What is a bootloader and how does it work?

Bootloaders ensure that all relevant operating system data is loaded into the main memory when a device is started. During the boot process, the firmware transmits the required information.

Data of an operating system must be loaded into the working memory during device start-up. This is made possible by a so-called bootloader, also known as a boot program or bootstrap loader. For this purpose, immediately after a device starts, a bootloader is generally launched by a bootable medium like a hard drive, a CD/DVD, or a USB stick. The boot medium receives information from the computer’s firmware (e.g. BIOS) about where the bootloader is. The whole process is also described as “booting”.

When you press the start button on a computer, the very first thing you see on the screen is information about the hardware installed. The software responsible for this notification is the device firmware mentioned above, which is usually implemented by manufacturers in flash memory on the computer’s motherboard. With most desktop PCs and notebooks this will be the BIOS (Basic Input/Output System) or the more modern UEFI (Unified Extensible Firmware Interface). Both applications collect the most diverse hardware data and create a complete list of all of the device’s available drives.

When this process is complete, the firmware goes through the data carriers found in sequence, checking for a bootloader by means of a special signature – the so-called boot signature (or “boot record”). The search always starts on the removable media (CD/DVD, USB stick, external hard drive, etc.), followed by the hard-coded drives. With the latter, the bootloader and its signature is generally in the Master Boot Record (MBR), which also contains the data carrier’s partition tables. When a bootloader is found, it is loaded and the system start is initiated. If the search is unsuccessful, the firmware will return an error message.

Bootloaders can be stored in two different places:

1. The bootloader is stored in the first block of the bootable medium.
2. The bootloader is stored on a specific partition of the bootable medium.

The first variant is closely connected to the principle of master boot records, which don’t just contain the link to the bootloader required by the firmware, but the boot software itself. The first available memory block or sector in the medium is always reserved for the record. Because of this important function, it is also known as the boot block or boot sector.

In the second case, the operating system uses a selected partition as the storage location for the bootloader, though the underlying file system and the partition tables used can vary greatly. The decisive factor here is always the firmware, which stipulates a specific file format for the boot manager in this bootloader storage model. For devices with UEFI, for example, the format is PE/COFF (Portable Executable / Common Object File Format).

Bootloaders serve as a mediator between hardware and the operating system. As soon as a bootloader has been initialized by the respective firmware, it has system responsibility to get the boot process going. The first task is to load the main memory, which is essential for the processor to work.

In the second step, the bootloader loads the kernel of the operating system, that is, the primary component of the system software that controls all storage and processor permissions and contains all important drivers. The bootstrap loader also processes different routine tasks and commands, e.g. integrating data storage. Some bootloaders also perform tasks beyond starting up software, including:

• Identifying and starting other available bootloaders
• Launching application programs (frequently used in the 1980s to launch computer games directly from a disk)
• Correcting or expanding missing functions and entries in the firmware
• Loading alternative firmware

After successfully completing all tasks, the bootloader returns the system responsibility to the kernel.

The following table lists the most important bootloaders: