Boot process - AIX

During the boot process, the system tests the hardware, loads and runs the
operating system, and configures devices. To boot the operating system, the
following resources are required:
_ A boot image that can be loaded after the machine is turned on or reset.
_ Access to the root and /usr file systems.
There are three types of system boots:
_ Hard Disk Boot
A machine is started for normal operations with the key in the normal position.
On PCI-based systems with no key locking, this is the default startup mode.
Chapter 2. System startup problem handling 13
_ Diskless Network Boot
A diskless or dataless workstation is started remotely over a network. A
machine is started for normal operations with the key in the normal position.
One or more remote file servers provide the files and programs that diskless
or dataless workstations need to boot.
_ Service Boot
A machine is started from a hard disk, network, tape, or CD-ROM with the key
set in the service position. This condition is also called maintenance mode. In
maintenance mode, a system administrator can perform tasks, such as
installing new or updated software and running diagnostic checks.
During a hard disk boot, the boot image is found on a local disk created when the
operating system was installed. During the boot process, the system configures
all devices found in the machine and initializes other basic software required for
the system to operate (such as the Logical Volume Manager). At the end of this
process, the file systems are mounted and ready for use.
The same general requirements apply to diskless network clients. They also
require a boot image and access to the operating system file tree. Diskless
network clients have no local file systems and get all their information by way of
remote access.
The system finds all necessary information for the boot process on its disk drive.
When the system is started by turning on the power switch (a cold boot) or
restarted with the reboot or shutdown commands (a warm boot), a number of
events must occur before the system is ready for use. These events can be
divided into the following phases:
1. Read Only Storage (ROS) Kernel Init Phase
During this phase, problems with the motherboard are checked, and the ROS
initial program load searches for the bootlist. Once the bootlist is found, the
boot image is read into memory and system initialization starts.
2. Base Device Configuration Phase
All devices are configured in this phase, with the help of the cfgmgr command.
3. System Boot Phase
In this phase of the boot process, all the logical volumes are varied on, paging
is started, and the /etc/inittab file is processed.