This article discusses address errors or Illegal instruction errors that occur during startup and how to help prevent them.

• damaged hard disk driver
• damaged or incompatible extension
• damaged Finder
• damaged System file

1. Start up with the Shift key down; this disables system extensions.
If the computer starts up and displays the desktop, the problem is not with the Finder or System file but likely with an extension in the Extensions folder within the system folder. Use the Extensions Manager control panel to selectively enable extensions to determine which is causing the problem. Once the problem returns, delete and reinstall the last extension that was enabled.

2. Next, try starting up into the Extensions Manager (hold the Space Bar down and start up the computer).
When the Extension Manager window appears, select the "Mac OS 8.x All" set of extensions and allow the system to continue starting up. If that fixes the problem, then you know the problem is an extension conflict.

3. If the error still occurs, the next thing you should try is starting up from a different source, such as the iMac install CD or any other bootable system software volume.
If the computer starts up properly from the CD (or other startup volume), then you can be confident that the computer does not have a hardware problem.

4. The next thing you should try is updating the hard disk driver using Drive Setup.
After updating the hard disk driver on the suspect hard disk, try restarting. If the computer still exhibits the problem continue with the steps below.
For more specifics on the hard disk driver, please see Knowledge Base article 24585: " Hard Disk Driver: Identifying It And Ensuring Compatibility "

5. Consider resetting Parameter RAM (PRAM); sometimes the issue can be resolved by performing this procedure.
For instruction on resetting Parameter RAM, please see Knowledge Base article 2238: " Macintosh: How To Reset PRAM And NVRAM "

6. If the computer started up and ran properly from a different volume (as described in step 3 above), and the computer still exhibits the problem then the last step is to perform a clean install of the system software.