Extension conflicts were sometimes a problem on Apple Macintosh computers running the classic Mac OS, especially System 7. Extensions were bundles of code that extended the operating system's capabilities by directly patching OS calls, thus receiving control instead of the operating system when applications made system calls. Generally, once an extension completed its task, it was supposed to pass on the system call to the operating system's routine. If multiple extensions want to patch the same system call, they end up receiving the call in a chain, the first extension in line passing it on to the next, and so on, until the last extension passes to the operating system. If an extension does not hand the next extension in line what it is expecting, problems occur; ranging from unexpected behavior to full system crashes. This is triggered by several factors such as carelessly programmed and malicious extensions that change or disrupts the way part of the system software works. In addition, extensions sometimes competed for system resources with applications, utilities and other extensions, leading to crashes and general instability. Many users happily loaded every extension they could find on their computer, with little or no impact. Others fastidiously avoided any non-essential extensions as a way of avoiding the problem. Others were judicious in the addition of extensions. This problem increased during the mid-1990s as resource-hungry multimedia technologies such as QuickTime were installed as extensions. In addition, a number of applications, especially Microsoft Office, required a large number of extensions. Many Macintosh users had hundreds of extensions running on their systems, all of varying age and quality. Buggy, damaged and outdated extensions were the most common cause of problems. Some users had to remember to turn off problematic extensions when running certain programs. Later versions of System 7 included the Extensions Manager, which allowed users to turn off specific extensions or groups of extensions at startup when troubleshooting the conflict by pressing the Spacebar while the computer boots. This tool was also accessible by heading to the Extensions panel in the Control Panels found in the Apple menu. Conflict Catcher, a third party utility, automatically detected conflicts and problematic extensions, otherwise a time-consuming task that required users to turn off extensions in sets until they found the conflict. Extensions were only loaded at startup time, meaning that any attempted change required a reboot. The most common time for extension conflicts to start was the release of a new version of the operating system, followed closely by the installation of a new application that either conflicted with existing extensions, or installed extensions that conflicted with the existing set. All of this was mitigated by the ease with which extensions and the operating system itself could be swapped in and out: Instead of modifying configuration files as on other operating systems, extensions simply had to reside in a particular folder to be picked up. In addition, the Mac was perfectly happy to have two system folders present on a hard drive. Only the "blessed" system folder would be loaded at startup. So, when a new version of the operating system was to be installed, or a new application, the user could easily duplicate the system folder, perform the install, and then fall back if a problem resulted. Extension conflicts came to an end with the release of Mac OS X, which uses a different extension mechanism. Software can still add new features to the Mac but instead of extensions, the new method entailed writing startup applications, which is considered safer and more organized approach that cannot destabilize the machine.
