The RAM file system, part of the boot image, is totally memory-resident and contains all programs that allow the boot process to continue. The files in the RAM file system are specific to the type of boot.

A maintenance boot RAM file system might not have the logical volume routines, because the rootvg might not need to be varied on. During a hard disk boot, however, it is desirable that the rootvg be varied on and paging activated as soon as possible. Although there are differences in these two boot scenarios, the structure of the RAM file system does not vary to a great extent.

The init command on the RAM file system used during boot is actually the simple shell (ssh) program. The ssh program controls the boot process by calling the rc.boot script. The first step for rc.boot is to determine from what device the machine was booted. The boot device determines which devices are to be configured on the RAM file system. If the machine is booted over the network, the network devices need to be configured so that the client file systems can be remotely mounted. In the case of a tape or CD-ROM boot, the console is configured to display the BOS installation menus. After the rc.boot script identifies the boot device, then the appropriate configuration routines are called from the RAM file system. The rc.boot script itself is called twice by the ssh program to match the two configuration phases during boot. A third call to rc.boot occurs during a disk or a network boot when the real init command is called. The inittab file contains an rc.boot stanza that does the final configuration of the machine.

The RAM file system for each boot device is also unique because of the various types of devices to be configured. A prototype file is associated with each type of boot device. The prototype file is a template of files making up the RAM file system. The bosboot command uses the mkfs command to create the RAM file system using the various prototype files. See the bosboot command for more details.