From https://en.wikipedia.org/wiki/DR-DOS

DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-DOS (which were the same product sold under different names).

DR-DOS was developed by Gary A. Kildall's Digital Research and derived from Concurrent PC DOS 6.0, which was an advanced successor of CP/M-86. As ownership changed, various later versions were produced with names including Novell DOS and Caldera OpenDOS.

Though DR-DOS was a descendant of CP/M (and indeed MS-DOS started life as QDOS, a clone of CP/M), achieving full compatibility with MS-DOS in the late eighties was nontrivial; applications depended on all sorts of quirks of the system. Some of these were documented in a book called Undocumented DOS, but this was only published in 1990.

It seems to me therefore that DR, to achieve full compatibility without legal vulnerability, must have used clean room reverse engineering: one team disassembles MS-DOS and fully documents their findings, another never looks at the MS-DOS code, but writes DR-DOS based on the documentation.

(This would be similar to the way IBM-compatible BIOSes were written, but with the difference that IBM published the source code for their BIOS, so in that case, the first team didn't need to use a disassembler.)

Are there historical records confirming that this was done, and giving any details of the process, like how long it took, and who worked on each team?

  • 7
    Considering that quite a number of commands and principles MSDOS used were originally been copied from CP/M, any copyright claims Microsoft could have set up against DR would likely have been considered a "good joke".
    – tofro
    Commented Feb 28, 2023 at 17:20
  • 2
    @wizzwizz4 Historically, This was the same kind of evidence that forced MS to settle a copyright lawsuit from DEC over Windows’(based on the similarity of some internal data structures to an OS the lead designer had previously worked on for DEC). however, in some other cases, judges ruled that IP law couldn’t be used to prevent reverse-engineering.
    – Davislor
    Commented Feb 28, 2023 at 17:49
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    @tofro Interestingly, this was adjudicated in court, in a way. Tim Paterson of Seattle Computer Products sued the author of a book for claiming that he "[took] 'a ride on' [Gary] Kildall's operating system, appropriated the 'look and feel' of [Kildall's] CP/M operating system, and copied much of his operating system interface from CP/M." The judge ruled in the author’s favor, saying, the evidence “provides a research picture tellingly close to the substance of the final chapter.” This doesn’t mean DR would’ve won a suit.
    – Davislor
    Commented Feb 28, 2023 at 17:55
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    It is telling that, when Caldera sued MS in an antitrust lawsuit for allegedly trying to monopolize the OS market, MS did not countersue and allege any infringement by DR-DOS against MS-DOS.
    – Davislor
    Commented Feb 28, 2023 at 18:28
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    @Davislor: Microsoft's settlement with DEC had a little more basis: it wasn't just similarity of "some internal data structures". Rather, much of the core VMS team outright moved from DEC to MS. Internal similarities were strong enough that when Windows NT first came out, if you wanted to write device drivers, you generally did it working from manuals that said "DEC VMS" on the covers. DEC's programmer's reference manuals described the internals of Windows more accurately and (especially) completely than anything available from Microsoft at the time. Commented Feb 28, 2023 at 19:01

1 Answer 1


Short Answer

Sort of. Digital Research had two teams, in different countries, that developed the product that became DR-DOS in two stages. The company that purchased DR-DOS only claimed in court that the second team had done no reverse-engineering of MS-DOS. However, it also said that there was code written by the first team in DR-DOS, so the final product was not entirely the work of the clean room.

Long Answer

The DR side of the story came out in Caldera v. Microsoft. That was an interesting story in itself: Novell purchased Digital Research in 1991, and then, according to the judgment in a lawsuit Novell filed against Caldera,

Novell owned the source code for DR DOS, a computer operating system that was the target of anticompetitive practices by Microsoft in the early 1990s. Novell's board of directors worried that, if they brought suit against Microsoft in a private antitrust action, Microsoft would retaliate with further unfair practices that could neutralize the value of any antitrust recovery. At the same time, however, Novell's board of directors recognized that Novell's shareholders would not permit Novell to simply walk away from such a significant cause of action and potential recovery.

