System Management Concepts:
Operating System and Devices

Understanding the Maintenance Boot Process

Occasions might arise when a 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 such as a CD-ROM, tape drive, network, or disk drive.

The maintenance boot sequence of events is similar to the sequence of a normal boot.

1. The firmware checks to see if there are any problems with the system motherboard.
2. Control is passed to ROS, which performs a power-on self-test.
3. 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 in maintenance mode.
4. 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.
5. The boot image is read sequentially from the boot device into memory, starting at the location specified in NVRAM.
6. Control is passed to the kernel, which begins running programs in the RAM file system.
7. The 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.
8. If a CD-ROM, tape, or the network is used to boot the system, the rootvg volume group (or rootvg) is not varied on, because the rootvg 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 maintenance boot is performed.

At the end of this process, the system is ready for installation, maintenance, or diagnostics.