Section 8.4. Initial RAM Disk

8.4. Initial RAM Disk

LILO, GRUB, and Yaboot support the loading of the initial RAM disk (initrd). initrd acts as a root filesystem before the final root filesystem is loaded and initialized. We refer to the loading of the final root filesystem as pivoting the root.

This initial step allows Linux to initially come up with certain modules precompiled and then dynamically load other modules and drivers from initrd. The major difference to the bootloader is that it loads a minimal kernel and the RAM disk during Stage 2. The kernel initializes using the RAM disk, mounts the final root filesystem, and then removes the initrd.

initrd allows for

• Configuring a kernel at boot time

• Keeping a small general-purpose kernel

• Having one kernel for several hardware configurations

The previously referenced stanzas are the most common for loading Linux with Yaboot, GRUB, and LILO. Each bootloader has a rich set of commands for their configuration files. For a customized or special function boot process, a quick Web search on GRUB and LILO configuration files yields good information on the subject.

Now that we have seen how the kernel is loaded and how memory initialization starts, let's look at the process of kernel initialization.