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 operating 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 reconfiguration 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 system. 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 specified 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 system 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

Administrator's Guide