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All Classic Mac OS versions beginning with System 7.5 have had a handy control panel utility called Extensions Manager. System extensions are small software patches that piggy-back the Mac OS, enabling extra functionality and/or supporting third-party applications and peripherals. They are also arguably the most troublesome aspect of the Classic Mac OS, and nobody was sorry to see them disappear with the advent of Mac OS X. Of course they are still there in Classic Mode,

The usual sort of system extension problem is an extension conflict, where two extensions decide they can’t get along with each other, and take down whole system in their tussle, most often during startup. If your Mac locks up when booting and/or announces a system error, and especially when this happens right after you’ve installed and new application or other software item, an extension conflict is the likely culprit.

In such cases, you can use Extensions Manager to turn off the offending extension to see if that fixes the startup problem. Just reboot while holding down the spacebar and the Extensions Manager window will appear. Scroll down the list of items in search of something (or things) associated with the software you just installed. Select the item(s) in the On/Off check box, and click the Restart button. If the Mac now boots happily, you have found the problem.

Of course, your new software application or peripheral now won’t work, which is usually not a satisfactory state of affairs. However, it’s entirely possible that the conflict was with an extension you don’t really need anyway. The problem is to find out which one.

The way to approach this is to open Extensions Manager, which you can access from the Control Panels submenu in the Apple Menu, or by opening the Control Panels folder, which lives in the System Folder. At the top of the Extensions Manager window he will see a pull-down menu labeled “Selected Set.” The set showing will likely be called “My Settings.”

If you pull down the menu, you will see at least two other extensions set selections -- e.g.: Mac OS 9. x base” and “Mac OS 9. x all.” You can’t modify either of these predefined sets of extensions and control panels, but you can use them as a jumping off point for your extensions conflict resolution detective work.

Choose one or the other (for our purposes here, Mac OS 9.x base is best), and click the Duplicate Set button. This will create a custom set with just the Mac OS extensions preselected, but that can be modified. You will be prompted to name the new set. Choose whatever name you like.

All the extensions and control panels that Extensions Manager can identify will also appear in the scroll window, but non-Apple extensions will be turned off. The procedure now is to gradually turn on your non-Apple extensions and try restarting to see if the extension conflict is still in abeyance. I usually turn on batches of four items at a time, which speeds the process up considerably. When/if the conflict returns, you know it was one of the last four items you enabled causinf the problem. When this happens, returnd to Extensions Manager, turn off half of the most recently selected batch, and restart again. Through a few trial and error attempts, you should be able to identify the culprit extension.

I said “if” above because by starting with Mac OS x.x base, you may have eliminated a potential conflicting extension among the Apple-installed extensions that is not necessary for your computing purposes. Once you have all the extensions you need turned on, and the Mac remains happily bootable and stable, you’re finished. Just leave your new system extension set selected.

As a general rule, it is advantageous to turn off any extensions you don’t need, and for most users that means a substantial portion of the Mac OS extensions. With a lean extension set, your Mac will start faster, use less memory for the system, be more stable, and have less potential for extension conflicts. Printer drivers for printers you don’t use are a good and obvious example of items that can and should be turned off. The problem for most people is determining which extensions and control panels they can get along without. However, if you started your custom sat with Mac OS x.x base, it shouldn’t be to larded up too badly with deadwood. You can also create as many custom extension sets as you like to use in different circumstances.

For example, If you have a really troublesome extension that is needed for a piece of software or peripheral that you only use occasionally, just create a special set for it and the Mac base set and reboot with this set selected only when necessary.

Charles W. Moore

Cool Mac Gear