How did Microsoft start, what did IBM miss, and why does almost every PC in the world run Windows?

Few business origin stories combine a college dropout's audacity, a contract clause that IBM's lawyers missed, and a hardware decision that accidentally created Microsoft's biggest market. Here is how it all happened — and why its effects are still felt on nearly every office desk today.

Two teenagers and a bluff that launched an empire

Bill Gates and Paul Allen were childhood friends from Lakeside School in Seattle who shared an early enthusiasm for computers. Their partnership began with a venture called Traf-O-Data, aimed at processing traffic data, which was not a commercial success but gave them valuable experience in running a business and in software development. The real break came from a magazine cover. The true catalyst for Microsoft's founding was the Altair 8800, considered the first commercially successful personal computer; Allen saw it on the cover of the January 1975 issue of Popular Electronics and proposed that they develop software for it.

Gates and Allen contacted MITS, the manufacturer of the Altair, claiming they had already built a BASIC interpreter for it — a bluff: "We hadn't written a line of code," Gates later admitted. They then wrote the interpreter from scratch, it worked, and MITS bought it. On April 4, 1975, Gates and Allen officially founded Microsoft (initially spelled "Micro-Soft") in Albuquerque, New Mexico — Gates was just 19 years old and took a 64% ownership stake, with Allen owning the remaining 36%. Gates had left Harvard University to do it.

IBM's fateful knock on Microsoft's door

By 1980, Microsoft was a modestly successful BASIC-language shop. Then IBM came calling. During the summer of 1980, IBM was originally interested in licensing the popular CP/M operating system, but the inability to come to an agreement with Digital Research led IBM to ask Microsoft if they could develop something similar. Microsoft was already going to work with IBM on BASIC, but they did not have an operating system.

Microsoft knew that a small company, Seattle Computer Products, had developed an operating system called QDOS (Quick-and-Dirty Operating System). Microsoft suggested to IBM that QDOS could work as the IBM PC's operating system, and IBM asked Microsoft to license and further develop it — leading to a formal contract on November 6, 1980. It was just two weeks before the IBM PC launched that Microsoft acquired full rights to 86-DOS, renaming it MS-DOS, which it licensed to IBM to be sold under the name PC DOS.

The clause IBM's team failed to notice

This is where the story becomes one of history's most consequential contract negotiations. Although IBM was under the impression that it had sole rights, Microsoft had included a clause that allowed the firm to sell the operating system to other companies. Microsoft did not sell IBM an exclusive license to the operating system. Microsoft's agreement with IBM actually prohibited IBM from licensing DOS, but placed no restrictions on Microsoft itself. This enabled Microsoft to license the operating system generically as "MS-DOS" to clone PC manufacturers. In the months that followed, other PC manufacturers, especially those from Japan, approached Gates about using DOS — and thanks to the terms of the deal, Microsoft was free to sublicense DOS to them.

Within a year, Microsoft had licensed MS-DOS to 70 other companies, many of whom had already begun cloning IBM's hardware platform. IBM had paid roughly $50,000 for what turned into the software foundation of the entire PC industry — and Microsoft kept the rights to sell it to everyone else.

How IBM accidentally built the clone market

IBM's second fateful mistake was architectural. A defining characteristic of IBM's PC was its open architecture and reliance on off-the-shelf components. IBM published extensive technical documentation, which allowed third-party developers to create compatible hardware and software — from expansion cards to complete clone systems. IBM assumed its brand name would be a sufficient moat. IBM had published the bus and BIOS specifications, wrongly assuming that this would not be enough to facilitate unlicensed copying of the design, but enough to encourage the add-on market.

Compaq successfully reverse-engineered the IBM PC's BIOS through a legally clean process in 1982, enabling fully compatible computers without copying IBM's protected code. The copyrighted firmware BIOS was legally duplicated in function by several companies, starting with Compaq in 1982, and Phoenix Technologies in 1984 made it available for sale to any manufacturer. IBM effectively surrendered control over the direction of the PC market: clone manufacturers undercut prices, software dominance shifted away from hardware ownership, and IBM itself would eventually lose its central role in personal computing. Every clone needed to license MS-DOS from Microsoft. IBM had inadvertently turned Microsoft into the unavoidable gatekeeper of the entire IBM-compatible world.

The self-reinforcing network effect that locked Windows in

Once MS-DOS — and later Windows — became the standard on the majority of the world's PCs, a powerful feedback loop took hold. In the operating system market, clusters of software applications grew up around the dominant operating system, and the gravity pull of that dominant platform became so intense that independent software vendors had little choice but to write to the APIs of the platform on which nearly all software would run.

Microsoft's own internal documents, surfaced during its 1990s antitrust trial, described the dynamic bluntly. Microsoft needed a "significant user installed base" to attract developers to Windows. Without that: "The industry would simply ignore our standards. Few would write Windows apps without the Windows user base." The logic runs in both directions: users go where the software is, and developers write for where the users are. The most obvious way Microsoft leveraged this was the very nature of software and the need for compatibility — users sharing files are better off on the same format, and businesses are incentivized to standardize on compatible operating systems within office environments.

The result has proved remarkably durable. For desktop systems, as of late 2025, Windows holds approximately 72% of the global desktop OS market share — running on 1.6 billion active devices. macOS, Linux, and ChromeOS collectively share the rest. Microsoft faced antitrust action from the U.S. Department of Justice in the late 1990s for anti-competitive business practices, ultimately reaching a settlement in 2001. Even that did not break the cycle: switching costs, application compatibility, enterprise IT infrastructure, and developer habit had already made Windows the path of least resistance for the vast majority of PC buyers.

What the story really illustrates

Microsoft's rise is a masterclass in identifying where value actually sits in a technology stack. IBM separated "the hardware" from "the standard," and by documenting interfaces and relying on a third-party OS, IBM helped create a stack where the most valuable control points shifted away from IBM's own manufacturing. Gates understood — almost uniquely at the time — that software licensing, not hardware sales, was where durable wealth would be created. A single non-exclusive clause in a contract, and a competitor's decision to publish its technical specifications, were enough to redirect the entire trajectory of personal computing.

This answer is for informational and educational purposes only. Business history involves contested accounts and incomplete records; details may vary across sources. Nothing here constitutes business, legal, or investment advice. Verify facts against primary sources and official histories for research purposes.

References

• 1. History of Microsoft
• 2. Microsoft, global leader in technology, founded in 1975 by Bill Gates, Paul Allen, originally $21 per share
• 3. History of Microsoft
• 4. Microsoft's Journey to Becoming the World's Most Valuable Company
• 5. Timeline of Microsoft
• 6. Founder Story: Bill Gates of Microsoft
• 7. This Day in History (4-Apr-1975)
• 8. Microsoft Founders: Bill Gates and Paul Allen (1975)
• 9. 0% found this document useful (0 votes)
• 10. The DOS Story Part II. Microsoft’s first meeting with IBM…