Extension (Mac OS)

In the classic Mac OS (versions 1 through 9), an extension was a small piece of software that added functionality to the operating system. Extensions loaded during the boot process and modified the system's behavior. They could perform a wide variety of tasks, such as adding support for new hardware, modifying the user interface, or implementing new system services.

Extensions were stored in the System Folder, typically in a subfolder specifically named "Extensions". The order in which extensions loaded was determined alphabetically, which could sometimes lead to conflicts between extensions attempting to modify the same system resources. This was a common source of system instability and crashes, often referred to as "extension conflicts."

Control Panels were often used to manage extensions, allowing users to enable or disable them. This was a common troubleshooting technique when experiencing system problems; disabling extensions one by one (or in groups) could help identify the conflicting extension.

Apple introduced the Extensions Manager utility to streamline the process of enabling and disabling extensions and control panels. This utility also allowed users to create sets of extensions for different purposes.

The concept of extensions as described above ceased to exist with the introduction of Mac OS X (now macOS), which uses a more robust and stable system for adding functionality, relying on frameworks, kernel extensions (kexts), and applications rather than the more loosely managed extension model of classic Mac OS. Kernel extensions require significantly more security clearance from the OS.