What does boot mean?
To boot (to boot up, to start up or booting) a computer is to load an operating system (OS) into the computer's main memory or RAM. Once the OS is loaded (for example, on a PC, you will see the initial Windows or Mac desktop screen), it's ready for users to run applications.
Sometimes, a user may be instructed to reboot the operating system, which will reload the OS. The most familiar way to reboot on PCs is to press the Ctrl, Alt and Delete keys at the same time.
On larger computers (including mainframes), the equivalent term for boot is initial program load (IPL); for reboot, it is re-IPL. Boot is also used as a noun for the act of booting, as in a system boot.
What is bootstrap?
The term boot is derived from bootstrap, a small strap or loop at the back of a leather boot that enables you to pull the boot on. There is also the expression "pulling yourself up by your own bootstraps," which means to achieve success from a modest beginning. The booting of an OS works by loading a small program into the computer and then giving that program control so that it, in turn, loads the entire operating system.
Booting or loading an OS is different from installing it, which is generally an initial one-time activity. These days, most computers come with an OS preloaded. But users may install a different OS that is compatible with the computer hardware.
When installing an OS, users may be asked to identify specific options or configuration choices. At the end of the installation, the OS is stored on a hard disk or solid-state drive and is ready to be booted (loaded) into RAM. A computer's memory is closer to the microprocessor and is faster to work with compared to accessing data on storage drives. Typically, when an OS is installed, it is set up so that when you turn the computer on, the system is also automatically booted.
If memory resources become unavailable or an application program encounters an error, an error message may appear, or the screen may freeze, preventing further operation. When those events occur, it may be necessary to reboot the OS.
What is a bootloader?
A bootloader is a program that runs on a computer or device before the OS starts. Its primary purpose is to load the OS kernel into memory and start it.
The bootloader is typically stored on the device's firmware, such as the BIOS or unified extensible firmware interface or in a dedicated boot partition on the storage device. When the computer starts up, the firmware looks for the bootloader and loads it into memory. Once the bootloader is in memory, it takes control of the boot process and loads the OS kernel.
It's important to note that bootloader is a term that is mainly used in the context of traditional computers and servers. Still, the concept of a bootloader also exists in embedded systems, mobile devices, IoT devices and other embedded systems.
What is a boot menu?
A boot menu is a menu that may be displayed by the bootloader when a computer starts up. It typically provides a list of bootable devices or OSes from which a user can make selections. A user can select one of the options to continue the boot process and load the corresponding operating system.
The boot menu may allow a user to select an OS or boot device different from the defaults or select other options or configurations. For example, a different OS may be selected to boot if the computer has multiple OSes installed.
A boot menu listing may include options such as these:
- Network boot
- Rescue mode
How booting works
When a computer is turned on, it's likely that the OS has been set up to boot automatically in this sequence:
- As soon as the computer is turned on, the BIOS on the system's read-only memory (ROM) chip is "awakened" and takes charge. BIOS is already loaded because it's built-in to the ROM chip. Unlike RAM, ROM contents don't get erased when the computer is turned off.
- BIOS first does a power-on self-test to ensure that all components are operational. Then, the BIOS's boot program looks for the special boot programs that load the OS from the drive.
- The boot process searches through all possible storage configurations until it finds the specific place where OS boot files are located.
- Having identified the drive where boot files are located, the BIOS next looks at the first sector and copies information from it into specific locations in RAM. This information is known as the boot record or master boot record.
- The boot record is then loaded into a specific place in RAM (hexadecimal address 7C00).
- The boot record contains a program to which BIOS now branches to give the boot record control of the computer.
- The boot record loads the initial system file (for example, Windows NTFS) into RAM from the diskette or hard disk.
- The rest of the OS is then loaded into RAM. At this point, the boot record is no longer needed and can be overlaid with other data.
- The initial file loads a system file that knows how to work with the BIOS.
- One of the first OS files loaded is a system configuration file that tells the loading program which specific OS files are needed such as a device driver.
- Another special file tells which specific applications or commands the user wants to have included or performed as part of the boot process. In DOS, this file is named AUTOEXEC.BAT.; in Windows, it's called WIN.INI.
- After all OS files have been loaded, the OS controls the computer. It performs the requested initial commands and waits for the first interactive user input.