# IBM PC compatible
An IBM PC compatible (often shortened to "PC compatible" or just "PC") is a personal computer that can run the software written for the [IBM Personal Computer](https://www.edgepedia.org/ibm-personal-computer), matching the hardware interfaces that software relied on even though the machine itself was made by another company.[1](https://gunkies.org/wiki/IBM_PC_compatible) The category began as a description of imitators of IBM's 1981 machine and grew into the dominant hardware standard of personal computing, eventually separating from IBM entirely.
| Key fact | Detail |
|---|---|
| Origin | IBM PC Model 5150 introduced August 12, 1981, priced at USD 1,565 with 16 KB of RAM and no disk drive[2](https://www.ibm.com/history/personal-computer) |
| Defining test | "It will run all the software for the IBM Personal Computer"[1](https://gunkies.org/wiki/IBM_PC_compatible) |
| First clone | Columbia Data Products MPC 1600, June 1982[3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html) |
| Legal method | Clean-room reverse engineering of IBM's BIOS, first done commercially by Compaq for about $1 million[4](https://www.computinghistory.org.uk/det/1322/Compaq-Portable-Computer/) |
| Bus standard | IBM's expansion bus adopted as ISA; the 16-bit version, the AT bus, ran at 8 MHz with 16 data and 24 address lines[5](https://www.ibmfiles.com/ibmfiles/powerpc/itso_power_series.pdf) |
| IBM's counterattack | PS/2 (1987) with the proprietary Micro Channel bus; nine clone makers answered with EISA within a year[6](https://www.ibm.com/history/ps-2) • [3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html) |
| End of IBM's control | IBM's PC market share fell from roughly 80% in 1982–1983 to 20% a decade later; the PC division was sold to Lenovo in 2005 for USD 1.75 billion[2](https://www.ibm.com/history/personal-computer) |
## What compatibility meant
The IBM PC, launched in August 1981, was built quickly from off-the-shelf parts to meet a one-year deadline, and IBM published a technical reference containing circuit designs and source code so that other companies could develop software and peripherals.[2](https://www.ibm.com/history/personal-computer) This openness cut both ways. It created a large third-party ecosystem, with more than 750 software packages available within a year of launch, but it also gave competitors the specifications they needed to build functionally identical machines.[2](https://www.ibm.com/history/personal-computer)
Compatibility operated at several levels. A machine could run MS-DOS and programs that used only DOS system calls, yet fail on software that addressed IBM's hardware directly. Full compatibility required matching the ROM BIOS, the low-level firmware through which DOS reached the hardware: a compatible system had to implement the same functionality and interfaces as IBM's proprietary ROM BIOS code without copying it, in a way transparent to the user's hardware and software.[7](https://ftp3.us.freebsd.org/pub/misc/bitsavers/pdf/dataDecisions/1984_Microcomputer_Systems/Vol1_400.pdf) Systems that fell short at this level could not run IBM's own PC DOS, because DOS made calls to specific BIOS functions that had to behave identically.[8](https://vtda.org/books/Computing/Hardware/Upgrading%20and%20Repairing%20PCs/URP_1st_edition.pdf) In practice, so much popular software bypassed DOS and wrote directly to the hardware that manufacturers had little choice but to reproduce IBM's design closely.[1](https://gunkies.org/wiki/IBM_PC_compatible)
## The BIOS problem and clean-room reverse engineering
IBM built the PC from standard components, and Microsoft had kept the right to license the operating system to other computer makers, so the only part a cloner had to duplicate was the BIOS.[4](https://www.computinghistory.org.uk/det/1322/Compaq-Portable-Computer/) Copying IBM's BIOS code outright was copyright infringement and commonly led to legal action by IBM.[9](https://wiki.softhistory.org/wiki/PhoenixBIOS)
