The first level is powering on the server... from here POST (Power On Self Test) will start.
After you've turned on the power and the server is starting, the server's hardware is verified and checked for possible issues. This step is called power-on self-test (POST). While the server is running through its process, POST is checking the memory, keyboard, sound card, and network devices. During this time, if you wanted to enter stand-alone mode (single-user maintenance), you would click F5 or F6 after the keyboard has been initialized. However, in this article, no keystrokes are entered, and the server boots into its normal boot mode.
After the POST process has finished, the bootstrap —or a smaller program used to load a larger program—is loaded into memory. The bootstrap then loads the Boot Logical Volume (BLV) into memory. After the BLV is loaded, the kernel takes over the boot process.
Boot Logical Volume and the bosboot command
The BLV is the location that contains AIX's bootable images. Typically, the BLV can be found on the local disk of the server. The BLV contains the AIX kernel, the rc.boot file, commands required during the boot process, and a trimmed-down version of the Object Data Manager (ODM).
To create bootable images, you use the bosboot command. Using bosboot, you create a boot file (that is, a bootable image) from a RAM disk, a file system, and a kernel. The bootable image is created along with interfaces with the server's boot Read-Only Storage (ROS) and Erasable Programmable Read-Only Memory (EPROM).
The AIX kernel stored in the BLV creates the / (root), /usr, and /var file systems in RAM. Keep in mind that these file systems as well as the kernel are stored in RAM initially during the operating system boot process. Because they are in RAM, they are not accessible to anything outside the BLV.
After the file systems have been loaded into RAM, the kernel executes the init process, which now takes over the boot process.
The AIX kernel loads the process init as process identifier (PID) 1. This process is the parent, or root, process to all other processes running on AIX. After the init process has been loaded and is running the boot process, init calls rc.boot.
The rc.boot file has three important cases of execution during the AIX boot-up process. The first section of rc.boot initializes the system's hardware to prepare it for the operating system to boot. A limited amount of devices needed to start the system are configured at this time with the Configuration Manager command cfgmgr.
During the second section of rc.boot, the file systems /, /usr, and /var as well as the paging space are mounted. After these file systems have been mounted, init is replaced with init on the disk as PID 1, and the RAM is cleared.
In the third and final section of rc.boot, the actual init process is executed from disk. When init is executed, the /etc/inittab file is read, and each item is executed. During this time, the /tmp file system is now being mounted to disk. Now that the system is in the last leg of the boot process, the cfgmgr command is run again on the remaining devices that were not configured in the first section of rc.boot.
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