The AARD code was a segment of code in a beta release of Microsoft Windows 3.1 that would determine whether Windows was running on MS-DOS or PC DOS, rather than a competing workalike such as DR-DOS, and would result in a cryptic error message in the latter case. This XOR-encrypted, self-modifying, and deliberately obfuscated machine code used a variety of undocumented DOS structures and functions to perform its work, and appeared in the installer, WIN.COM and several other executables in the OS.
The AARD code was originally discovered by Geoff Chappell on 17 April 1992 and then further analyzed and documented in a joint effort with Andrew Schulman. The name was derived from Microsoft programmer Aaron R. Reynolds (1955–2008), who used "AARD" to sign his work; "AARD" was found in the machine code of the installer. Microsoft disabled the AARD code for the final release of Windows 3.1, but did not remove it, so that it could have become reactivated anytime later by the change of a single byte in an installed system, thereby constituting a "smoking gun".
The rationale for the AARD code came to light when internal memos were released during the United States v. Microsoft antitrust case in 1999. Internal memos released by Microsoft revealed that the specific focus of these tests was DR-DOS. At one point, Microsoft CEO Bill Gates sent a memo to a number of employees, reading "You never sent me a response on the question of what things an app would do that would make it run with MSDOS and not run with DR-DOS. Is there feature [sic] they have that might get in our way?" Microsoft Senior Vice President Brad Silverberg later sent another memo, stating: "What the [user] is supposed to do is feel uncomfortable, and when he has bugs, suspect that the problem is DR-DOS and then go out to buy MS-DOS."
Following the purchase of DR-DOS by Novell and its renaming to "Novell DOS", Microsoft Co-President Jim Allchin stated in a memo, "If you're going to kill someone there isn't much reason to get all worked up about it and angry. Any discussions beforehand are a waste of time. We need to smile at Novell while we pull the trigger."
What had been DR-DOS changed hands again. The new owner, Caldera, Inc., began a lawsuit against Microsoft over the AARD code, Caldera v. Microsoft, which was later settled. It was believed that the settlement ran in the order of $150 million, but was revealed in November 2009 with the release of the Settlement Agreement to be $280 million.
- Geoff Chappell. Record of AARD Research. Web article published by Geoff Chappell on 3 September 1999 about an e-mail sent to Andrew Schulman on 17 April 1992 ().
- Geoff Chappell. "First Public AARD Details". Archived from the original on 2 April 2013.
- Schulman, Andrew (September 1993). "Examining the Windows AARD Detection Code - A serious message--and the code that produced it". Dr. Dobbs Journal. Archived from the original on 2005-12-10. Retrieved 2013-10-05.
- Schulman, Andrew; Brown, Ralf; Maxey, David; Michels, Raymond J.; Kyle, Jim (1994). Undocumented DOS - A programmer's guide to reserved MS-DOS functions and data structures - expanded to include MS-DOS 6, Novell DOS and Windows 3.1 (2 ed.). Addison Wesley. ISBN 0-201-63287-X. ISBN 978-0-201-63287-3.
- Dellert, Brian (21 October 1998). "Microsoft Plays Hardball". Eat the State!. 3 (7). Archived from the original on 20 April 2008. Retrieved 2008-08-21.
- Pournelle, Jerry (2000-04-01). "The Microsoft Monopoly Debates". Archived from the original on 29 August 2008. Retrieved 2008-08-21.
- wired.com: "How a Crypto ‘Backdoor’ Pitted the Tech World Against the NSA" (Zetter) 24 Sep 2013
- Paul, Matthias (2000-09-18). "25 years of DR DOS history - Digital Research DOS history". FreeDOS.org. Retrieved 2013-10-05.; see footnote #19 (BDOS 1067h "DR DOS 6.0 Windows 3.1 update, April 1992"; 1992-03, 1992-04-07: "This public DR DOS 6.0 update only includes patches addressing full Windows 3.1 compatiblity [sic]. There should have been a full "business update" for registered users, shipping a little bit later."), #27 (BDOS 1072h "Novell DOS 7 Panther/Smirnoff BETA 3", 1993-09: "This issue does not have workarounds for Windows 3.1 AARD code."), #29 (BDOS 1072h "Novell DOS 7 German release"; 1994-02-22: "This issue is known to have workarounds for Windows 3.1 AARD code. This should also apply to the earlier English issue.")
- Susman, Stephen D.; Eskridge III, Charles R.; Southwick, James T.; Susman, Harry P.; Folse III, Parker C.; Palumbo, Ralph H.; Harris, Matt; McCune, Phil; Engel, Lynn M.; Hill, Stephen J.; Tibbitts, Ryan E. (April 1999). "In the United States District Court - District of Utah, Central Division - Caldera, Inc. vs. Microsoft Corporation - Consolidated statement of facts in support of its responses to motions for summary judgement by Microsoft Corporation - Case No. 2:96CV 0645B" (Court document). Caldera Inc. Archived from the original on 1999. Retrieved 2013-05-12.
- Lea, Graham (1999-11-05). "How MS played the incompatibility card against DR-DOS - Real bear-traps, and spurious errors". The Register. Retrieved 2013-09-26.
- Goodin, Dan (1999-04-28). "Microsoft emails focus on DR-DOS threat". CNET News. Retrieved 2008-08-21.
- Susman, Stephen D.; Eskridge III, Charles R.; Susman, Harry P.; Southwick, James T.; Folse III, Parker C.; Borchers, Timothy K.; Palumbo, Ralph H.; Harris, Matthew R.; Engel, Lynn M.; McCune, Philip S.; Locker, Lawrence C.; Wheeler, Max D.; Hill, Stephen J.; Tibbitts, Ryan E. (May 1999). "In the United States District Court - District of Utah, Central Division - Caldera, Inc. vs. Microsoft Corporation - Case No. 2:96CV 0645B - Caldera, Inc.'s Memorandum in opposition to defendant's motion for partial Summary Judgment on plaintiff's "Technological Tying" claim" (Court document). Caldera Inc. Archived from the original on May 1999. Retrieved 2013-10-05.
- Ball, Lyle (1999-04-28). "Caldera submits evidence to counter Microsoft's motions for partial summary judgment" (Press release). Caldera, Inc. Archived from the original on May 1999. Retrieved 2013-10-05.
- Lea, Graham (2000-01-13). "Caldera vs Microsoft - the settlement". BBC News. Archived from the original on 5 October 2008. Retrieved 2008-08-21.
- Wilcox, Joe (2000-01-11). "Caldera settlement shows a new side of Microsoft". cnet. Retrieved 2009-01-19.
- "Exhibits to Microsoft's Cross Motion for Summary Judgment in Novell WordPerfect Case". Groklaw. 2009-11-23. Retrieved 2011-10-22.
- AARDvarks in your code
- So why didn't the Windows guys just remove the AARD code from the system?
- Details and initial discovery (archived site)
- Caldera v. Microsoft details
- Site with email excerpts from Microsoft and an example of tripping the AARD code (XMS error)