Understanding the Service Boot Process

Occasions may arise when a service boot is needed to perform special tasks such as installing new or updated software, performing diagnostic checks, or maintenance. In this case, the system starts from a bootable medium (CD-ROM, tape), a network, or from the disk drive with the key in the Service position.

The service boot sequence of events is similar to the sequence of a normal boot. The events can be outlined as follows:

1. The On-Chip Sequencer (OCS) 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 (POST).
3. ROS checks the user boot list, which can be altered to suit your requirements using the bootlist command. 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: The system maintains a default boot list, located in ROS, and a user boot list, stored in NVRAM, for normal boot. Separate default and user boot lists are also maintained for booting from the Service key position.

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 execution 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 the boot record.
6. Control is passed to the kernel, which begins executing programs in the RAM file system.
7. 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.
8. If CD-ROM, tape, or the network is used to boot the system, the rootvg volume group (RVG) is not varied on, since the RVG may not exist (as is the case when installing the operating system on a new system). Network configuration may 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's run level is set to "S". The system is then ready for maintenance, software updates, or running the bosboot command.