Enabling Mac Os ROM to boot on MacRisc3 computers (Os X only G4's):
Open the Mac OS ROM from the OS 9 system folder in Hex Editor. Be aware that this is written based on the ROM taken from the OS9General.dmg (Mirrored Drive Doors) version. Other versions of the ROM may be different or have different values.
First thing - and this is important - is to write the byte count down somewhere. If this changes, you will have problems. Of course it will inevitably change since we are adding some data to the ROM, but I'll show you how to correct this further down.
In the very first line, you will see the "COMPATIBLE" tags. In the case of the Mac OS ROM included with the OS9General.dmg system, there is only one value - MacRISC. In order to make this ROM detectable as a valid bootable system to the iBook (which is a MacRISC3 system), we need to add those as compatible systems. For good measure, I added both MacRISC3 (DDR-based G4s?) and MacRISC4 (G5-based?).
Next, you need to scroll down to the tag. You're looking for a line that looks like:
2dup " macrisc" $= r> or >r
Immediately after the $gt;r, I added the following (and spaces are important):
2dup " macrisc3" $= r> or >r 2dup " macrisc4" $= r> or >r
The idea here is to add those same additional models MacRISC3 and MacRISC4 as we added between the COMPATIBLE tags above, only here they're included in the "2dup" strings.
With these changes, the modified ROM will fail with a Checksum Error. To avoid this, you need to change the abort after checksum error to clear. It's possible another 5-letter command will work here too, but I used clear simply because it works.
Finally, after making the above modifications, the byte count of the ROM will have changed. Scroll down to the closing tag. After this tag is a large number of spaces (represented in hex as zeroes) for unused space in the ROM. Now, remember in the first step that you wrote down the byte count? What I did was delete some of these until the byte count matched exactly what I initially started with. Don't delete large blocks as pictured, delete them one by one. I only highlighted it to show the empty space.
Save the ROM and close Hex Fiend. If you're working in Target Disk Mode like I am, make sure you eject the OS 9 drive properly! Failing to do so will likely damage or fail to write the changes to the ROM file you just edited. I did this at least 5 or 6 times before I remembered to eject properly every single time.
The Result?
After making the modifications to the Mac OS ROM above, I restarted the machine with the Option key held down to enter the Startup Manager. Unlike before, the OS 9 drive and system folder is detectable as a valid boot device.
source:
https://www.thinkclassic.org/viewtopic.php?id=46