When a computer is turned off, its software—including operating systems, application code, and data—remains stored on non-volatile memory. When the computer is powered on, it typically does not have an operating system or its loader in random-access memory (RAM). The computer first executes a relatively small program stored in read-only memory (ROM, and later EEPROM, NOR flash) along with some needed data, to initialize RAM (especially on x86 systems), to access the nonvolatile device (usually block device, eg NAND flash) or devices from which the operating system programs and data can be loaded into RAM.
Some earlier computer systems, upon receiving a boot signal from a human operator or a peripheral device, may load a very small number of fixed instructions into memory at a specific location, initialize at least one CPU, and then point the CPU to the instructions and start their execution. These instructions typically start an input operation from some peripheral device (which may be switch-selectable by the operator). Other systems may send hardware commands directly to peripheral devices or I/O controllers that cause an extremely simple input operation (such as "read sector zero of the system device into memory starting at location 1000") to be carried out, effectively loading a small number of boot loader instructions into memory; a completion signal from the I/O device may then be used to start execution of the instructions by the CPU.
Smaller computers often use less flexible but more automatic boot loader mechanisms to ensure that the computer starts quickly and with a predetermined software configuration. In many desktop computers, for example, the bootstrapping process begins with the CPU executing software contained in ROM (for example, the BIOS of an IBM PC or an IBM PC compatible) at a predefined address (some CPUs, including the Intel x86 series are 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, typically starting at a fixed entry point such as the start of the sector.
First-stage boot loader
Boot loaders may face peculiar constraints, especially in size; for instance, on the earlier IBM PC and compatibles, a boot sector should typically work in only 32 KB (later relaxed to 64 KB) of system memory and only use instructions supported by the original 8088/8086 processors. The first stage of PC boot loaders (FSBL, first-stage boot loader) located on fixed disks and removable drives must fit into the first 446 bytes of the Master Boot Record in order to leave room for the default 64-byte partition table with four partition entries and the two-byte boot signature, which the BIOS requires for a proper boot loader — or even less, when additional features like more than four partition entries (up to 16 with 16 bytes each), a disk signature (6 bytes), a disk timestamp (6 bytes), an Advanced Active Partition (18 bytes) or special multi-boot loaders have to be supported as well in some environments. In floppy and superfloppy Volume Boot Records, up to 59 bytes are occupied for the Extended BIOS Parameter Block on FAT12 and FAT16 volumes since DOS 4.0, whereas the FAT32 EBPB introduced with DOS 7.1 requires even 87 bytes, leaving only 423 bytes for the boot loader when assuming a sector size of 512 bytes. Microsoft boot sectors therefore traditionally imposed certain restrictions on the boot process, for example, the boot file had to be located at a fixed position in the root directory of the file system and stored as consecutive sectors, conditions taken care of by the
SYS command and slightly relaxed in later versions of DOS.[nb 1] The boot loader was then able to load the first three sectors of the file into memory, which happened to contain another embedded boot loader able to load the remainder of the file into memory. When Microsoft added LBA and FAT32 support, they even switched to a boot loader reaching over two physical sectors and using 386 instructions for size reasons. At the same time other vendors managed to squeeze much more functionality into a single boot sector without relaxing the original constraints on only minimal available memory (32 KB) and processor support (8088/8086).[nb 2] For example, DR-DOS boot sectors are able to locate the boot file in the FAT12, FAT16 and FAT32 file system, and load it into memory as a whole via CHS or LBA, even if the file is not stored in a fixed location and in consecutive sectors.[nb 3][nb 2]
Second-stage boot loader
Second-stage boot loaders, such as GNU GRUB, rEFInd, BOOTMGR, Syslinux, NTLDR or iBoot, are not themselves operating systems, but are able to load an operating system properly and transfer execution to it; the operating system subsequently initializes itself and may load extra device drivers. The second-stage boot loader does not need drivers for its own operation, but may instead use generic storage access methods provided by system firmware such as the BIOS or Open Firmware, though typically with restricted hardware functionality and lower performance.
Many boot loaders (like GNU GRUB, rEFInd, Windows's BOOTMGR, Syslinux, and Windows NT/2000/XP's NTLDR) can be configured to give the user multiple booting choices. These choices can include different operating systems (for dual or multi-booting from different partitions or drives), different versions of the same operating system (in case a new version has unexpected problems), different operating system loading options (e.g., booting into a rescue or safe mode), and some standalone programs that can function without an operating system, such as memory testers (e.g., memtest86+), a basic shell (as in GNU GRUB), or even games (see List of PC Booter games). Some boot loaders can also load other boot loaders; for example, GRUB loads BOOTMGR instead of loading Windows directly. Usually, a default choice is preselected with a time delay during which a user can press a key to change the choice; after this delay, the default choice is automatically run so normal booting can occur without interaction.
The boot process can be considered complete when the computer is ready to interact with the user, or the operating system is capable of running system programs or application programs.
