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.
Boot disks are used for:
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. Floppy disks and 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.
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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 file 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. The process is vulnerable to abuse; data floppies could have a virus written to their first sector which silently infect the host computer if switched on with the disk in the drive.
Bootable floppies for PCs usually contain MS-DOS or miniature versions of Linux. The most commonly available floppy disk can hold only 1.4 MB of data in its standard format, making it impractical for loading large operating systems. The use of boot floppies is in decline, due to the availability of other higher-capacity options, such as CD-ROMs or USB flash drives.
A modern PC is configured to attempt to boot from various devices in a certain order. If your computer is not booting from the device you desire, such as the floppy drive, you may have to enter the BIOS setup function by pressing a special key when the computer is first turned on -- Delete, F1, F2, F10 or F12 -- and then changing the boot order. More recent BIOSes permit the interruption of the final stage of the boot process by pressing a function key (usually F11). This results in a list of bootable devices being presented, from which a selection may be made.
Modern Apple computers will boot from an appropriate disk if the user presses the C key while the machine is starting.
Different operating systems use different boot disk contents. All boot discs must be compatible with the computer they are designed for.
All files must be for the same version; although MS-DOS 5 and 6 use a file called COMMAND.COM, they are not interchangeable. Complete boot disks can be prepared in one operation by an installed operating system; details vary.
Windows 8 has a feature called Windows To Go, that allows the system to run from a USB flash drive, requiring only the extraction of files into the USB drive and the installation of a bootloader.