Generating a Non-Maskable Interrupt (NMI)

Q:  I need to do remote debugging on a Mac computer which is hung in the kernel. How do I interrupt this system so I can attach to it from a kernel debugging session?

A: If OS X is started with the DB_NMI bit set in the debug boot argument, momentarily pressing the system's power button will generate a non-maskable interrupt instead of sleeping or waking the system.

The system reads the debug flags at boot time from the boot-args firmware 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 boot arguments. Now, add debug=0x4 to the current settings with this command:

$ sudo nvram boot-args="<current settings> debug=0x4"

You must restart the computer for the new settings to take effect.

For other useful debug flags, please see Table 20-1 'Debugging flags' of Kernel Programming Guide: When Things Go Wrong: Debugging the Kernel.

The power button will retain this functionality until OS X is restarted without the DB_NMI bit set. You can clear this bit by issuing the nvram command with the -d option:

$ sudo nvram -d boot-args

Document Revision History