Boot Definition

To boot a computer is to start the computer.

A boot sequence, also called a boot process, boot routine or bootstrap routine, is the set of operations a computer begins performing when the electric power is switched on and continues until it is ready to use. The main thing that occurs is the copying the operating system from a storage device, typically the hard disk drive (HDD), into main memory (which is composed of random access memory chips, or RAM) so that it can be directly accessed by the central processing unit (CPU).

Bootable means that a computer can be started and attain a state sufficient that any desired application programs can be run on it. The term is also used to refer to any removable storage device or any software that contains sufficient components of an operating system and other necessary utilities (e.g., for file decompression) such that it can be loaded into a computer's main memory and allow the computer to start up.

A boot sector is a region of a HDD, floppy disk or other storage device (usually the first sector) that is loaded into memory and executed as a part of the boot sequence. It usually contains a very small program (a few hundred bytes) that loads the operating system into memory and then transfers control to it. A sector is is a segment of a track (i.e., a concentric circle around a disk) that constitutes the smallest unit of storage that can be accessed on a HDD or floppy disk by the disk drive mechanism.

A multibooting system is one which can be started with either of two or more operating systems. The operating systems can be stored on different partitions on the same HDD or on separate HDDs. A partition is a logically independent section of a HDD. Multibooting can offer several advantages, including the convenience of having several operating systems on a single computer and the ability to boot into an alternative operating system should one system unbootable (due to file corruption, damage to the magnetic media, etc.).

In a broader sense, almost any computer is a multibooting computer in that it can be booted from some external storage medium or device, such as a live CD, or via a network. A live CD is is a bootable, read-only compact disk (CDROM) that contains an operating system (and often application programs as well) that can be loaded into memory so that the computer can be operated without the need to install anything on the HDD.

Perhaps the best known example of a bootable CDROM is the Knoppix liveCD. Knoppix is a distribution (i.e., version) of Linux that is based on the highly regarded Debian distribution. Thus, the Knoppix allows virtually any computer with an x86 (i.e., Intel-compatible) processor to be converted almost immediately into a Linux computer. Although designed specifically for live CD use, it is also a relatively easy matter to install the Knoppix distribution on a HDD so that the computer can boot directly into it without without need to insert the CDROM1. There are also bootable floppies, among the most popular of which is muLinux, which is a miniaturized distribution of Linux, inclusive of numerous command line programs, that fits on a single floppy disk.

To reboot means to restart a computer. A cold reboot is restarting computer by turning the power off and then back on. A warm reboot is restarting a computer that is already on by just reloading the memory and without turning the power off.

The booting or rebooting process is completed when the operating system is capable of running ordinary application programs. Typical modern personal computers require roughly a minute to boot, of which about a quarter is consumed by the boot loader and the remainder by the loading of the operating system, although the time can vary considerably according to the operating system and the hardware. Large servers will likely require several minutes to boot and commence all services. Most embedded systems (i.e., single chip or single circuit board computers built into other products) boot nearly instantaneously because they store their operating system or other programs in flash memory or read-only memory (ROM), both of which retain their contents when the power supply is disconnected.

The continued growth in the size of operating systems has been a factor in increasing the time required for booting of personal computers. However, this has been offset by the large increases in the speeds of the hardware, particularly the HDDs, memories and microprocessors. It is likely that boot times will be dramatically reduced in the future by storing major components of the operating system in flash memory chips so that they do not need to be copied from the HDD each time the computer is turned on. This will become practical as the price for flash memory continues to fall and reduces the gap with the price of conventional RAM.

The word boot, when used in a computer context, is short for the word bootstrap, which is a strap that was attached to the top of a boot to help pull the boot on2. Bootstrap utilities are small programs that help the computer get started and load the operating system.

1Among the advantages of installation directly on the HDD are faster boot times, faster operation and a complete freeing of the CDROM drive for other uses. For a tutorial on how to install Knoppix on a HDD from a bootable CD, see How to Install Knoppix on a Hard Drive, The Linux Information Project, March 2004.

2This is the origin of the somewhat archaic expression to pull oneself up by the bootstraps.

Created January 23, 2006.
Copyright © 2006 The Linux Information Project. All Rights Reserved.