Apple Hardware Test now in ROM?

Well, I guess I've answered my question, or part of it anyway. I went to the Apple Manuals page, downloaded the last several manuals for the MacBook and MacBook Pro, and checked what they say about the Apple Hardware Test. The Mid-2007 MacBook manual says to "Insert the Mac OS X Install Disc 1 that came with your computer," while the Late-2007 manual simply says to "Restart your MacBook while holding down the D key." For the MacBook Pro, it's the Early-2008 model that no longer needs the Install Disc. The same research could be done for other models (which I deal with rarely).

Whether the Apple Hardware Test is in ROM or on some hidden partition on the HD I don't know; but the latter seems unlikely, as it would then be deleted if you erase the HD and install from an off-the-shelf OS X package (rather than doing a software restore from the discs that came with the computer), or if you replace the drive with a non-Apple label one. It's not much code, I'm sure (the interface is primitive, and it does only one thing); it could easily be in ROM.

This is a great idea I wish Apple had thought of years ago, as old Macs seldom come to me with their original Install discs. Apple has made AHT disc images for some older models available for download, but only those whose AHT was on a separate CD; for models where AHT is on the Install DVD, you have to have it to do the test. I suppose it might be possible to dig into the DVD and find the invisible AHT partition and copy it to a CD, but I don't have the skill to do so.

Well, I've tried some experiments:

1) Removed the hard disk from the MacBook, and tried starting up while holding down the D key. Question mark icon. So apparently the AHT is not in ROM, but ensconced somewhere invisibly on the HD.

2) Put the HD back in, started the MacBook from a standard, late-2007 retail 10.5.1 Install DVD (not the Install/Restore DVD that came with it). No AHT of course. And when it started up, the backlight went dark. Shining a lamp on the screen and looking from an angle, I was able to see enough to erase the HD. And tried starting again with the D pressed. Question mark icon. So apparently the AHT is not in a special HD partition that persists even when the HD is erased.

3) Installed 10.5.1 on the MacBook HD using the standard 10.5 Install DVD (not the Install/Restore DVD that came with it); once again, running the MacBook from that DVD turned the display backlight off. Then started the MacBook with the D key pressed. Question mark icon. So the standard, retail 10.5 Installer doesn't install the AHT.

4) Started the MacBook from its own Install/Restore DVD, and restored the factory HD setup. When setup was finished, shut down and started again pressing the D key. Apple Hardware Test (v.3A143) appeared.

Conclusion: Apple Hardware Test is on the hard disk, not in ROM, installed as part of the software specific to this model. If the disk is erased and System software is installed from a generic 10.5 DVD - for instance, because the original Install/Restore DVDs have been lost - AHT will not be installed. So the instructions in the manual apply only so long as the original disk setup (or a restoration of same using the original DVDs) remains. Otherwise the original Install Disc #1 will be required to run Apple Hardware Test.

Came across this Apple article, which clarifies the matter:
http://docs.info.apple.com/article.html?artnum=306568

Some Intel-based Macs ship with Apple Hardware Test and Mac OS X 10.4.x preinstalled on the hard drive. If an Erase and Install installation of Mac OS X 10.5 Leopard is performed using a retail Leopard installation disc, Apple Hardware Test will be erased and not reinstalled on these Macs.

If you're in this situation and need to use Apple Hardware Test, simply start from the DVD that came with your computer (not the Leopard installation disc) while holding the "D" key. Apple Hardware Test will run from the DVD.

+If you install OSX from the install DVD that came with the computer (not a retail copy), it should install AHT on the hard drive along with OSX.+

This is true only for the latest model Macs, e.g. MacBook late 2007, MacBook Pro early 2008. As I noted above, when this feature was introduced in each Mac model can be deduced by checking the instructions in the manuals, which can be downloaded in PDF form (if you need to know about models other than your own).

Well, I just started a new MacBook, with nothing in the drive, held down the D, and after a delay (slightly longer than with the DVD in the drive) AHT appeared. And that's what the manual says to do; no mention of the DVD. The manual for my 2006 MacBook Pro says to insert the DVD first, but that instruction is missing in the manual for the new MacBook.

P.S.: Also, of course, if you're running the Apple Hardware Test because the MacBook won't start up, starting with the D key but without the DVD may not work. The AHT software may well work even when the main System is messed up, but if a serious hard disk problem is involved it may not. Thus I'd have to say the instructions in the manual are inadequate; they should say "If the Apple Hardware Test does not appear, try again after inserting Install Disc #1" or something similar. More complete (though not entirely clear) instructions are in the AHT Read Me on the Install DVD, but of course if you can't start the computer, you can't read the AHT Read Me.