The solution was clean-room reverse engineering: one team studied IBM's BIOS and wrote a functional specification, and a separate team with no exposure to IBM's code wrote a new implementation from that specification. Compaq used this method, at a cost of $1 million, to produce a legally distinct BIOS for its first machine.[4](https://www.computinghistory.org.uk/det/1322/Compaq-Portable-Computer/) [Phoenix Technologies](https://www.edgepedia.org/phoenix-technologies) then developed an IBM PC-compatible BIOS that it could license to any manufacturer, which removed the last barrier to entry; competitors such as [American Megatrends](https://www.edgepedia.org/american-megatrends) with AMIBIOS and Award Software with AwardBIOS followed.[9](https://wiki.softhistory.org/wiki/PhoenixBIOS) • [3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html)
## The early clone market
Columbia Data Products introduced the first PC clone, the MPC 1600, in June 1982; Dynalogic and Compaq followed with their own machines in 1983.[3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html) The Hyperion, an IBM PC-compatible portable, actually shipped before the better-known Compaq Portable but lost out by being more expensive, less reliable, and unable to run all of the PC's programs.[10](https://www.computerhistory.org/revolution/personal-computers/17/302)
**Compaq set the template.** Announced in November 1982 and released in March 1983 at $2,995, the 28-pound Compaq Portable was advertised as the first 100% IBM PC-compatible computer, capable of running all software that would run on an IBM PC.[4](https://www.computinghistory.org.uk/det/1322/Compaq-Portable-Computer/) • [11](https://www.computerhistory.org/timeline/computers/?curator=upstract.com) It sold 53,000 units in its first year, and Compaq recorded first-year sales of $111 million, at the time the most ever by an American business in a single year.[4](https://www.computinghistory.org.uk/det/1322/Compaq-Portable-Computer/) • [11](https://www.computerhistory.org/timeline/computers/?curator=upstract.com)
The clone industry then began setting the pace itself. In 1986 Compaq beat IBM to market with the Deskpro 386, the first computer to use Intel's new 32-bit 80386 processor, whose backward-compatibility mode preserved the software base while making graphical operating environments practical on PC compatibles.[11](https://www.computerhistory.org/timeline/computers/?curator=upstract.com) [Michael Dell](https://www.edgepedia.org/michael-dell) founded PCs Limited in 1984 to sell made-to-order PC-compatible computers by mail, and the company became one of the world's largest PC sellers.[10](https://www.computerhistory.org/revolution/personal-computers/17/302)
## Buses and standards
Because clones reproduced IBM's slot design, the expansion bus itself became a standard. IBM's bus architecture was adopted as the [Industry Standard Architecture](https://www.edgepedia.org/industry-standard-architecture) (ISA); the original 8-bit version passed 8 bits at a time, and the 16-bit version introduced with the IBM PC/AT in 1984, sometimes called the AT bus, ran at 8 MHz with 16 data and 24 address lines.[5](https://www.ibmfiles.com/ibmfiles/powerpc/itso_power_series.pdf) Any PC-compatible machine could use almost all the cards created for other PC compatibles.[3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html)
**IBM tried to retake control.** The Personal System/2, released in 1987, was IBM's response to losing the PC market to cloners.[6](https://www.ibm.com/history/ps-2) Its higher-end models carried the proprietary Micro Channel Architecture (MCA), a 32-bit bus whose design IBM strictly controlled and licensed, limiting the industry's ability to clone the PS/2 without paying a toll to IBM.[6](https://www.ibm.com/history/ps-2) • [3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html) The PS/2 line also introduced lasting standards: VGA graphics with 640×480 resolution in 16 colors and 320×200 in 256 colors, 3.5-inch floppy drives, the [Model M keyboard](https://www.edgepedia.org/model-m-keyboard), and 72-pin SIMM memory.[6](https://www.ibm.com/history/ps-2)
The clone industry rejected the power play. One year after the PS/2's launch, a consortium of nine clone manufacturers announced EISA, which extended ISA to 32 bits at minimal licensing cost.[6](https://www.ibm.com/history/ps-2) • [3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html) Few desktop PCs used EISA; the market stayed with 16-bit ISA slots until Intel's PCI bus arrived in the early 1990s.[3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html)
