The AARD code was a segment of obfuscated machine code that was included in several executables, including the installer and WIN.COM, in a beta release of Microsoft Windows 3.1. It was XOR-encrypted, self-modifying, and deliberately obfuscated, using a variety of undocumented DOS structures and functions to determine whether or not Windows really was running on MS-DOS. It was originally discovered by Geoff Chappell on 17 April 1992 and then further analyzed and documented in a joint effort with Andrew Schulman.[1][2][3][4]
The AARD code ran several functional tests on the underlying DOS that succeeded on MS-DOS and PC-DOS, but resulted in a technical support message on competing disk operating systems such as DR-DOS. 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.)[5][6] 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".
Digital Research released a patch to enable the AARD tests to pass on DR-DOS in 1992.[7]
The rationale for the AARD code came to light when internal memos were released during the United States Microsoft antitrust case. 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 [sic] feature they have that might get in our way?"[8] 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."[8]
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."[8]
What had been DR-DOS changed hands again. The new owner, Caldera, Inc., began a lawsuit against Microsoft over the AARD code, Caldera vs. Microsoft, which was later settled.[8][9] It was believed that the settlement ran in the order of $150m,[10] but was revealed in November 2009 with the release of the Settlement Agreement to be $280m.[11]