Accordingly, Novell entered into negotiations with Caldera, Inc., the predecessor in interest to Canopy, to sell DR DOS to Canopy. The main purposes of this sale were to obligate Canopy to bring suit against Microsoft, to allow Novell to share in the recovery, and at the same time to obfuscate Novell's role in the action against Microsoft. Novell insisted that its role be completely undetectable to avoid retaliation from Microsoft.

To accomplish this, Novell and Canopy executed two separate documents: the first was a contract of sale, obligating Canopy to pay $400,000 for rights to the source code; the second was a temporary license obligating Canopy to pay $600,000 in license fees and "royalties." The royalties included provisions for payment to Novell of a percentage of any recoveries from lawsuits.

This all came out when Novell sued Caldera (by then part of the Canopy Group) for not giving them a big enough cut of their legal settlement it received from Microsoft. Novell would later file its own lawsuit against Microsoft, alleging antitrust violations against its suite of office software.

But in that Caldera v. Microsoft lawsuit, which alleged that Microsoft broke the law to stop DR-DOS, the former boss of Digital Research gave its version of the history of DR-DOS. (Emphasis added.)

A dynamic new leader, Dick Williams, took the helm DRI on January 5, 1987. Prior to Williams’ arrival, DRI had been approached by several OEMs who, unhappy with MS-DOS and Microsoft’s control of the operating system market, had urged DRI to develop a competing product. Within 30 days of taking control of the company, Williams had begun the development of DR DOS.

John Constant, a DRI engineer in Hungerford, England, began development of DR DOS using an existing DRI product, Concurrent DOS. Constant did no “reverse engineering” of MS-DOS. Rather, he built upon the DOS compatibility DRI had already developed in Concurrent DOS.

In other documents it filed in the suit, Caldera provided some more detail. (The same document has some information relevant to your other recent question about sales of DR-DOS in Japan.)

Although development of DR DOS was undertaken at DRI’s European Development Center (the “EDC”) in Hungerford, England, significant portions of the code included in the initial version of DR DOS came directly from Concurrent DOS, which was developed by DRI engineers at DRI’s facilities in Monterey, California. EDC engineers developed DR DOS for DRI as a work-for-hire, and DRI has at all times owned the product. [...] (“the EDC was the only facility outside of the United States that ever [(illegible)]-aged in software development [for DRI] other than for product translation”)

Notably, Caldera’s lawyers never claim that Digital Research’s Concurrent DOS project had not reverse-engineered MS-DOS, only that John Constant did not do so on when turning their code into DR-DOS, in his separate room on another continent. The lawsuit is, in fact, largely about undocumented interfaces and data structures needed for 100% compatibility, which it would have been impossible to duplicate without reverse-engineering. What keeps this from being a clean-room implementation is that Caldera admits DR-DOS incorporates some code written in Monterey.

However, just as significantly, Microsoft did not countersue or raise a copyright claim against DR-DOS. Indeed, when it came out, Microsoft attempted to purchase it.

The lawsuit also revealed, among other things, that Steve Ballmer of Microsoft approached Digital Research in December 1988. “Steve Ballmer had a promising discussion with DRI re: them licensing MS-DOS vs. competing with us.” Joachim Kempin, MS’ Microsoft’s head of OEM sales, testified that Microsoft wanted to license DR-DOS, and give DR a license to distribute the new MS-DOS. Caldera claimed that DR did not want an ongoing relationship with MS, asked for a one-time cash payment instead, and MS refused.

  • So Microsoft wanted to cross-license operating systems with DR? Curious! Does it say why Microsoft wanted to do this?
    – rwallace
    Commented Feb 28, 2023 at 20:26
  • 1
    @rwallace Caldera alleged that Microsoft wanted to buy the superior technology of DR-DOS.
    – Davislor
    Commented Feb 28, 2023 at 20:27
  • 1
    @rwallace Caldera claimed in its lawsuit, ‘In essence, Microsoft was “trying to buy technology” it did not have.’
    – Davislor
    Commented Feb 28, 2023 at 20:29

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