An IBM PC compatible computer, a short-hand way of designating, especially during the period 1981–1997, a computer generally similar to the original IBM PC, XT and AT. Such computers used to be referred to as PC clones, since they almost exactly duplicated all the significant features of the PC, XT or AT internal design. IBM-compatible is a historical term since IBM no longer manufactures personal computers. Pragmatically, the operational definition of "compatible" is now "capable of running the current edition of Microsoft Windows", while the term PC has largely supplanted IBM compatible for computers in this line of descent.
The original clones of the IBM PC were created without IBM's participation or approval. As the market evolved however (and despite the failure of the MCA bus), IBM derived a considerable income stream from license fees—companies who cloned the PC 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.
Descendants of the IBM PC compatibles make up the majority of microcomputers on the market today, although interoperability with the bus structure and peripherals of the original PC, XT or AT may be non-existent.
History
The origins of this platform came with the decision by IBM in 1980 to market a low-cost single-user computer that they dubbed a personal computer as quickly as possible in response to Apple Computer's success in the burgeoning market. On 12 August 1981, the first IBM PC went on sale. There were three operating systems (OS) available for it but the most popular and least expensive was PC DOS. IBM licensed MS DOS from Microsoft and named it PC-DOS. In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS, for non-IBM platforms. The only proprietary component of the PC was the BIOS (Basic Input/Output System).
Columbia 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. Compaq Computer Corp. announced its first IBM PC compatible a few months later in November 1982—the Compaq Portable. The Compaq was not only the first "sewing machine-sized" portable PC but, even more important, was the first essentially 100% PC-compatible computer. The company 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, but was bought out by Hewlett-Packard in 2002.
Origins
Simultaneously, many manufacturers such as Xerox, HP, Digital, Sanyo, Texas Instruments, Tulip and Wang introduced personal computers that were — although x86 and MS-DOS-based — not completely hardware-compatible with the IBM PC. 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 Application programming interfaces (or APIs) 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 Original Equipment Manufacturer (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.
This expectation seemed reasonable, in the light of the computer marketplace as it existed then. At that time, Microsoft was primarily focused on computer languages, such as BASIC. The established model for small system operating software was CP/M from Digital Research, which was in use both at the hobbyist level and at the more professional end of the microcomputer spectrum. To achieve this spectrum of use, the OS had to operate across a range of machines that had widely varying hardware, although mostly based on the 8080 and Z-80 architectures. Many CP/M-based computers came with a suite of software (often including MicroPro's WordStar, CalcStar, and DataStar). Those customers who needed additional applications beyond the starter pack could expect publishers to offer their products in several media formats for a variety of computers.
Microsoft's competing OS was initially targeted to run on a similar varied spectrum of hardware, although all based on the 8086 architecture. Thus, MS-DOS was for many years 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, very early in the development of the PC, its applications evolved beyond the simple terminal applications that MS-DOS supported directly. Spreadsheets, WYSIWYG Word processors, presentation software and remote communication software 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 pieces of popular commercial software wrote directly to the hardware, for a variety of reasons:
At first, other than Compaq's models, few "compatibles" really lived up to their claim. "95% compatibility" was seen as excellent. 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." 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. 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.
Meanwhile, IBM damaged its own franchise by failing to appreciate the importance of "IBM compatibility", when they introduced products such as the IBM Portable (which was outperformed by the Compaq Portable launched at the same time, which went on to become the market leader), and later the PCjr, which had significant incompatibilities with the mainline PCs. 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, the incompatible Xeroxes and Digitals and Wangs did not succeed in the marketplace. Although they ran MS-DOS, the inability to run off-the-shelf software written for the IBM PC and true compatibles resulted in poor sales.
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 in the original IBM design brief. 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 to achieve acceptable speed; this 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 PC prices fell.
Even for standard business applications, speed of execution was a significant competitive advantage. This was shown dramatically by Lotus 1-2-3's competitive knockout of rival 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, 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.
