Was DOS copied from CP/M?

August 06, 2016

zeidman-August 06, 2016

Was There Copyright Infringement?
Here are my conclusions about copying. And because many people are interested in whether DRI could have brought a copyright lawsuit against Microsoft, I will tie in my conclusions with that possibility. Keep in mind that while I have extensive experience in copyright law, I’m not a lawyer and the law is constantly changing.

Source Code
There was no copied source code and so there was no copyright infringement of the code.

Commands were not copied, but even so, the commands are no copyrightable because they are simple and descriptive of the functionality. Only creative expression that is not simple description and not functional can be copyrighted.

System Calls
The system call numbers were copied. While a list of numbers is not by itself creative and thus not copyrightable, a list of numbers that arbitrarily represent specific functions is creative and thus copyrightable. Furthermore, DRI appears to have indicated its copyright by putting a copyright notice on the CP/M Interface Guide[14] that describes the system calls.

On the other hand, Microsoft could have prevailed by showing that it was a fair use to copy the system calls. According to copyright law, fair use is determined by the following factors[15]:

  1. the purpose and character of the use, including whether such use is for nonprofit educational purposes;
  2. the nature of the copyrighted work, especially whether it benefits the public;
  3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
  4. the effect of the use upon the potential market for or value of the copyrighted work.

It’s clear that the copying would not pass the first two factors. DOS was a commercial product sold at a profit and it would be hard to argue that the copying served a public benefit. Therefore to defeat a copyright infringement charge, Microsoft would have had to show that the amount of copyrighted material copied into DOS was minimal and that copying the CP/M system calls did not, by itself, cause DRI any financial harm.

I believe that DRI could have brought a legitimate copyright claim against Microsoft and that Microsoft would have had a good chance of avoiding liability by claiming a fair use defense.

The Zeidman Challenges
I’m confident in my conclusion, so I’ve decided to offer two cash rewards to back it up. The first Zeidman Challenge is an offer of $100,000 reward to anyone who can use accepted forensic techniques to prove that Microsoft copied MS-DOS source code from DRI’s CP/M source code. The second Zeidman Challenge is an offer of $100,000 reward to anyone who can demonstrate or find source code for a secret function in MS-DOS that prints Gary Kildall’s name or a copyright notice for DRI, as was claimed by science fiction author and computer pundit Jerry Pournelle to John C. Dvorak on the podcast This Week in Tech (TWiT) on October 15, 2006[16]. The award details and specific criteria will be announced shortly.


  1. Steve Hamm and Jay Greene, “The Man Who Could Have Been Bill Gates,” BusinessWeek, October 25, 2004,
  2. Computer History Museum, What Was The First PC?, retrieved 8/7/2013
  3. James Wallace and Jim Erickson, 1992. Hard Drive, John Wiley & Sons. ISBN 0-471-56886-4
  4. Mary Bellins, “Putting Microsoft on the Map: History of the MS-DOS Operating Systems, IBM & Microsoft,” com Guide,, retrieved 12/5/2011
  5. Clive Akass, “The man who gave Bill Gates the world,” Computeract!ve, 11/8/2006,
  6. Tony Smith, “Microsoft's MS-DOS is 30 today,” The Register, July 27, 2011,
  7. Mat Honan, “Bill Gates Spent the Best Money of His Life 30 Years Ago Today,” Gizmodo,, July 27, 2011
  8. Doug Conner, “Father of DOS Still Having Fun at Microsoft,” MicroNews, 4/10/98,
  9. Bob Zeidman, “A Code Correlation Comparison of the DOS and CP/M Operating Systems” Journal of Software Engineering and Applications,, May 27, 2014.
  10. Bob Zeidman, “Did Bill Gates Steal the Heart of DOS?” IEEE Spectrum,, July 2012.
  11. Bob Zeidman, The Software IP Detective’s Handbook, Prentice Hall; May 8, 2011.
  12. Babcock, Charles, “VMS Operating System Is 30 Years Old; Customers Believe It Can Last Forever,” InformationWeek, November 11, 2007,
  13. Roberge, D.G., “DOS 3.1,” everything2,, retrieved August 1, 2016.
  14. CP/M Interface Guide, © Digital Research 1976, 1978.
  15. Tysver, Daniel A., “Fair Use In Copyright Law,” Bitlaw,, retrieved August 1, 2016.
  16. Podcast This Week in Tech (TWiT), Episode 73: Hello Armenia, October 15, 2006,, originally at, retrieved 12/10/2011.

Bob Zeidman is considered a pioneer in the fields of analyzing and synthesizing software source code. He is the president and founder of Zeidman Consulting, a premier contract research and development firm in Silicon Valley that provides engineering consulting to law firms regarding intellectual property disputes, and he is the president and founder of Software Analysis and Forensic Engineering Corporation, the leading provider of software intellectual property analysis tools, having pioneered the field. His book The Software IP Detective’s Handbook is one of the main books for engineers and lawyers on software intellectual property

Editor's note: Updated for corrections on 7 Aug 2016.

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