With regard to Linux, a rootboot disk is a floppy disk which is both a boot disk and a root disk -- that is, it contains a boot loader and kernel and, separately, an image of a root filesystem. When one boots a computer from such a disk, the boot loader loads the kernel, which then creates a RAM disk and loads the root filesystem image into it. It can then mount the RAM disk, execute init from it, and run normally.

Most commonly, a rootboot disk is a rescue disk; that is to say, it contains utilities useful in recovering a hard-disk-based system which has crashed and become unbootable. tomsrtbt is an example of one such. However, a rootboot disk can also be a fully functional system on its own -- usually a special-purpose one. The Linux Router Project, for instance, produces toolkits and images for making Linux network routers that boot from rootboot disks. When you install a Linux distribtion from floppy disks, the disk you boot from is also (usually) a rootboot disk.

Why do Linux users refer to this by the awkward name of "rootboot disk" when the equivalent under DOS or Mac OS is simply called a "boot disk"? Early Linux systems did not allow for the two functions to be combined on a single floppy. In order to start up without a bootable hard disk, one needed a "boot disk" containing the boot loader and kernel, and a "root disk" containing the root filesystem. The term "rootboot" stands for the combination of these functions into a single disk.

The oldest surviving Linux distribution, Slackware, still uses separate boot and root disks when performing a floppy-based installation; younger systems such as Red Hat and Debian begin with rootboot disks.

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