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Boot disk

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A boot disk (formerly known as a PC booter, or booter) is a removable digital data storage medium from which a computer can load and run (boot) an operating system or program directly, bypassing any operating system that might be installed on the hard disk of the computer.

CD-ROMs are now 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.

Video games were the type of application most commonly distributed as booters.

History

Some booters include a customized subset or variant of a "standard" operating system for the platform (for example, DOS for IBM PC compatible, Apple DOS or ProDOS for Apple II, etc.).

Amiga games and those for other computers were often distributed as bootable floppies using a custom boot block which would consist of a custom loader. These disks contained no filesystem; instead, the custom loader would read the tracks directly. Many Amiga games were released as such in order to thwart piracy, and to utilize the RAM otherwise occupied by the AmigaOS. In early to mid-1990s, disks with a custom boot block became very popular for making so-called "trackmos" by demo groups.

An example computer system where this technique was not used is the Commodore 64, since it was not designed to read any boot block from external storage media prior to starting operating system routines and entering the BASIC interpreter. However, there were cartridges that mapped their program code directly into the Commodore 64's addressable memory (no loading required) and which would start immediately, though these are not considered booters due to no loading taking place.

While booters provided a safe form of copy protection[citation needed], programs such as Locksmith and Copy II PC existed that provided a method for copying of these disks; these were known as bit nibblers.

Today, IBM PC compatible computers can still boot from floppies, CD-ROMs/DVDs, USB storage devices etc., if a corresponding drive is connected to the system. However, it may require to change boot device priority in the BIOS setup utility.

Uses

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)

Process

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 infects the host computer if switched on with the disk in the drive.

Media

Bootable floppy disks ("boot floppies") for PCs usually contain MS-DOS, a workalike such as FreeDOS, 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.

Device selection

A modern PC is configured to attempt to boot from various devices in a certain order. If a computer is not booting from the device desired, such as the floppy drive, the user may have to enter the BIOS setup function by pressing a special key when the computer is first turned on (such as 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.

Requirements

Different operating systems use different boot disk contents. All boot discs must be compatible with the computer they are designed for.

MS-DOS/PC DOS

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.

FreeDOS
  • A valid boot sector on the disk
  • COMMAND.COM
  • KERNEL.SYS
Linux

Benefits

  • Ease of use (the software would start automatically, without any further action required by the user).
  • Insulation (few chances to manually alter program files). In a way, the booter acted as a sandbox, even though the concept didn't exist yet.[weasel words]
  • Copy prevention (the booter floppies can be hard to read and copy with a regular operating system – mostly because they often used a nonstandard filesystem or formatting).
  • Bypassing the normal operating system (to use some specialized replacement).

The last benefit was critical for games, as it allowed to use specialized replacements such as lightweight filesystems and memory access.[citation needed] This allowed performance boosts, especially for graphical applications.[citation needed]

Drawbacks

  • The application cannot co-exist with other installed applications (because of the insulation described in the previous section).
  • Bypassing the normal operating system forced the developers to anticipate every hardware-related or driver-related issue. They would have to write drivers themselves. This could cause compatibility issues on unusual equipment.

Examples

  • A scaled down version of GeoWorks was used by America Online for their DOS-based AOL client software until the late 1990s. AOL was distributed on a single 3.5-inch floppy disk, which could be used to boot GeoWorks as well.
  • In 1998 Caldera distributed a demo version of their 32-bit DPMI extended DOS web-browser and mail client DR-WebSpyder on a bootable fully self-contained 3.5-inch floppy. On 386 PCs with a minimum of 4 MB of RAM, the floppy would boot a minimal DR-DOS 7.02 system complete with memory manager, RAM disk, dial-up modem, LAN, mouse and display drivers and automatically launch into the graphical browser, without ever touching the machine's hard disk. Users could start browsing the web immediately after entering their access credentials.

See also

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