Booting the system

1 Boot sequence

1. POST (Power On Self Test) from the BIOS or UEFI firmware.

2. Then, it finds the bootloader, usually GRUB.

3. Once GRUB is loaded, the bootloader finds initramfs (modern)/initrd (legacy), loading a kernel image with some basic drivers on memory. The initrd or initramfs file exists in /boot and is specified in your grub.cfg or menu.lst.

4. Once the image is loaded, the root FS is mounted. The system starts with PID 1 (systemd).

5. Depending on the configuration, a specific runlevel or target is started.

• Legacy: The default runlevel is stored in /etc/inittab.

• Modern: systemctl get-default, will show you the default target.

2 Installing GRUB Legacy

GRUB Legacy syntax for installation:

• First hard drive in MBR: grub-install '(hd0)'

• First hard drive, first partition: grub-install '(hd0,0)'

Other available syntax for installation:

• First hard drive in MBR: grub-install /dev/sda

• First hard drive, first partition: grub-install /dev/sda1

3 "Legacy" and "Modern" ways of storing boot information

1. Legacy (BIOS/MBR): The boot code is hidden in the first 512 bytes (MBR). It is hard to manage and easy to overwrite. It covers the stage 1 of the bootloader. Its only job is to point to the next stage.

2. Modern (UEFI/ESP): The boot code is stored as standard files within the ESP (EFI System Partition). This allows you to have multiple bootloaders (e.g., Windows and Linux) living side-by-side in their own folders (e.g., /EFI/ubuntu/ and /EFI/Microsoft/) without fighting for the same 512-byte space.

Quick Command: If you want to see if your system is using an ESP right now, you can run lsblk or fdisk -l. Look for a small partition (usually 100MB–500MB) that it usually appears as "/boot/efi".

4 Summary Table

Feature BIOS + MBR UEFI + GPT

GRUB Location Sector 0 (MBR) EFI System Partition (FAT32)

Storage Capacity Extremely limited (512 bytes) Large (can hold many .efi files)

Primary Command grub-install /dev/sda grub-install (auto-detects ESP)

grub-install '(hd0)'

Note: In the context of the EFI System Partition (ESP), GPT is the modern standard for organizing how a disk is carved into sections. While the old MBR (Master Boot Record) was limited and a bit fragile, GPT was designed specifically to work with UEFI firmware. If you run fdisk -l, look at the Disklabel type row:

• dos = MBR (Legacy)

• gpt = GPT (Modern/UEFI)