The first step of the Linux boot process really has nothing whatever to do with Linux and is the same whether the boot process uses GRUB or GRUB2, or SystemV start scripts or systemd. This is the hardware portion of the boot process and is the same for any operating system. When power is first applied to the computer it runs the Power On Self Test (POST) which is part of BIOS. BIOS stands for Basic I/O System and POST stands for Power On Self-Test.

When IBM designed the first PC back in 1981, BIOS was designed to initialize the hardware components. POST is the part of BIOS whose task is to ensure that the computer hardware functioned correctly. If POST fails, the computer may not be usable and so the boot process does not continue.

BIOS POST checks basic operability of the hardware and then it issues a BIOS interrupt, INT 13H, which locates the boot sectors on any attached bootable devices.The first boot sector it finds that contains a valid boot record is loaded into RAM and control is then transferred to the boot sector.

The boot sector is really stage 1 of the boot loader. There are three boot loaders currently in use by most Linux distributions, LILO, GRUB and GRUB2. GRUB2 is the newest and is used much more frequently these days. The DataBook does not cover LILO because it is so much older than either version of GRUB, but it does cover both GRUB and GRUB2.

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