Concept
A boot disk is a removable digital data storage medium from which a computer can load and run (boot) an operating system or utility program. The computer must have a built-in program which will load and execute a program from a boot disk meeting certain standards. While almost all modern computers can boot from a hard drive containing the operating system and other software, they would not normally be called boot disks (because they are not removable media). CD-ROMs are the most common forms of media used, but other media, such as magnetic or paper tape drives, ZIP drives, and more recently USB flash drives can be used. The computer's BIOS must support booting from the device in question. One can make one's own boot disk (typically done to prepare for when the system won't start properly). Boot disks are used for: Operating system installation Data recovery Data purging Hardware or software troubleshooting BIOS flashing Customizing an operating environment Software demonstration Running a temporary operating environment, such as when using a Live USB drive. Administrative access in case of lost password is possible with an appropriate boot disk with some operating systems Games (e.g. for Amiga home computers, running MS-DOS video games on modern computers by using a bootable MS-DOS or FreeDOS USB flash drive). The term boot comes from the idea of lifting oneself by one's own bootstraps: the computer contains a tiny program (bootstrap loader) which will load and run a program found on a boot device. This program may itself be a small program designed to load a larger and more capable program, i.e., the full operating system. To enable booting without the requirement either for a mass storage device or to write to the boot medium, it is usual for the boot program to use some system RAM as a RAM disk for temporary storage. As an example, any computer compatible with the IBM PC is able with built-in software to load the contents of the first 512 bytes of a floppy and to execute it if it is a viable program; boot floppies have a very simple loader program in these bytes.