Bootloader

A bootloader, also spelled as boot loader or called bootstrap loader, is a computer program that is responsible for booting a computer and booting an operating system. If it also provides an interactive menu with multiple boot choices then it's often called a boot manager.

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 the boot ROM, which is read-only memory (ROM, and later EEPROM, NOR flash) along with some needed data, to initialize hardware devices such as CPU, motherboard, memory, storage and other I/O devices, to access the nonvolatile device (usually block device, e.g., 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/basic input output system 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.

Examples of first-stage bootloaders include BIOS, UEFI, coreboot, Libreboot, and Das U-Boot. It initializes hardware devices such as CPU, motherboard, memory, storage and other I/O devices.

Second-stage boot loaders, such as GNU GRUB, rEFInd, BOOTMGR, Syslinux, and NTLDR, 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.

Second-stage implementations can include interactive user interfaces, allowing boot option selection and parameter modification. They handle kernel loading, including processing of initrd/initramfs images, and can pass boot parameters to the kernel. Many implement modular designs supporting loadable modules for additional functionality. 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. They may also handle compression, cryptographic verification, and chain-loading of other bootloaders. 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.

In x86 computers, after the BIOS executes Power-On Self Test, then a first-stage bootloader is a compact 512-byte program that resides in the master boot record (MBR) is executed. Running in 16-bit real mode at address 0x7C00, it locates the second-stage bootloader. Its primary challenge lies in accomplishing these tasks within strict size constraints while handling potential hardware failures. The bootloader must navigate disk structures, often implementing FAT file system support, and manage the delicate transition from the BIOS startup state to a stable environment for the next boot stage.