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When Apple designed the Mac OS back in the 1980's, it was a very stable system. My Mac Plus running System 6 virtually never crashed. However, as the Mac OS evolved, new functions and features were added on by way of system extensions -- patches of computer code that modified and enhanced System or application functions. Unfortunately, these little guys don't always peacefully coexist.

If you are running System 7 or later versions of the classic Mac OS, you will have almost certainly experienced crashes, lockups, and freezes from time to time. Not all of these are caused by extension conflicts, and in fact most are not. Running out of Finder memory (which can and does happen even if you have plenty of RAM installed) is a prime culprit, as are memory collisions between programs.

However, if you begin having a lot of crashes, especially after installing a new system version upgrade or a new software application version, (third-party software now frequently installs System extensions), extension conflicts may be to blame.

Fortunately, there is a tool in the Mac OS that can help you track down extension conflicts, and manage extensions in general. It's in the Control Panels Folder and called Extensions Manager. You can open it from the Apple Menu like any other control panel, or at startup by holding down the while you boot your Mac. In the latter case, handy if your Mac is crashing at startup, Extensions Manager will appear after the startup screen is displayed. The spacebar at start up method is preferable because in that mode you will also get a menu that adds a number of additional features to Extensions Manager, including a File menu with commands that let you duplicate, rename, delete, or create a text report about extension sets you create (see below); an Edit menu with a Find Item command that takes you to the actual icon in your System Folder; and a View menu that lets you rearrange the list of control panels and extensions, making packages or clusters of them alphabetically.

In the Extensions Manager window you will see a list of control panels, followed by a list of system extensions, shutdown items, and startup items. Items that are enabled will have an "X" in the check box to the left of their name. Unchecked items will be ignored by the Mac OS at startup.

There is also a pull-down menu that allows you to select from several choices of startup configurations, including Mac OS all, Mac OS basic, and also a extension sets that you have created yourself. If you are having problems at startup, your first diagnostic step using Extensions Manager should be to select Mac OS All, and try a restart. Only the extensions and control panels that came with your Mac OS will load at startup, and if the Mac boots up happily and runs smoothly, your problem may have been with a third party system extension.

However, now any third-party software you have that requires system extensions will not work. In order to restore functionality and isolate the culprit that had been causing the problem, proceed as follows:

1. Open your Mac's System Folder (in the hard drive window), find the extensions and control panels folders, open them respectively, select all items (Command > A), and select Label from the File Menu. Apply a bright color (e.g.: red) to all the items in these folders.

2. Now restart and open Extensions Manager again. All the items in the basic Mac OS extensions and control panel sets should be highlighted in the color you chose, which will be helpful in distinguishing basic extensions and control panels from third party ones.

3. Click the "Duplicate Set" button and and create a new custom set with a name like "My Custom Set One" or whatever you like.

4. Select "By Package" from the View menu (provided that you opened the Extensions Manager by holding the spacebar down on startup). That will sort extensions and control panels in groups according to the program that they are associated with.

5. There are now several ways to proceed, but the one I recommend is to enable the packages of extensions and control panels one application at a time, restarting with each one, to see if things are still working properly.

6. If an extension conflict was the problem, it should reappear after enabling a particular package of extensions. Disable that package began, and restart to double-check. If all is now well again, you can be reasonably sure that these extensions were the ones doing in dirt.

The reason could be that the third-party extensions are incompatible with the Mac OS version you're using, especially if you have just upgraded to a new Mac OS version; possibly they are just buggy, or perhaps corrupted.

Check the Read Me document for the problematic application for compatibility notes, and you could also try trashing the troublesome extensions and reinstallng the program (or just the extensions in a custom install).

If that doesn't work, you may have to settle for doing without that piece of software, or upgrading to a later version.

I should also note here that while the procedure I've described will cover most extension conflicts, there are other potential issues such as extension load order or the possibility that two third-party applications will work fine in isolation from one another, but not with their respective extensions enabled together. However these are beyond the scope of a basic tutorial for beginners.

System extensions are a thing of the past with OS X, which is no doubt a good thing.