Did Microsoft Steal MS-DOS?

The history of the computer industry is filled with fascinating tales of riches that appear to practically fall from the sky.

Along with stories of riches won, there are stories of opportunities missed. Take that of Ronald Wayne, who cofounded Apple Computer with Steve Wozniak and Steve Jobs but sold his shares for just US $2300. And John Atanasoff, who proudly showed his digital computer design to John Mauchly—who later codesigned the Eniac, often defined as the first electronic computer, without credit to Atanasoff.

But by far the most famous story of missed fame and fortune is that of Gary Kildall. A pioneer in computer operating systems, Kildall wrote Control Program for Microcomputers (CP/M), the operating system used on many of the early hobbyist personal computers, such as the MITS Altair 8800, the IMSAI 8080, and the Osborne 1, before IBM introduced its own machine, the PC. Kildall could have virtually owned the personal computer operating system business, had he sold that system to IBM. He didn’t. Why is a matter of speculation, mundane gossip, and urban legend?

What Is MS-DOS?

Whether you realize it or not, you’ve probably used MS-DOS. Short for “Microsoft Disc Operation System,” MS-DOS was the code almost every version of Windows has used “under the hood,” so to speak. It wasn’t until Windows XP that Microsoft transitioned away from using DOS, and it’s not a stretch to say MS-DOS is the software that built Microsoft. To prove it was stolen would probably shut down the company.

The basic crux of the legend is that MS-DOS was somehow a rip-off of the operating system CP/M, and that Microsoft essentially pulled off a massive scam. But why would anybody believe this? The answer has, ironically, nothing to do with Microsoft, but rather how MS-DOS came to be, as a stopgap solution from a tiny computer company.

How Microsoft Built MS-DOS

MS-DOS, despite the name, wasn’t made by Microsoft originally. It was engineered by Tim Paterson of Seattle Computer Products, (SCP), a computer company in the Pacific Northwest. In the late ’70s, the idea of buying a pre-built computer was unusual and exotic, and it was a tiny market. SCP’s computers weren’t selling because they didn’t have an operating system, so it decided to make one. At the time, the most popular system was CP/M, short for “Control Program/Monitor.” CP/M, however, wasn’t designed for the type of processor SCP used in their computers, much like even today you can’t use an Android app on an iPhone and vice versa. So SCP employee Tim Paterson wrote, from the ground up, Quick And Dirty Operating System, or Q-DOS.

That would likely have been the end of it if it hadn’t been for Bill Gates. Gates realized that Q-DOS, unlike CP/M, worked on IBM processors, and IBM was trying to break into the consumer market. So they bought the rights to Q-DOS and hired Paterson to update it to run on IBM’s computers. IBM, as we all know, began building the PC industry, putting Microsoft on the ground floor of one of the greatest industrial revolutions of all time. Or, at least, that’s the official version.

Did Microsoft Steal MS-DOS?

The urban legend goes that in reality, there’s more of someone else’s work in MS-DOS than Microsoft cares to admit. The rumors range from Paterson using a manual as a programming guide to his outright rewriting the code and passing it off as his own work. One rumor even claimed that CP/M mastermind Gary Kildall hid a “signature” of sorts, a command popularly called the “Kildall command” that when punched into his code reeled off a message that named him specifically as the copyright holder.

The charge of outright plagiarism has repeatedly been proven false. Most recently, in 2012, Bob Zeidman, an expert in software and intellectual property, ran MS-DOS and CP/M through forensic software tools and found that there wasn’t any copying of the source code. That said, Zeidman found new code from CP/M recently, compared it to MS-DOS, and found something fascinating.

It turns out that MS-DOS and CP/M use the same “system calls,” pieces of code that talk to things like printers and modems, and those calls even have the same number. It’s enough of a similarity that Zeidman has put up $200,000 in prize money to whoever can prove MS-DOS was plagiarized. But is it?

It seems unlikely. As we’ve noted, brilliant forensic minds and hobbyists alike have tackled this question and haven’t found a scrap of evidence. And there are some crucial technical differences between even Q-DOS and its supposed inspiration that simply would have to exist; otherwise, Q-DOS would never have worked in the first place. Similarly, despite nearly four decades of searching, nobody’s found the Kildall Command.

One of the fundamental problems in software is that you can only solve the same problem so many ways. But as long as there’s somebody with a grudge against Microsoft, there’s likely going to be someone willing to believe it’s built on a stolen product.