The declining influence of IBMOne of the strengths of the PC compatible platform is its modular hardware design. This meant that if a component became obsolete, only an individual component had to be upgraded and not the whole computer as was the case with many of the microcomputers of the time. As long as applications used operating system calls and did not write to the hardware directly, the existing applications would work. However, MS-DOS (the dominant operating system of the time) did not have support for many calls for multimedia-hardware, and the BIOS was also inadequate. 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 the PC compatible platform advanced much faster than other competing platforms of the time.
Expandability
In the 1990s, IBM's influence on PC architecture became increasingly irrelevant. Instead of focusing on staying compatible with the IBM PC, vendors began to focus on compatibility with the evolution of Microsoft Windows. In 1993, Windows NT was released that in fact could run on processors other than x86. It did require a recompile, however, and many applications weren't recompiled. Still, its hardware independence was taken advantage of when the SGI x86 workstations were released that were not PC compatible. Thanks to NT's HAL, NT and more importantly, its applications could run on them. Windows 2000 were also released for them (support is right on the Windows 2000 CD, minus some drivers). No vendor dares to be incompatible with the latest version of Windows, and Microsoft's annual WinHEC conferences provides 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. Microsoft and Windows have become so important to the ongoing development of the PC hardware that industry writers have taken to using the term "Wintel architecture" ("Wintel" being a portmanteau combination of "Windows" and "Intel") to refer to the combined hardware-software platform. This terminology itself is becoming a misnomer though as Intel has lost absolute control of the direction of the development of this hardware platform as AMD has become a major player and in some aspects a leader, with Intel starting to copy AMD technologies such as x86-64.
"IBM PC Compatible" becomes "Wintel"
The original IBM PC was not designed with games as its primary focus. Although color graphics adapters and joystick adapters were available from the beginning, the more widely-adapted monochrome adapter and simple sound capabilities made it unsuitable for multimedia applications such as entertainment. It was also priced as a business computer (its primary design focus), well outside of the entertainment market.
The games that were available for the dawn of the PC used the real strength of the machine, 16-bit processing at a faster clock speed, to overcome the lack of multimedia capabilities. One of the most impressive titles for the machine, available less than a year after launch, was Microsoft Flight Simulator. Although Flight Simulator was available for other platforms, it ran at a faster framerate and with more detail on the PC.
As the technology of the PC advanced, more advanced games were developed. As early as 1988, VGA cards were available for PC clones. These offered 256-colour graphics out of a palette of 262,144. Also in 1988, sound cards such as the Adlib and Creative Music System (precursor to the Sound Blaster) were available. These developments brought the PC up to a sufficient level such that it could support arcade games ported to the platform as well as other home computers of the time. Another advantage was that many consumers opted to equip their PCs with a hard drive, while relatively few home computer platforms were so equipped. This allowed PC games to be more ambitious in their use of resources, and made playing games faster and more convenient.
By 1990, the PC had comparable hardware to competing entertainment platforms of the time, such as the Commodore Amiga, but was still was not taken "seriously" as a games machine. This could have been caused by the higher price, or that the hardware was very awkward to program for, and required the development of different drivers for all the multimedia hardware options available to the consumer. As before, the PC's main strength -- raw processing power -- was used as leverage, and this led to impressive 3D or pseudo-3D titles such as Wing Commander, Ultima Underworld, Stunts, Wolfenstein 3D and of course the subsequent Doom. Doom in particular had the most wide-spread success, with awareness crossing over into mainstream media.
