boot(8)               MAINTENANCE COMMANDS                boot(8)



NAME
     boot - bootstrap procedures

DESCRIPTION
     Bootstrapping is the process  of  loading  and  executing  a
     standalone  program.   For  the  purpose of this discussion,
     bootstrapping means the process of loading and executing the
     bootable operating system, but any standalone program can be
     booted instead.  The diagnostic monitor for a machine  is  a
     good  example of a standalone program other than the operat-
     ing system that can be booted.  The bootstrap  procedure  on
     most  machines  consists  of  the  following  basic  phases.
     First, the machine is either turned on, or brought  down  to
     firmware  mode  in  any  of a number of ways (hardware reset
     button, a shutdown or init command, etc.).  On powerup,  the
     boot  process  is  generally  begun  automatically:  a small
     firmware program is loaded and  executed,  and  the  process
     moves  into  the second phase.  From firmware mode, however,
     the boot process is not automatic and the user  can  request
     the  running  of  a  firmware  command, a standalone program
     (such as the bootable operating system), or the reconfigura-
     tion  of  the  operating  system.   See firmware(8) for more
     information on firmware operations.   For  the  purposes  of
     this  discussion,  assume  that  an operating system boot is
     requested from firmware;  a small firmware program is loaded
     and  executed,  and the process moves into the second phase.
     Second, the firmware  boot  program  loads  and  executes  a
     larger micro-boot program;  the sole purpose of this program
     is to load and execute a larger boot  program.   Third,  the
     boot  program loads and executes the bootable operating sys-
     tem.  It is at this point that the UNIX System  is  started,
     necessary file systems are mounted [see vfstab(4)], and init
     is run to bring the system to the initdefault  state  speci-
     fied  in  /sbin/inittab  [see inittab(4)].  For the AT&T 3B2
     Computers, the micro-boot program is called  mboot  and  the
     boot  program is called boot.  These programs are taken from
     the boot partition on disk, and loaded and executed at  boot
     time.   Copies  of  these  programs  exist  in the directory
     /usr/lib, for the purpose of copying them  to  another  hard
     disk using the newboot command.  The bootable operating sys-
     tem file is /stand/unix.

NOTES
     The boot program isn't smart enough to know which files  can
     be used as bootable programs.  If the booting of a file that
     is not bootable is requested from firmware, the boot program
     loads  it  and  branches  to it.  What happens after that is
     unpredictable.

SEE ALSO
     init(1M), newboot(1M), shutdown(1M), inittab(4),  vfstab(4),
     firmware(8)   The   ``Machine''   chapter   in   the  System



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boot(8)               MAINTENANCE COMMANDS                boot(8)



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