IBM PC compatible

IBM PC compatible computers are computers similar to the original IBM PC, XT, and AT, able to use the same software and expansion cards. Such computers used to be referred to as PC clones, or IBM clones. They duplicate almost exactly all the significant features of the PC architecture, facilitated by IBM's choice of commodity hardware components and various manufacturers' ability to reverse engineer the BIOS firmware using a "clean room design" technique. Columbia Data Products built the first clone of the IBM personal computer by a clean room implementation of its BIOS.[citation needed]

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 (ISA) bus by manufacturers of compatible computers. The term "IBM PC compatible" is now a historical description only, since IBM has ended its personal computer sales.

Descendants of the IBM PC compatibles comprise the majority of personal computers on the market presently with the dominant operating system being Microsoft Windows, although interoperability with the bus structure and peripherals of the original PC architecture may be limited or non-existent. This is a result of technology drift over the years. However, some non-PC models, even in the 1980s, did run MS-DOS but had enough hardware differences that IBM compatible software could not be used; examples include slight differences in the memory map, serial ports or video hardware. Only the Macintosh kept significant market share without compatibility with the IBM PC.


The original IBM PC (Model 5150) motivated the production of clones during the early 1980s.

IBM decided in 1980 to market a low-cost single-user computer as quickly as possible in response[citation needed] to Apple Computer's success in the burgeoning microcomputer market. On 12 August 1981, the first IBM PC went on sale. There were three operating systems (OS) available for it. The least expensive and most popular was PC DOS made by Microsoft. In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS, for non-IBM computers. The only component of the original PC architecture exclusive to IBM was the BIOS (Basic Input/Output System).

IBM at first asked developers to avoid writing software that addressed the computer's hardware directly, and to instead make standard calls to BIOS functions that carried out hardware-dependent operations.[1] This software would run on any machine using MS-DOS or PC-DOS. Software that directly addressed the hardware instead of making standard calls was faster, however; this was particularly relevant to games. Software addressing IBM PC hardware in this way would not run on MS-DOS machines with different hardware. 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 cards, and peripherals as the PC. The 808x computer marketplace rapidly excluded all machines which were not hardware- and software-compatible with the PC. The 640 KB barrier on "conventional" system memory available to MS-DOS is a legacy of that period; other non-clone machines, while subject to a limit, could exceed 640 kB.

Rumors of "lookalike", compatible computers, created without IBM's approval, began almost immediately after the IBM PC's release.[2][3] InfoWorld wrote on the first anniversary of the IBM PC that[4]

The dark side of an open system is its imitators. If the specs are clear enough for you to design peripherals, they are clear enough for you to design imitations. Apple ... has patents on two important components of its systems ... IBM, which reportedly has no special patents on the PC, is even more vulnerable. Numerous PC-compatible machines—the grapevine says 60 or more—have begun to appear in the marketplace.

By June 1983 PC Magazine defined "PC 'clone'" as "a computer [that can] accommodate the user who takes a disk home from an IBM PC, walks across the room, and plugs it into the 'foreign' machine".[5] Because of a shortage of IBM PCs that year, many customers purchased clones instead.[6][7] Columbia Data Products produced the first computer more or less compatible with the IBM PC standard during June 1982, soon followed by Eagle Computer. Compaq announced its first IBM PC compatible in November 1982, the Compaq Portable. The Compaq was the first sewing machine-sized portable computer that was essentially 100% PC-compatible. The company could not copy the BIOS directly as a result of the court decision in Apple v. Franklin, but it could reverse-engineer the IBM BIOS and then write its own BIOS using clean room design.

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