Booting refers to the loading of the operating system of a computer, as it is usually required after powering up. The word booting is a short form of bootstrap loading. The phrase originated in 1950s. In computer science, bootstrapping refers to a process that activates a more complex system on a simple system. It is a solution to the chicken-and-egg problem of starting a system by itself. The term is often used for the startup process of a computer, where a mechanism is needed to start the software that is responsible for starting additional software (operating system). The start of a software installation, in which an upgrade of the installer is carried out first, is also referred to as bootstrapping.
The boot process of a computer proceeds in several stages. After switching on, a simple program is first read from a small fixed-value read only memory (ROM). This program allows you to start a more complex program, which then starts an operating system, for example. Early computers (before the 1970s) often had no fixed value memory (ROM) available, so the first stage of the boot process had to be written to the memory by hand using the machine console (keyboard) so that the operating system could then be read by external storage devices. On all currently used computers and computer-controlled devices/systems, the boot process starts automatically after switching on.
When booting a PC, the processor begins to process the system firmware stored at a specified memory address in the ROM. In general, it performs a test of the connected devices (POST) and checks storage devices such as hard disks or CD/DVD drives, whether they are bootable or contain bootable media. Depending on the specific firmware implementation, the search order according to which these devices are accessed can be changed, e.g. via firmware setup or via boot menu.
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For IBM PC-compatible computers up to the 2010s, this firmware is usually the BIOS. For PCs after about 2010, it was increasingly replaced by the (Unified) Extensible Firmware Interface, EFI or UEFI for short. Apple’s Intel Macs use an Apple-specific EFI.
Bootable media requires a valid boot sector on a PC with BIOS. On larger data storage devices such as hard disks, this is usually the Master Boot Record (MBR), which also contains the partition table. The process of booting begins when the boot sector is loaded and executed by the BIOS. Since the boot sector is limited to a block size of 512 bytes, it usually contains a boot loader that loads further data, for example by searching for special files on the storage medium and then loading and executing them until, for example, the kernel and finally the entire operating system has been started. This process, in which one program loads the next, is also known as chain loading. With boot managers that hook into this chain at an early stage, it is possible to change the boot process and, for example, implement an additional boot menu for multi-boot systems.
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The EFI loader is also called a boot loader. Bootloaders are computer programs whose task is to advance the boot process. In a few cases, the bootloader is already the last stage in the boot process, for example in PC booters.
When installing an operating system, the first thing to boot from is bootable media such as a CD or DVD in an optical drive or from a USB stick. However, the firmware must support this type of bootable media. This usually contains a customized version of the operating system to be installed – if the boot process of this installation medium is successful, it automatically loads the installation program of the operating system. This sets up a working boot configuration for the respective system on the selected installation target, a data storage device such as a hard disk, such as boot sectors and boot files on the corresponding partitions.
