The RAMdisk is a very useful kernel facility that lets you load the files you need on the system into RAM along with the kernel. It take the form of a compressed filesystem. The kernel automatically allocates RAM for it and uncompresses it, then mounts it as the root filing system.
Again, there are many variations possible on this theme, but the conventional setup is to format the RAMdisk as ext2, the normal Linux disk filesystem. For this to work you need to specify the correct kernel options: RAMdisk support (CONFIG_BLK_DEV_RAM, CONFIG_BLK_DEV_RAM_SIZE, CONFIG_BLK_DEV_INITRD) and support for the filesystem used (normally ext2 - CONFIG_EXT2_FS).
There are suitable example RAMdisks provided for the devices covered in this manual on the CD. There are also several available on-line, where developers have made their own available for others to use. We recommend that you use one of these to get started, however for many applications you will need to make your own. It's not difficult, and the process is described in the section called Making a RAMdisk in Chapter 7.