Many embedded systems must boot immediately. For example, waiting a minute for a digital television or a GPS navigation device to start is generally unacceptable. Therefore, such devices have software systems in ROM or flash memory so the device can begin functioning immediately; little or no loading is necessary, because the loading can be precomputed and stored on the ROM when the device is made.
Large and complex systems may have boot procedures that proceed in multiple phases until finally the operating system and other programs are loaded and ready to execute. Because operating systems are designed as if they never start or stop, a boot loader might load the operating system, configure itself as a mere process within that system, and then irrevocably transfer control to the operating system. The boot loader then terminates normally as any other process would.
Most computers are also capable of booting over a computer network. In this scenario, the operating system is stored on the disk of a server, and certain parts of it are transferred to the client using a simple protocol such as the Trivial File Transfer Protocol (TFTP). After these parts have been transferred, the operating system takes over the control of the booting process.
As with the second-stage boot loader, network booting begins by using generic network access methods provided by the network interface's boot ROM, which typically contains a Preboot Execution Environment (PXE) image. No drivers are required, but the system functionality is limited until the operating system kernel and drivers are transferred and started. As a result, once the ROM-based booting has completed it is entirely possible to network boot into an operating system that itself does not have the ability to use the network interface.
- The PC DOS 5.0 manual incorrectly states that the systen files no longer need to be contiguous. However, for the boot process to work the system files still need to occupy the first two directory entries and the first three sectors of IBMBIO.COM still need to be stored contiguously. SYS continues to take care of these requirements.
- As an example, while the extended functionality of DR-DOS MBRs and boot sectors compared to their MS-DOS/PC DOS counterparts could still be achieved utilizing conventional code optimization techniques up to 7.05, for the addition of LBA, FAT32 and LOADER support the 7.07 sectors had to resort to self-modifying code, opcode-level programming, controlled utilization of side effects, multi-level data/code superpositioning and algorithmic folding techniques to squeeze everything into a single physical sector, as it was a requirement for backward- and cross-compatibility with other operating systems in multi-boot and chain load scenarios.
- There is one exception to the rule that DR-DOS VBRs will load the whole IBMBIO.COM file into memory: If the IBMBIO.COM file is larger than some 29 KB, trying to load the whole file into memory would result in the boot loader to overwrite the stack and relocated Disk Parameter Table (DPT/FDPB). Therefore, a DR-DOS 7.07 VBR would only load the first 29 KB of the file into memory, relying on another loader embedded into the first part of IBMBIO.COM to check for this condition and load the remainder of the file into memory by itself if necessary. This does not cause compatibility problems, as IBMBIO.COM's size never exceeded this limit in previous versions without this loader. Combined with a dual entry structure this also allows the system to be loaded by a PC DOS VBR, which would load only the first three sectors of the file into memory.
- "GNU GRUB - GNU Project - Free Software Foundation (FSF)". www.gnu.org. Retrieved 2021-09-27.
- "systemd-boot". www.freedesktop.org. Retrieved 2021-09-27.
- Paul, Matthias R. (1997-10-02) [1997-09-29]. "Caldera OpenDOS 7.01/7.02 Update Alpha 3 IBMBIO.COM - README.TXT and BOOT.TXT - A short description of how OpenDOS is booted". Archived from the original on 2003-10-04. Retrieved 2009-03-29. 
- Sakamoto, Masahiko (2010-05-13). "Why BIOS loads MBR into 7C00h in x86?". Glamenv-Septzen.net. Retrieved 2012-08-22.
- Compaq Computer Corporation; Phoenix Technologies Ltd; Intel Corporation (1996-01-11). "BIOS Boot Specification 1.01" (PDF). Retrieved 2017-12-21.
