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a small program stored in PROM (or ROM), responsible for both loading the OS and providing some means to analyse OS crashes. It may also have some sort of user interface which can be used to examine and change the contents of memory, control the boot process (specifying arguments to the kernel, or changing where to look for the it), and so forth.
The main difference between a PROM monitor and a bootstrap loader is that the PROM monitor regains control when the OS terminates. This may enable a wizard to find out what went wrong if the OS crashed, although it is usually of little help for the average sysadmin.