To breakin a VMS system if f.ex. the SYSTEM password has been forgotten
use:

1)  conversational boot
    SYSBOOT> SET/STARTUP _OPA0:
    SYSBOOT> CONTINUE
    fix the problem
    $ MCR SYSGEN
    SYSGEN> USE CURRENT
    SYSGEN> SET/STARTUP SYS$SYSTEM:STARTUP.COM
    SYSGEN> WRITE CURRENT
    SYSGEN> EXIT
    $ @SYS$SYSTEM:STARTUP

2)  conversational boot
    SYSBOOT> SET UAFALTERNATE 1
    SYSBOOT> CONTINUE
    fix the problem
    $ MCR SYSGEN
    SYSGEN> USE CURRENT
    SYSGEN> SET UAFALTERNATE 0
    SYSGEN> WRITE CURRENT
    SYSGEN> EXIT
    $ @SYS$SYSTEM:SHUTDOWN