About Apple Hardware Test

“Whose fault is it?” One of the major issues you'll face when troubleshooting is determining whether the problem is due to software or hardware. With the increasing power and complexity of computer systems, making this determination has become increasingly difficult. Software problems can create symptoms that appear to be hardware-related. Hardware problems can sometimes appear to be due to unusable or incorrect software.

Misdiagnosing a hardware or software issue wastes time and may lead to unnecessary replacement of service parts. Worst of all, it may not fix the problem.

AHT gives you a way to identify hardware issues and test only the hardware. It is useful in any situation in which you need to confirm that compatible Macintosh computer hardware is operable—particularly when a system turns on but does not boot to the Finder successfully. It's also extremely useful to confirm that you've properly performed a hardware upgrade or repair.

In this section, you learn how to use the AHT diagnostic tool efficiently and effectively as a part of the troubleshooting process for various Apple computers.

Getting AHT

As of early 2000, Apple includes an Apple Hardware Test disc with the software supplied to consumers with each new Mac. AHT is model-specific, so you must be sure you are using the version designed for the particular Mac being tested. For a list of the available versions, check Knowledge Base document 112125, “Service Diagnostics Matrix.” If your customers can't locate the copy of the AHT CD that corresponds to their Mac, keep in mind that Apple Authorized Service Providers (AASPs) can download all versions from the CD images section of Apple Service Source.

Required Tools and Equipment

To complete the AHT section of this lesson, you need an Apple Hardware Test CD that is compatible with the computer you are testing. There are AHT versions for most iBook, PowerBook, Power Mac, iMac, and eMac models.

ROM-in-RAM and AHT

With the introduction of the first iMac, Apple dramatically revised how ROM operates in its computer products.

The ROM-in-RAM architecture (also called NewWorld ROM) separates the hardware-specific and higher-level system software into two pieces:

• BootROM, which holds most of the hardware-specific components needed to start up the computer

• Mac OS ROM, which contains boot-time toolbox routines and hardware-specific components common to many Macintosh computers

BootROM is part of the computer hardware. The Mac OS ROM file is part of the Mac OS software.

This more efficient approach to the startup process also enables AHT to work with code in the BootROM, eliminating the need to perform diagnostic procedures through the Mac OS. That gives the user a diagnostic tool that tests only the computer hardware and that works even when the operating system is not functional. As a result, it doesn't matter if the computer you want to test is running Mac OS 9 or Mac OS X.