To get around the root password, start 'inst' from an IRIX CD or tape
  as you would if you wanted to install software. (See chapter 3 of the
  Software Installation Administrator's Guide.) If you've set a PROM
  password, you'll need to provide it or circumvent it first; see below.
  Say 'admin shroot' to get a root shell. You can then do any of the
  following:

  - use 'passwd' to change root's password
  - 'setenv TERM iris-tp' and 'vi/etc/passwd'
  - if /etc/passwd is really hosed, 'mv' the remains out of the way and
    'echo root::0:0:root:/:/bin/sh > /etc/passwd'.

  Alternatively, if your machine is an NIS client you can change the uid
  of an NIS account to 0 from the server and do a 'ypmake'.

  If you've lost your PROM password but can still log in as root, you
  can zero the PROM password with 'nvram passwd_key ""'. If not, you'll
  have to disable the PROM password via the hardware. On a 4D/35 or
  Crimson, find the battery which maintains the nvram ("non-volatile
  RAM") and remove it. On an Indigo or Indy, find the nvram chip itself
  and remove it. On an Indigo^2, remove the jumper described in the
  owner's manual. This may be a good time to call SGI.