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#1 2014-12-17 12:22:25

iMic
Administrator
From: Adelaide, Australia
Registered: 2014-05-12
Posts: 895
Website

An Unusual Method for Fixing Open Firmware NVRAM / No Boot Issues

Here's one for the more technically minded to discuss.


I have an iMac G4 (17-inch Flat Panel, 1GHz) that serves as my home entertainment system. Occasionally though I use it to test new software that I have in the works. Some of the software I test is rather dangerous. It writes commands into the NVRAM, it changes low level system settings... and occasionally it goes bad.

Picture the scenario. The computer reboots, it chimes, but the video never kicks in. You attempt to hold down the PRAM reset keys, but the keyboard is completely unresponsive. There's no network activity on the Ethernet port. You listen closely, and there's no hard drive activity. For all intents and purposes, the machine is in an electronic coma - powered up and running, but unable to activate any of its vital I/O subsystems. (Think a MITS Altair, but without the lights and switches.)

It stems from writing a command into the NVRAMRC (and setting "use-nvramrc? true") that the computer is seemingly unable to handle on startup. It halts the system even before the I/O devices can be activated.

I've managed to brick a Power Mac G4 this way several years ago during testing of the then current LeopardAssist 1.2. I was never able to resolve that issue, and I had to replace the Logic Board. The exact same thing happened this evening to the iMac G4.



This time however, I managed to revive it.

What I don't understand is how.


You could consider this an act of desperation, but somehow these acts came together to allow me a chance to revive the iMac's Logic Board from its comatose state.


1. Turn off the machine.

2. Press and hold the power button. The computer will turn on, and after a few seconds you will hear a long beep (like performing a firmware update). Continue holding the button.

3. When you hear the normal Macintosh boot chime, release the power button and start tapping the enter key as rapidly as possible.


I'm aware of how ridiculous this fix sounds, but it worked - the computer's display came on and the machine dropped into an Open Firmware prompt.

If you do somehow manage to reach an Open Firmware prompt, don't forget to type the following (pressing return after each line):


reset-nvram
set-defaults
reset-all

This resets the NVRAM and clears whatever was stored there to cause the issue in the first place. The computer will boot normally once again.


I have no idea how the steps above - holding the power button until it beeps, continue holding until it chimes normally, release the button and frantically bash enter/return - come together to bring the machine back to life. Perhaps someone more knowledgeable about Open Firmware and the low level startup operations of a PowerPC Mac can shed some light on this?


Even so, I had to document it. If someone here ever finds themselves in this scenario, don't be afraid to get creative.



Cheers,

iMic.


Resident Professor of Alternative Methodology
Faculty of Macintosh Restorations & Modifications - "It works, let's fix it!"

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#2 2014-12-17 15:53:02

ClassicHasClass
Member
From: Electron Alley
Registered: 2014-05-26
Posts: 1,108
Website

Re: An Unusual Method for Fixing Open Firmware NVRAM / No Boot Issues

The hold-in-the-power-button part I get: it probably is attempting to flash the firmware. I'm assuming the RETURN key bit aborts it and forces a factory reset, but I don't know.

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