The best information I've seen on this yet.
Date: Tue, 04 Mar 2003 14:28:04 -0500
From:
Stavros Karatsoridis Subject: Re: Mac OS 9 issues with new Power Macs
Does anyone know of a way to get the new 17" G4 Imacs and/or the new Tower models, which are "not compatible" with OS9 to boot into OS9? I understand that there has been a change with the ROMs to prevent this and make them OSX bootable only, even though they have the "Classic" option. It seems to me that being able to boot these new machines into OS9 would be a great advantage, even if Apple doesn't think so.
It is a common misconception that Apple has "intentionally crippled the ROMs," and that a simple hack will magically enable Mac OS 9 to boot on the machines. This is absolutely NOT the case.
When Apple makes a new motherboard design (and that has been done with all the new 2003 macintosh models), they have to update the Mac OS with motherboard-specific drivers to be able to run the operating system on the machine. Without those drivers, the machine won't boot (this is similar to the System Enabler system Apple used to use back in the days of System 7.1). This is also why in Mac OS 9.2.2 there were updated versions of some files (like the Mac OS ROM file, for instance), and why a Mac OS 9.2.1 disk for an earlier macintosh won't boot a newer Macintosh that boots into Mac OS 9.2.2
Apple decided that in 2003, they would not spend the effort to keep updating Mac OS 9.2.2 to run on the new machines. This requires quite a bit of extra time and effort separate from getting Mac OS X to work on the machines, that could be better spent elsewhere. Therefore, since they're not updating the operating system to support the new hardware and motherboard designs, they have removed the Mac OS 9 boot code from the ROM as well (there's no need for it).
In short, to get Mac OS 9 to boot on the new machines, one would not only have to add Mac OS 9 boot code to the machine's boot ROM, but they would also have to figure out how to reverse engineer the operating system itself and the new motherboard designs to figure out how to write the low-level drivers necessary for them to talk to each other. Then they would need to figure out how to write drivers to support new features (Firewire 800, for example). They would need to "roll a new version" of the Mac OS ROM file, along with updating some of the control panels, extensions, and other code needed to run on the new machines. Needless to say, this is a major undertaking.
Some may attempt it, but I honestly don't think it will happen. Even if people should be able to do it, it won't run as well as it could, unless the people that do it are Apple engineers themselves with access to the necessary technical info and Mac OS 9 source code. Besides, I wouldn't be surprised if Apple tried to invoke the DMCA to prevent any such efforts either.
http://www.macintouch.com/g4-2003pt02.html#mar05