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This article is about booting, in the sense of starting a computer. For other meanings, see Booting (disambiguation).

In computing, booting is a bootstrapping process that starts operating systems when the user turns on a computer system. A boot sequence is the set of operations the computer performs when it is switched on which load an operating system.

Boot loader

Most computer systems can only execute code found in the memory (ROM or RAM); modern operating systems are mostly stored on hard disks (occasionally LiveCDs, USB flash drives, and the like). Just after a computer has been turned on, it doesn't have an operating system in memory. The computer's hardware alone cannot perform complicated actions of the operating system, such as loading a program from disk; so a seemingly irresolvable paradox is created: to load the operating system into memory, one appears to need to have an operating system already installed.

The solution to the paradox involves using a special small program, called a bootstrap loader or boot loader. This program doesn't have the full functionality of an operating system, but is tailor-made to load enough other software for the operating system to start. Often, multiple-stage boot loaders are used, in which several small programs summon each other, until the last of them loads the operating system. The name bootstrap loader comes from the image of one pulling oneself up by one's bootstraps (see bootstrapping).

Early programmable computers had toggle switches on the front panel to allow the operator to place the bootloader into the program store before starting the CPU. This would then read the operating system in from an outside storage medium such as paper tape, punched card, or an old fixed head disk drive.

Pseudo-assembly code for the bootloader might be as simple as the following eight instructions:

0: set the P register to 8
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: if end of tape, jump to 8
5: store accumulator to address in P register
6: increment the P register
7: jump to 1

A related example is based on a loader for a 1970's Nicolet Instrument Corporation minicomputer. Note that the bytes of the second-stage loader are read from paper tape in reverse order.

0: set the P register to 106
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: store accumulator to address in P register
5: decrement the P register 
6: jump to 1

The length of the second stage loader is such that the final byte overwrites location 6. After the instruction in location 5 executes, location 6 starts the second stage loader executing. The second stage loader then waits for the much longer tape containing the operating system to be placed in the tape reader. The difference between the boot loader and second stage loader is the addition of checking code to trap paper tape read errors, a frequent occurrence with the hardware of the time, which in this case was an ASR-33 teletype.

In modern computers the bootstrapping process begins with the CPU executing software contained in ROM (for example, the BIOS of an IBM PC) at a predefined address (the CPU is designed to execute this software after reset without outside help). This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section (most commonly the boot sector) of the most promising device.

Boot loaders may face peculiar constraints, especially in size; for instance, on the IBM PC and compatibles, the first stage of boot loaders must fit into the first 446 bytes of the Master Boot Record, in order to leave room for the 64-byte partition table and the 2-byte AA55h 'signature', which the BIOS requires for a proper boot loader.

Some operating systems, most notably pre-1995 Macintosh systems from Apple Computer, are so closely interwoven with their hardware that it is impossible to natively boot an operating system other than the standard one. A common solution in such situations is to design a bootloader that works as a program belonging to the standard OS that hijacks the system and loads the alternative OS. This technique was used by Apple for its A/UX Unix implementation and copied by various freeware operating systems and BeOS Personal Edition 5.

Second-stage boot loader

File:NTLDR Menu.png

NTLDR – A second stage boot loader

The small program is most often not itself an operating system, but only a second-stage boot loader, such as NTLDR, LILO or GRUB. It will then be able to load the operating system proper, and finally transfer execution to it. The system will initialize itself, and may load device drivers and other programs that are needed for the normal operation of the OS.

The boot process is considered complete when the computer is ready to interact with the user or the operating system is capable of running ordinary applications. Typical modern PCs boot in about a minute (of which about 15 seconds are taken by the preliminary boot loaders, and the rest by loading the operating system), while large servers may take several minutes to boot and to start all services - to ensure high availability, they bring up some services before others.

Most embedded systems must boot almost instantly -- for instance, waiting a minute for the television to come up is not acceptable. Therefore they have their whole operating system in ROM or flash memory, so it can be executed directly.

The FreeBSD Booting Process

Loading a kernel
Determine the root filesystem
Initialize user-land things
Interesting combinations

BIOS boot devices

A boot device is any device that must be initialized prior to loading the operating system. This includes the primary input device (keyboard), the primary output device (display), and the initial program load device (floppy drive, hard drive, CD-ROM, USB flash drive, etc.). (An IPL device is any device in the system that can boot and load an operating system, a stand alone utility (i.e. memtest86+) or even a boot loader; in old AT machines, this is the floppy drive or hard drive.)

In a modern BIOS, the user can select one of several interfaces from which to boot. These include: hard disk, floppy, SCSI, CDROM, Zip, LS-120, a network interface card using PXE, or USB (USB-FDD, USB-ZIP, USB-CDROM, USB-HDD).

For example, one can install Microsoft Windows on the first hard disk and Linux on the second. By changing the BIOS boot device, the user can select the operating system to load.

Boot sequence on standard PC (IBM-PC compatible)

File:Pc boot.jpg

A PC going through its boot sequence

Upon starting, a personal computer's x86 CPU runs the instruction located at the memory location F000:FF00 (on 286s and 386SXs, the base of the code segment is actually 0xFF0000 and on 386s it is 0xFFFF0000) of the BIOS. This memory location is close to the end of system memory. It contains a jump instruction that transfers execution to the location of the BIOS start-up program. This program runs a Power-On Self Test (POST) to check that devices the computer will rely on are functioning; it also initializes these devices. Then, the BIOS goes through a preconfigured list of devices until it finds one that is bootable. If it finds no such device, an error is given and the boot process stops. If the BIOS finds a bootable device, it loads and executes its boot sector. In the case of a hard drive, this is referred to as the master boot record (MBR) and is often not operating system specific. Usually, the MBR code checks the partition table for an active partition. If one is found, the MBR code loads that partition's boot sector and executes it. The boot sector is often operating system specific, however in most operating systems its main function is to load and execute a kernel, which continues startup. If there is no active partition or the active partition's boot sector is invalid, the MBR may load a secondary boot loader and pass control to it and this secondary boot loader will select a partition (often via user input) and load its boot sector, which usually loads the corresponding operating system Kernel.

Other kinds of boot sequence

Some other processors have other kinds of boot modes; most digital signal processors have the following boot modes:

Serial mode boot
Parallel mode boot
HPI boot
Warm boot or soft reboot (as opposed to hard reboot) refers to an abridged start up which does not require that power be removed and reapplied.

Random reboot

Random reboot is an informal reference to an unintended reboot whose cause is not immediately evident. It may occur because of:

software problems, including
- incompatible software
- improper software or driver configuration
- buggy code
- computer virus or malicious software
hardware problems, including
- incompatible hardware
- improper hardware configuration
- Not enough power (electricity needed exceeds the rating of the PSU).
- loose connections
  - improper memory timing
- defective/damaged hardware
  - failing hard drive (especially in TiVos)
- overheating
  - CPU overheating
other problems, including
- electrical problems.
  - power surge
- electromagnetic interference
- space radiation (very unlikely)

A good resource for troubleshooting random reboots is located at http://www.hardwareanalysis.com/content/topic/57410/ [1]