Q: I need to do remote debugging on a Power Macintosh G4 (Mirrored Drive Doors) which is hung in the kernel. This machine doesn't have a programmer's switch, so how can I attach to this system from a remote GDB session? A: Starting with Mac OS X 10.1.2, if Mac OS X is started while the Open Firmware debug flags have the DB_NMI bit set, pressing the system's power button will generate a non-maskable interrupt (NMI) instead of sleeping or waking the system. This feature works on systems that do not have a physical programmer's switch.
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Note:
On portables such as the iBook or PowerBook, hold down the command key while pressing the power button to generate an NMI.
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The system reads the debug flags at boot time from the boot-args configuration variable.
To set the DB_NMI bit, start by entering this command in Terminal:
% nvram boot-args
This will display the current setting of the debug flags. Now, add debug=0x4 to the current settings with this command:
% sudo nvram boot-args="<current settings> debug=0x4"
For more information about the debug flags, please see Kernel Programming .
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Note:
If the power button is pressed for more than five seconds, the system will immediately power off.
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The power button will retain this functionality until Mac OS X is restarted without the DB_NMI bit set. You can clear this bit by issuing the nvram command omitting debug=0x4 :
% sudo nvram boot-args=""
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Note:
This bit will be cleared if you use System Preferences to change the startup disk. It may be cleared if you perform an installation that requires a restart.
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[Apr 22, 2004]
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