co-founder, chairman, and chief executive officer of Apple Inc...

 said of his new NeXT
NeXT
Next, Inc. was an American computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets...

 platform, "It will either be the last new hardware platform to succeed, or the first to fail." In 1993 NeXT announced it was ending production of the NeXTcube
NeXTcube
The NeXTcube was a high-end workstation computer developed, manufactured and sold by NeXT from 1990 until 1993. It superseded the original NeXT Computer workstation and was housed in a similar cube-shaped magnesium enclosure. The workstation ran the NeXTSTEP operating system.- Hardware :The...

 and porting NeXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...

 to Intel processors.

On the hardware front, Intel initially licensed their technology so that other manufacturers could make x86
X86 architecture
The term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...

 CPUs
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

. As the "Wintel" platform gained dominance Intel abandoned this practice. Companies such as AMD and Cyrix
Cyrix
Cyrix Corporation was a microprocessor developer that was founded in 1988 in Richardson, Texas as a specialist supplier of high-performance math coprocessors for 286 and 386 microprocessors. The company was founded by former Texas Instruments staff members and had a long but troubled relationship...

 developed alternative CPUs that were functionally compatible with Intel's. Towards the end of the 1990s, AMD was taking an increasing share of the CPU market for PCs. AMD even ended up playing a significant role in directing the evolution of the x86 platform when its Athlon line of processors continued to develop the classic x86 architecture as Intel deviated with its "Netburst" architecture for the Pentium 4 CPUs and the IA-64 architecture for the Itanium line of server CPUs. AMD developed AMD64, the first major extension not created by Intel, which Intel later adopted. In 2006 Intel began abandoning Netburst with the release of their line of "Core" processors that represented an evolution of the earlier Pentium III.

The IBM PC compatible today


The term 'IBM PC compatible' is not commonly used today because all current mainstream computers are based on the PC architecture, and IBM no longer makes PCs. The competing platforms have either died off or, like the Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...

, have been relegated to niche, enthusiast markets. One notable exception was Apple Inc., whose Macintosh
Macintosh
The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...

 line of computers used non-Intel processors from its inception; first the Motorola 68000 family, then the PowerPC
PowerPC
PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...

 architecture until 2006, when Apple adopted the Intel x86 architecture.

The processor speed and memory capacity of modern PCs are many orders of magnitude
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...

 greater than they were on the original IBM PC and yet backwards compatibility has been largely maintained - a 32-bit operating system can still run many of the simpler programs written for the OS of the early 1980s without needing an emulator
Emulator
In computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...

, though an emulator like DOSBox
DOSBox
DOSBox is emulator software that emulates an IBM PC compatible computer running MS-DOS. It is intended especially for use with old PC games. DOSBox is free software....

 now has near-native functionality at full speed.

See also

  • AT (form factor)
  • ATX form factor
  • Baby AT form factor
  • Computer hardware
    Computer hardware
    Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

  • Computer software
    Computer software
    Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....

  • Computing platform
  • History of computing hardware (1960s-present)
    History of computing hardware (1960s-present)
    The history of computing hardware starting at 1960 is marked by the conversion from vacuum tube to solid state devices such as the transistor and later the integrated circuit. By 1959 discrete transistors were considered sufficiently reliable and economical that they made further vacuum tube...

  • Homebuilt computer
    Homebuilt computer
    A homebuilt computer is a computer assembled from available components, usually commercial off-the-shelf components, rather than purchased as a complete system from a computer system supplier.- History :...

  • IBM Personal Computer
  • Influence of the IBM PC on the personal computer market
  • PC speaker
    PC speaker
    A PC speaker is a loudspeaker, built into some IBM PC compatible computers. The first IBM Personal Computer, model 5150, employed a standard 2.25 inch magnetic driven speaker. More recent computers use a piezoelectric speaker instead. The speaker allows software and firmware to provide...

  • Personal computer
    Personal computer
    A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

  • x86 architecture
    X86 architecture
    The term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...

  • MS-DOS
    MS-DOS
    MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

  • CP/M
    CP/M
    CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...


External links

  • http://www.allgame.com/cg/agg.dll?p=agg&sql=5:2
  • http://www.ibscorporation.com/pc2.htm
  • http://oldcomputers.net/compaqi.html