Extension (Mac OS)

On the Apple Macintosh operating system prior to OS X, extensions were small pieces of code that extended the system's functionality. They were run initially at start-up time, and operated by a variety of mechanisms, including trap patching and other code modifying techniques. Initially an Apple developer hack, extensions became the standard way to provide a modular operating system. Large amounts of important system services such as the TCP/IP network stacks (MacTCP and Open Transport) and USB and FireWire support were optional components implemented as extensions. The phrase "system extension" later came to encompass faceless background applications as well.