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IWishTodayWasStill2004's picture
Joined: 2012 May 25
Hacking Mac OS 7.5.3 to run on a New World Mac

I'm envisioning the possibility of an iMac G3 emulator, but as far as I know, there is one very first step needed - capturing the ROM code and firmware stored in flash memory from the iMac G3, but that comes to the problem that such a tool which directly accesses hardware will not work on Mac OS 8 or later but the EARLIEST iMac G3 will only boot Mac OS 8.5 or later. However, I have a method for hacking Mac OS 7.5.3 to possibly run on a New World Mac, but I'm not sure if it will work:

  1. Download and "expand" the Mac OS 7.5.3 files and ResEdit 2.1.3
  2. Back up all data you have on your New World Mac
  3. Insert your Mac OS 8.5 CD-ROM into your New World Mac's CD tray and reboot your Mac into the CD-ROM
  4. Reformat your Mac's hard drive in "Mac OS Standard" format (once again, before doing this is your last chance to back up all data)
  5. Restore just the Mac OS 7.5.3 files and ResEdit 2.1.3 from your backup
  6. Mount Mac OS 7.5.3 01of19.smi and install Mac OS 7.5.3
  7. Copy the "Mac OS ROM" file from the System Folder of your Mac OS 8.5 CD-ROM to your newly installed Mac OS 7.5.3 System Folder
  8. Open the Mac OS 7.5.3 System file and Mac OS 8.5 System file with ResEdit
  9. Replace the boot ID = 2 and gbly ID = -16385 resources in the Mac OS 7.5.3 System file with those in the Mac OS 8.5 System file
  10. Open the gusd ID = 1 resource in the Mac OS 8.5 System file using the hex editor and look for "0196 00**"
  11. As an example, I found "0196 0025" in the gusd ID = 1 resource in the Mac OS 8.5 System file. With this example, copy the gtbl ID = 37 (0x25 is 37) resource from the Mac OS 8.5 System file to the Mac OS 7.5.3 System file and replace any random eight hex digits in the gusd ID = 1 resource in the Mac OS 7.5.3 System file with "0196 0025" (I'm not sure which exact eight hex digits have to be replaced with "0196 00**" for a New World Mac to boot Mac OS 7.5.3 without refusal, so if you manage to get Mac OS 7.5.3 to boot on an iMac G3 using this method, please tell me the exact eight hex digits you replaced with "0196 00**")
  12. Save the Mac OS 7.5.3 System file and quit ResEdit. You can then reboot into the hacked Mac OS 7.5.3 and restore all your other data from your backup

(this method was partly inspired by the method used at http://www.vintagebox.de/mac68k/macos_p2-en.html to hack Mac OS 8.1 so it will boot on any Macintosh computer with a 32-bit clean ROM instead of only a 68040-based Macintosh model)

Edit: Replaced 'xxx 7' by 'Mac OS 7' - IIGS User

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Temporary Joe's picture
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Joined: 2009 Nov 14

I have an iMac G3 from 2000. If and when I get repaired (I recently rebuilt my Game Boy Advance SP, so it can be done), it would be a neat thing to try out. I want to see if anyone's actually tried to do this, though.

Ilovetheoldones's picture
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Joined: 2012 Jun 3

Interesting, I'll give it a shot.

cessna's picture
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Joined: 2011 Mar 29

Does this work? I'd like to try it out if it does!

IWishTodayWasStill2004's picture
Joined: 2012 May 25

I'm not sure it works. We still have to find out which 8 hex digits in the "gusd ID = 1" resource have to be replaced with "0196 00**"