Mixed-mode operation is a term that refers to a programming feature known as the domain controller. The function of the domain controller is to provide directory-like and administrative functions, such as password administration, to the operating environment. Additionally, in mixed-mode operation, the domain controller is able to run two operating systems in the same domain. Features from the operating systems are enabled or disabled so as to make the two operating systems compatible and, as much as possible, invisible to the user.

Mixed-mode operation allows files that were created and stored in one operating system to be read and manipulated in a normal fashion in the other operating system. In essence, the systems become transparent.

The ability to operate in mixed-mode is a ubiquitous feature of operating systems. For example, mixed-mode enables controller servers running on Microsoft Windows NT 4.0 and servers responsible for message functions running on Windows 2000 to communicate seamlessly with one another. As another example, the mixed-mode manager in the Macintosh operating system is designed to both permit the running of two operating systems and to hide, as much as possible, these dual operating environments from the user.