IBM PC compatible

IBM PC compatible refers to a class of computers which make up the vast majority of smaller computers (microcomputers) on the market today. They are based (without IBM's participation) on the original IBM PC. They use the Intel x86 architecture and are capable of using interchangeable commodity hardware.

The origins of this platform came with the decision by IBM in 1981 to market a personal computer as quickly as possible in response to Apple Computer's rapid success (50% marketshare) in the burgeoning PC market. In licensing an operating system from Microsoft, IBM's agreements allowed Microsoft to sell MS-DOS for non-IBM platforms (the IBM version was called PC-DOS). Also, in creating the platform, IBM used only one proprietary component: the BIOS.

Columbia produced the first IBM PC compatible in 1982. Compaq Computer Corp. produced an early IBM PC compatible (which was also the first sewing machine-sized portable PC) a few months later in 1982. Compaq could not directly copy the BIOS 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. Compaq became a very successful PC manufacturer, and was bought out by Hewlett-Packard in 2002.

Simultaneously, many manufacturers such as Xerox, Digital, Sanyo, and Wang introduced PCs that were, although x86- and MS-DOS-based, not completely hardware-compatible with IBM. While such decisions seem foolish in retrospect, it is not always appreciated just how fast the rise of the IBM clone market was, and the degree to which it took the industry by surprise.

Microsoft's intention, and the mindset of the industry from 1981 to as late as the mid-1980s, was that application writers would write to the API's in MS-DOS, and in some cases to the firmware BIOS, and that these components would form what would now be called a hardware abstraction layer. Each computer would have its own OEM version of MS-DOS, customized to its hardware. Any piece of software written for MS-DOS would run on any MS-DOS computer, regardless of variations in hardware design.

During this time MS-DOS was sold only as an OEM product. There was no Microsoft-branded MS-DOS, MS-DOS could not be purchased directly from Microsoft, and the manual's cover had the corporate color and logo of the PC vendor. Bugs were to be reported to the OEM, not to Microsoft. However, in the case of the clones, 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 minicomputer and a Digital VT100 terminal. Had the bulk of commercially important software fallen within these bounds, hardware compatibility might not have mattered. However, from the very beginning, many significant pieces of popular commercial software wrote directly to the hardware, for a variety of reasons:

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.
Graphics capability was not taken seriously. It was considered to be an exotic or novelty function. MS-DOS didn't 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); having 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, arc-drawing, and blitting had to be performed by the application, and this was usually done by bypassing the BIOS and accessing video memory directly. Games, of course, used graphics. They also performed any machine-dependent trick the programmers could think of in order to gain speed. Thus, games were machine-dependent—and games turned out to be important in driving PC purchases.
Even for staid business applications, speed of execution was a significant competitive advantage. This was shown dramatically by Lotus 1-2-3's competitive knockout of rival spreadsheet Context MBA. The latter, now almost forgotten, preceded Lotus to market, included more functions, was written in Pascal, and was highly portable. It was also too slow to be really usable on a PC. Lotus was written in pure 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, popular at the time, made direct access to the disk drive hardware precisely in order to write nonstandard data patterns, patterns that were illegal from the point of view of the OS and therefore could not be produced by standard OS calls.
The microcomputer programming culture at the time was hacker-like, and enjoyed discovering and exploiting undocumented properties of the system.

At first, other than Compaq's models, few "compatibles" really lived up to their claim. "95% compatibility" was seen as excellent. Gradually vendors discovered, not only how to emulate the IBM BIOS, but the places where they needed to use identical hardware chips to perform key functions within the system. Reviewers and users developed suites of programs to test compatibility, generally including Lotus 1-2-3 and Microsoft Flight Simulator, the two most popular "stress tests." Meanwhile, IBM damaged its own franchise by failing to appreciate the important of "IBM compatibility," when they introduced products such as the IBM Portable (essentially a Compaq knockoff), and later the PCjr, which had significant incompatibilities with the mainline PCs. 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.

By the mid-to-late 1980s buyers began to regard PCs as commodity items, and became skeptical as to whether the security blanket of the IBM name warranted the price differential. Meanwhile, of course, the incompatible Xeroxes and Digitals and Wangs were left in the dust. Nobody cared that they ran MS-DOS; the issue was that they did not run off-the-shelf software written for IBM compatibles.

Since 1982, IBM PC compatibles have conquered both the home and business markets of commodity computers so that the only notable remaining competition comes from Apple Macintosh computers with a market share of only a few per cent. Meanwhile, IBM has long since lost its leadership role in the market for IBM PC compatibles; currently the leading players include Dell and Hewlett-Packard.

Despite advances in computer technology, 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 modes used only during a system boot.

"IBM PC Compatible" becomes "Wintel"

In the 1990s, IBM's influence on PC architecture became increasingly irrelevant. Instead of focusing on staying compatible with IBM, vendors began to focus on compatibility with the evolution of Microsoft Windows. As of 2004, no vendor dares to be incompatible with the latest version of Windows, and Microsoft's annual WinHEC conferences provide a setting in which Microsoft can lobby for and in some cases dictate the pace and direction of the hardware side of the PC industry.

The term "IBM PC Compatible" is on the wane. Ordinary consumers simply refer to the machines as "PCs," while programmers and industry writers are increasingly using the term "Wintel architecture"("Wintel" being a contraction of "Windows" and "Intel") to refer to the combined hardware-software platform.

IBM PC compatible

"IBM PC Compatible" becomes "Wintel"

See also