System boot process:

Power on
PDC executes CPU, Memory and other tests
PDC initialize the console to display messages
Looks for Primary boot path
It loads and executes Initial System Loader (ISL). Roll of boot ROM ends.
ISL loads the secondary system loader called “hpux”. It resides on boot area of the disk
“hpux” loads “/stand/vmunix” into memory and hands control over […]

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