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System Management Concepts: Operating System and Devices
Occasions might arise when a
service boot is needed to perform special tasks such as installing new or
updated software, performing diagnostic checks, or for maintenance. In
this case, the system starts from a bootable medium like the CD-ROM or tape
drive, a network, or from the disk drive with the key in the Service
The service boot sequence of
events is similar to the sequence of a normal boot.
- The On-Chip Sequencer (OCS)
checks to see if there are any problems with the system motherboard.
- Control is passed to ROS,
which performs a power-on self-test (POST).
- ROS checks the user boot
list. You can use the bootlist command
to alter the user boot list to suit your requirements. If the user boot
list in NVRAM is not valid or if no valid boot device is found, the default
boot list is checked. In either case, the first valid boot device found
in the boot list is used for system startup.
Note: For a normal boot, the system maintains a default boot
list located in ROS, and a user boot list stored in NVRAM. Separate
default and user boot lists are also maintained for booting from the Service
- When a valid boot device is
found, the first record or program sector number (PSN) is checked. If
it is a valid boot record, it is read into memory and is added to the initial
program load (IPL) control block in memory. Included in the key boot
record data are the starting location of the boot image on the boot device,
the length of the boot image, and the offset to the entry point to start
running when the boot image is in memory.
- The boot image is read
sequentially from the boot device into memory, starting at the location
specified in the boot record.
- Control is passed to the
kernel, which begins running programs in the RAM file system.
- The Object Data Manager
(ODM) database contents determine which devices are present, and the cfgmgr command dynamically configures all devices
found, including all disks which are to contain the root file system.
- If a CD-ROM, tape, or the
network is used to boot the system, the rootvg volume group (RVG) is not
varied on, since the RVG might not exist (as is the case when installing the
operating system on a new system). Network configuration can occur at
this time. No paging occurs when a service boot is performed.
At the end of this process, the system is ready for installation,
maintenance, or diagnostics.
Note: If the system is booted from the hard disk, the RVG is
varied on, the hard disk root file system and the hard disk user file system
are mounted in the RAM file system, a menu is displayed which allows you to
enter various diagnostics modes or single-user mode. Selecting
single-user mode allows the user to continue the boot process and enter
single-user mode, where the init run level is set to "S".
The system is then ready for maintenance, software updates, or running the bosboot command.
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