If you are running on an IBM pSeries machine that is HMC controlled, here are the simple steps to start a debug boot.
1 - Enter the open firmware prompt at boot time:
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1 = SMS Menu 5 = Default Boot List
>> 8 = Open Firmware Prompt 6 = Stored Boot List
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2 - At the firmware prompt - For AIX 5.3 and above.
boot -s verbose -s debug
For AIX 5.2 I believe command is: boot -s trap
Regards,
Steve.
