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Mac OS 8.6 for (some) Unsupported G3s and G4s


Well-known member
Evening folks.

Apple released the early G4s, Yikes and Sawtooth, with special, updated versions of Mac OS 8.6. I'm not sure if people are aware, but @LightBulbFun did some excellent work here to produce a modified version of the Mac OS ROM file, that goes in the System Folder, to enable this special version of 8.6 to be booted on unsupported machines including the Pismo, Gigabit Ethernet G4, some DA G4s, some extra iMacs that are not usually supported, some iBooks, probably the Cube and perhaps some early PowerBook G4s? The requirements are that the machine needs to have an old enough processor and be similar to an officially supported machine. Machines with the G4 7450 processor and newer are not immediately supported without some further fiddling in OpenFirmware.

I decided to take this modified ROM file and wrap it up in a modified version of the Mac OS 8.6 installer CD. I'm not going to lie, while most of it was easy, getting the Installer application on the CD to actually install the modified file was way more tricky than I expected. Yes I tried replacing the file in the two tomes it appeared in. That lost me about 5 hours of my day. Yes I tried editing the install scripts, it wasn't possible to convert them between their compiled state and editible state easily, given my poor knowledge of the Apple Installer SDK. Right. Anyway.

Make sure your computer is running the most up to date firmware. I forgot to and it caused me issues on my GigE G4.

Features of my CD image :

  • This CD should boot most supported unsupported machines, by holding the 'C' key.
  • The firmware updates you might need are included on the CD.
  • An Apple made Machine ID spoofer Control Panel is included in utilities in case you need it.
  • The installer works and provides a system with the modified Mac OS ROM file.
  • The restore application works and restores an image including the modified Mac OS ROM file.
Unintentional features :
  • For some reason, sometimes Pismos (perhaps other similar machines) power off when they start loading the desktop from a CD. Installed systems work just fine, but you might need to boot from another disk while you install.

Things I did :
  • I modified the restore image to add the Mac OS ROM file.
  • I modified the CD System Folder to add the Mac OS ROM file.
  • I added the Machine ID control panel in Utilities (but not the System Folder, I probably should have).
  • I removed the machine specific white-list from the installer program so it will launch on absolutely any machine, regardless of whether it is appropriate or not.
  • I made a new installer script, and tagged it on the end of the 8.6 update in the Installer, so that it silently swaps in the modified Mac OS ROM file without bothering the user.
  • I added a load of firmware updaters in a folder.
  • I added some ReadMes for clarity.
  • I messed up the order of the icons on the CD by accidentally clicking "Clean Up" when I didn't mean to.
I have tested on a Pismo and on a GigaE G4, but would appreciate feedback on what it works on. This isn't a magic bullet - this will not work on a G5, or a Plus, see the thread linked above for OpenFirmware hacks to get 8.6 running on a QuickSilver or MDD, but that won't work right out the box, so really only play with this if you have a Pismo, early TiBook, G3 iBook, Graphite G4, G3 iMac or Cube.

My hosting service isn't great sorry. People in the Americas might find the connection drops. All I can say is sorry, but if you keep trying it eventually should complete. The download is about 400MB.

Consider the software to be experimental. It might not work, but if it does 🎆

Please don't ask for changes, they're likely impossible and I've had a really crap day.
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Well-known member
Interesting, I wonder if this would also work for firmware updated clamshells, which can no longer run stock 8.6


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Cool. Do you mind if this gets uploaded to the Garden? It would take any bandwidth burden off your hosting service.


Well-known member
Sweet stuff! glad to see my modified ROM is being put to good use :)

a few things to note:

the MacOS8.6 ROM is quite picky about which Video card it will work with, for best results, use only an ATI Video card with a full ROM (this is why certain machines like Snow iBooks and iMacs fail to load the ROM I think) if you try a card without an NDRV in its ROM or an NVIDIA card the ROM might not load

for booting on 7450 CPU's and newer, you will need to spoof your CPU's PVR to be that of a 7400

this ROM does not sadly seem to work on FW800 MDD's, but will work on FW400 ones, however the ROM will only see drives on the ATA66 bus, otherwise ya get a blinking floppy (not a blinking folder)

as per the OP make sure your machines firmware is fully up to date, it was only in later BootROM's did apple add the MacRISC2 descriptor which is what the modified ROM looks for instead of "PowerMac3,1" etc

otherwise have fun :)

(I do wonder if the very last MacOS ToolBox ROM, 10.2.1 could be made to boot MacOS 8.6 but I would not know where to start with that, IIRC it does load 8.6's system file but then I think it complains about something, if that Toolbox ROM could be made to load 8.6 then that really would open 8.6 up to a load more machines)


Well-known member
How can you spoof the CPU's PVR?
Power on while holding Cmd+Opt+O+F
Then type...
dev /
" Rev 2.7 7400" encode-string " cpu-pvr" property

I'm not certain that you need the .property line, but this is what was written in the thread I linked above.

This isn't persistent.
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Well-known member
How can you spoof the CPU's PVR?
I have a guide I put together here :)



Well-known member
Right, I've made an updated version of the CD image. The download is available here, as is a description of the changes.

The big change is that I fixed an issue with the restore (turns out I somehow overlooked a checksum issue - Apple used a weird bodge of putting the CRC and file size in the comment of the image file... WTH Apple).

Otherwise, the installer and recovery now install an updated version of the Zip, FireWire and ATI graphics extensions.


Well-known member

Why does the checksum behave for the restore on my Pismo... But not on my G4?!

I literally just tested the restore on my Pismo, now on my Graphite? No. You shall not restore.

Edit - Even more weird, I restarted in 9.2.2 (instead of the 8.6 CD) and now the checksum passes on the exact same file.

Edit 2 - Ahhhhh no, just tried running the wrong version. I think we're all good :)
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