Bootloader: What you need to know about the system boot manager

For a computer processor to be able to launch applications it relies on the services of random-access memory (RAM), also known as the main memory. The data required is loaded into the main memory for this purpose, which is why its capacity, transmission rate and access speed play a decisive role in the maximum performance capacity of the entire computer.

One of the most important programs with data in the main memory is the operating system. Equipped with a suitable process management function, Windows, macOS, and others actually regulate the processor access to the main storage, among other things. Consequently, the relevant data in the system software has to be loaded into the main memory as soon as the device is started, and stays there for the whole time the computer is running. This is made possible by a so-called bootloader, which comes permanently integrated as a standard in most modern operating systems.

What is a bootloader?

A bootloader, also known as a boot program or bootstrap loader, is a special operating system software that loads into the working memory of a computer after start-up. 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”.

Note

The term “bootloader” is a shortened form of the words “bootstrap loader”. The term stems from the fact that the boot manager is the key component in starting up the computer, so it can be likened to the support of a bootstrap when putting a boot on.

How does a bootloader work?

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.

Tip

As soon as you have installed an operating system properly it will launch via a bootloader, which is typically located on the main partition. If the system is damaged, however, and ought to be installed again, you are dependent on the bootable removable media mentioned previously. This also applies if no operating system is installed. You can find out how to transform a standard USB stick into a fully functional boot medium in our guide.

Where exactly are bootloaders stored?

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).

Note

Bootloaders can also be spread across several layers that build upon each other. These multi-stage bootloaders consist of up to three different levels that are launched in sequence. They are typically used if the boot program is too big for the boot sector.

A summary of a bootloader’s functions

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.

What well-known bootloaders are there?

The following table lists the most important bootloaders:

Name of the bootloader Description
Bootmgr boot program for Microsoft systems since Windows Vista and Windows Server 2008
NT loader (NTLDR) boot program for Microsoft systems until Windows XP and Windows Server 2003
barebox bootloader for embedded systems in printers, cameras, cars, airplanes, and more
boot.efi EFI bootloader that has been used in Mac devices since 2006
BootX former bootloader for Mac operating systems
Grand Unified Bootloader (GRUB) free boot program for Unix-like operating systems such as Linux
ARM Core Bootloader bootloader for microcontrollers (used in iPhones among others)
OpenBIOS free, portable boot manager under a GNU-GPL license

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