The firmware prepares the system to load and run the operating system.
Its initialization phase involves the following steps:
- The firmware performs basic testing on the system resources that are required for starting the operating system.
- The firmware checks the user boot list, a list
of available boot devices. This boot list can be changed to suit your
requirements by using the bootlist command.
If the user boot list in non-volatile random access memory (NVRAM)
is not valid or if a valid boot device is not found, the default boot
list is then checked. In either case, the first valid boot device
found in the boot list is used for system startup. If a valid user
boot list exists in NVRAM, the devices in the list are checked in
order. If no user boot list exists, all adapters and devices on the
bus are checked. In either case, devices are checked in a continuous
loop until a valid boot device is found for system startup. Note: The system maintains a default boot list that is stored in NVRAM for normal mode boot. A separate service mode boot list is also stored in NVRAM, and you must refer to the specific hardware instructions for your model to learn how to access the service-mode boot list.
- 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 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 instructions on where to load the boot image in memory.
- The boot image is read sequentially from the boot device into memory starting at the location specified in NVRAM. The disk boot image consists of the kernel, a RAM file system, and base customized device information.
- Control is passed to the kernel, which begins system initialization.
- The kernel runs init, which runs phase 1 of the rc.boot script.
When the kernel initialization phase is completed, base device configuration begins.