- Zbikowski, Mark; Allen, Paul; Ballmer, Steve; Borman, Reuben; Borman, Rob; Butler, John; Carroll, Chuck; Chamberlain, Mark; Chell, David; Colee, Mike; Courtney, Mike; Dryfoos, Mike; Duncan, Rachel; Eckhardt, Kurt; Evans, Eric; Farmer, Rick; Gates, Bill; Geary, Michael; Griffin, Bob; Hogarth, Doug; Johnson, James W.; Kermaani, Kaamel; King, Adrian; Koch, Reed; Landowski, James; Larson, Chris; Lennon, Thomas; Lipkie, Dan; McDonald, Marc; McKinney, Bruce; Martin, Pascal; Mathers, Estelle; Matthews, Bob; Melin, David; Mergentime, Charles; Nevin, Randy; Newell, Dan; Newell, Tani; Norris, David; O'Leary, Mike; O'Rear, Bob; Olsson, Mike; Osterman, Larry; Ostling, Ridge; Pai, Sunil; Paterson, Tim; Perez, Gary; Peters, Chris; Petzold, Charles; Pollock, John; Reynolds, Aaron; Rubin, Darryl; Ryan, Ralph; Schulmeisters, Karl; Shah, Rajen; Shaw, Barry; Short, Anthony; Slivka, Ben; Smirl, Jon; Stillmaker, Betty; Stoddard, John; Tillman, Dennis; Whitten, Greg; Yount, Natalie; Zeck, Steve (1988). "Technical advisors". The MS-DOS Encyclopedia: versions 1.0 through 3.2. By Duncan, Ray; Bostwick, Steve; Burgoyne, Keith; Byers, Robert A.; Hogan, Thom; Kyle, Jim; Letwin, Gordon; Petzold, Charles; Rabinowitz, Chip; Tomlin, Jim; Wilton, Richard; Wolverton, Van; Wong, William; Woodcock, JoAnne (Completely reworked ed.). Redmond, Washington, USA: Microsoft Press. ISBN 1-55615-049-0. LCCN 87-21452. OCLC 16581341. (xix+1570 pages; 26 cm) (NB. This edition was published in 1988 after extensive rework of the withdrawn 1986 first edition by a different team of authors. )
- Chappell, Geoff (January 1994). "Chapter 2: The System Footprint". In Schulman, Andrew; Pedersen, Amorette (eds.). DOS Internals. The Andrew Schulman Programming Series (1st printing, 1st ed.). Addison Wesley Publishing Company. ISBN 978-0-201-60835-9. (xxvi+738+iv pages, 3.5"-floppy ) Errata: 
- Rosch, Winn L. (1991-02-12). "DR DOS 5.0 - The better operating system?". PC Magazine. Vol. 10 no. 3. p. 241-246, 257, 264, 266. Archived from the original on 25 July 2019. Retrieved 2019-07-26.
[…] SYS has been improved under DR DOS 5.0 so you don't have to worry about leaving the first cluster free on a disk that you want to make bootable. The DR DOS system files can be located anywhere on the disk, so any disk with enough free space can be set to boot your system. […](NB. The source attributes this to the SYS utility while in fact this is a feature of the advanced bootstrap loader in the boot sector. SYS just plants this sector onto the disk.)
- Paul, Matthias R. (2001-01-17). "FAT32 in DR-DOS". opendos@delorie. Archived from the original on 2017-10-06. Retrieved 2017-10-06.
[…] The DR-DOS boot sector […] searches for the IBMBIO.COM (DRBIOS.SYS) file and then loads the *whole* file into memory before it passes control to it. […]
- Paul, Matthias R. (2002-02-20). "Can't copy". opendos@delorie. Archived from the original on 2017-10-06. Retrieved 2017-10-06.
[…] The DR-DOS boot sector loads the whole IBMBIO.COM file into memory before it executes it. It does not care at all about the IBMDOS.COM file, which is loaded by IBMBIO.COM. […] The DR-DOS boot sector […] will find the […] kernel files as long as they are logically stored in the root directory. Their physical location on the disk, and if they are fragmented or not, is don't care for the DR-DOS boot sector. Hence, you can just copy the kernel files to the disk (even with a simply COPY), and as soon as the boot sector is a DR-DOS sector, it will find and load them. Of course, it is difficult to put all this into just 512 bytes, the size of a single sector, but this is a major convenience improvement if you have to set up a DR-DOS system, and it is also the key for the DR-DOS multi-OS LOADER utility to work. The MS-DOS kernel files must reside on specific locations, but the DR-DOS files can be anywhere, so you don't have to physically swap them around each time you boot the other OS. Also, it allows to upgrade a DR-DOS system simply by copying the kernel files over the old ones, no need for SYS, no difficult setup procedures as required for MS-DOS/PC DOS. You can even have multiple DR-DOS kernel files under different file names stored on the same drive, and LOADER will switch between them according to the file names listed in the BOOT.LST file. […]
- Paul, Matthias R. (2017-08-14) [2017-08-07]. "The continuing saga of Windows 3.1 in enhanced mode on OmniBook 300". MoHPC - the Museum of HP Calculators. Archived from the original on 2017-10-06. Retrieved 2017-10-06.
[…] the DR-DOS FDISK does not only partition a disk, but can also format the freshly created volumes and initialize their boot sectors in one go, so there's no risk to accidentally mess up the wrong volume and no need for FORMAT /S or SYS. Afterwards, you could just copy over the remaining DR-DOS files, including the system files. It is important to know that, in contrast to MS-DOS/PC DOS, DR-DOS has "smart" boot sectors which will actually "mount" the file-system to search for and load the system files in the root directory instead of expecting them to be placed at a certain location. Physically, the system files can be located anywhere and also can be fragmented. […]
- "Chapter 6 - Troubleshooting Startup and Disk Problems". Windows NT Server Resource Kit. Microsoft. Archived from the original on 2007-05-15.
- "Tint". coreboot. Retrieved 2010-11-20.