## How the standard outgrew IBM
The PS/2 sold reasonably at first, about 1.5 million units by January 1988, but its higher cost steered most buyers to cheaper compatibles, and nearly every PS/2 advance was matched or surpassed by clone vendors.[3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html) By 1990, IBM had lost its leadership role in the PC-compatible market, and by 1994 Compaq had overtaken it as the top-selling PC maker in the United States.[3](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html) IBM's share of the PC market had shrunk from roughly 80% in 1982–1983 to 20% a decade later, and in 2005 IBM sold its PC division to Lenovo for USD 1.75 billion.[2](https://www.ibm.com/history/personal-computer)
The paradox of the standard is that IBM's own architecture decisions mattered less over time than the compatibility target itself. The 5150 and its successor, the 5160 XT, created a hardware and software target that clone makers could reproduce, eventually separating the "IBM-compatible PC" from IBM.[12](https://vma-broadcast.com/emu/ibm.html) "PC" became a generic term, and the AT-derived standards persisted into the Pentium era and survive in descendants such as the ATX component standard.[2](https://www.ibm.com/history/personal-computer) • [13](https://www.tomshardware.com/pc-components/cpus/the-intel-286-cpu-was-introduced-on-this-day-in-1982-16-bit-x86-chip-introduced-protected-mode-memory-and-would-power-the-ibm-pc-at-and-a-tidal-wave-of-clones)
## References
1. [IBM-compatible PC, Computer History Wiki](https://gunkies.org/wiki/IBM_PC_compatible)
2. [The IBM PC, IBM history](https://www.ibm.com/history/personal-computer)
3. [The IBM PS/2: 25 Years of PC History, PCWorld](https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html)
4. [Compaq Portable Computer, Centre for Computing History](https://www.computinghistory.org.uk/det/1322/Compaq-Portable-Computer/)
5. [IBM Power Series System Architecture, IBM SG24-4592](https://www.ibmfiles.com/ibmfiles/powerpc/itso_power_series.pdf)
6. [The PS/2, IBM history](https://www.ibm.com/history/ps-2)
7. [IBM PC Compatibility, Data Decisions (1984), bitsavers](https://ftp3.us.freebsd.org/pub/misc/bitsavers/pdf/dataDecisions/1984_Microcomputer_Systems/Vol1_400.pdf)
8. [Upgrading and Repairing PCs, 1st edition, Vintage Technology Digital Archive](https://vtda.org/books/Computing/Hardware/Upgrading%20and%20Repairing%20PCs/URP_1st_edition.pdf)
9. [PhoenixBIOS, SoftHistory wiki](https://wiki.softhistory.org/wiki/PhoenixBIOS)
10. [Send in the Clones, Computer History Museum](https://www.computerhistory.org/revolution/personal-computers/17/302)
11. [Computers timeline, Computer History Museum](https://www.computerhistory.org/timeline/computers/?curator=upstract.com)
12. [IBM, VMA Virtual Computer Museum](https://vma-broadcast.com/emu/ibm.html)
13. [The Intel 286 CPU was introduced on this day in 1982, Tom's Hardware](https://www.tomshardware.com/pc-components/cpus/the-intel-286-cpu-was-introduced-on-this-day-in-1982-16-bit-x86-chip-introduced-protected-mode-memory-and-would-power-the-ibm-pc-at-and-a-tidal-wave-of-clones)
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*Topic: Encyclopedia › Technology and the built world › Computing and digital systems › Computer hardware › Personal computers & laptops*
*Initially written Sep 17, 2026 · Reviewed: Sep 19, 2026; Oct 15, 2026 · Edited: Sep 19, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license