Entertainment software
Although the IBM PC was designed for expandability, the designers could not anticipate the hardware developments of the '80s. To make things worse, IBM's choice of the Intel 8088 for the CPU introduced several limitations which were hurdles for developing software for the PC compatible platform. One example was the DOS 640 KiB barrier (memory below 640 KiB is known as conventional memory). This was due to the 20-bit memory addressing space of the 8088. In order to expand PCs beyond one mebibyte, Lotus, Intel, and Microsoft jointly created EMS, a scheme to allow access to additional memory provided by add-in hardware, available via a 64 KiB "window" inside the 20-bit addressing. Later Intel CPUs had larger address spaces and could directly address 16 MiB (80286) or more, leading Microsoft to develop an additional specification that did not require additional hardware, XMS. EMS and XMS have incompatible interfaces, so anyone writing software that used more than one mebibyte had to support both systems for the most compatibility. A protected mode OS can also be written for the 80286, but DOS application compatibility was harder than expected, owing not only to the fact that most DOS application directly access the hardware, but also that most BIOS interrupts were in reserved interrupt vectors.
Graphics cards suffered from their own incompatibilities. Once graphics cards advanced to SVGA level, the standard for accessing them was no longer clear. At the time, PC programming involved using a memory model that had 64 KiB 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 to not number the modes consistently. An attempt at creating a standard called VBE was made, but not all manufacturers adhered to it.
Due to the wide number of third-party adapters for the PC and no standard for interfacing with them, programming the PC could be difficult. When developing for the PC, a large test-suite of various hardware combinations was needed to make sure the software was compatible with as many PC configurations as possible. Even the PC itself had no clear application interface to the flat memory model the 386 and higher could provide in protected mode. Again a protected mode OS could be written for the 80386. This time, DOS compatiblity was much easier because of virtual 8086 mode. Unfortunately programs cannot switch directly to protected mode from that mode so eventually, some new memory-model APIs were developed, VCPI and DPMI, the latter becoming the most popular.
Meanwhile, consumers were overwhelmed by the many different combinations of hardware on offer. To give the consumer some idea of what sort of PC would be needed to run a given piece of software, the Multimedia PC standard (or MPC) was set in 1990. It meant that a PC that met the minimum MPC standard could be considered an MPC. Software that could run on a minimalistic 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 set.
Design limitations and more compatibility issues
The success of Microsoft Windows had driven nearly all other rival commercial operating systems into near-extinction, and had ensured that the "IBM-compatible" PC was the dominant computing platform. This meant that if a manufacturer only made their software for the Wintel platform, they would be able to reach out to the vast majority of computer users. By the mid to late 1990s, introducing a rival operating system had become too risky a commercial venture. Experience had shown that even if an operating system was superior to Windows, it would be a failure.
However, free operating systems were being developed by enthusiasts in their spare time — for example Linux. Despite the fact that Microsoft programmers were programming for a living and the programmers working on Linux were programming as recreation, Linux became used by a great number of people in a vast number of settings. The sheer number of contributors to the Linux project allowed development effort comparable to that of the Microsoft programmers. After a couple of years, Linux had become a very powerful operating system and, because it was free, it spread widely.
Linux had become a serious alternative operating system of technically adept users by the late 1990s. It was seen as an example of what could be achieved by the open source movement. While initially lacking in software and being incompatible with Windows, Linux (like Windows NT) was shown to be more stable than Windows 9x-based operating systems in many instances. Linux has so far failed to gain marketshare outside of technically adept users, mainly due to its relative complexity and lack of many user friendly traditions found in Windows.
On the hardware front, Intel initially licensed their technology so that other manufacturers could make x86 CPUs. As the "Wintel" platform gained dominance Intel abandoned this practice. Companies such as AMD and Cyrix 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 the first 64 bit extension of the x86 architecture (that forced intel to make a clean-room version of it, in all its latest cpus). In 2006 Intel began abandoning Netburst with the release of their line of "Core" processors that represent an evolution of the earlier Pentium III.
DirectX, while solving many of the problems in programming the PCs, was only compatible with Windows. OpenGL, which was available for several platforms, was ported to Windows, and offered a means of rapidly developing cross-platform 3D applications.
Challenges to Wintel domination
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