What a Bootloader Actually Does

linux1-5 walked through a real Debian install, including the moment the installer asks "Install the GRUB boot loader?" and offers to write it to a disk — then moved on, without ever explaining what that step actually does. This chapter is that explanation, from the ground up.

The Boot Sequence — From Power-On to a Running Kernel

1. The machine's own firmware (BIOS or UEFI) runs first, initializing hardware
2. Firmware locates and hands control to a bootloader
3. The bootloader loads the kernel — and usually an initramfs, a small temporary filesystem — into memory
4. The kernel takes over, mounts the real root filesystem, and starts the init system (systemd on Debian and most modern distros)

The bootloader's own job is narrow and specific: it's the bridge between "firmware has no concept of an operating system" and "the kernel is now running and can take it from here." It does not run the operating system itself — it hands off, once.

BIOS vs. UEFI — Two Different Firmware Models

BIOS (legacy) reads the very first sector of a disk and blindly executes whatever code is there — no awareness of filesystems, no real structure. UEFI (modern) is a genuinely more capable environment: it can read an actual filesystem — specifically a FAT-formatted EFI System Partition — and directly execute a real, named .efi file from it. This distinction gets its own full chapter later (grub1-5); for now, just know the two models exist and behave in fundamentally different ways.

MBR vs. GPT — Previewed

MBR (Master Boot Record) is the legacy partition table format — a maximum of four primary partitions, a 2TB disk-size ceiling, and the natural pairing for BIOS. GPT (GUID Partition Table) is the modern replacement — far more partitions allowed, no meaningful size limit, and the natural pairing for UEFI. Full depth on both is grub1-5's own job; this preview exists purely so the chapters between here and there make sense as you go.

What GRUB's Actual Job Is

GRUB (the GRand Unified Bootloader) is one specific bootloader implementation — the one Debian, and most major Linux distributions, install by default. Its job: find the kernel, find the initramfs, load both into memory with the right parameters, and hand off control. Along the way it offers a menu — letting you choose between multiple installed kernels, or multiple operating systems entirely — and lets kernel parameters be edited live at boot time, previewed here and covered fully in grub1-4.

Resolving linux1-5's Own Installation Screen

That "Install the GRUB boot loader?" prompt is the exact moment GRUB gets written to disk — into the MBR's own boot sector on a BIOS system, or as a real file inside the EFI System Partition on a UEFI system. linux1-5 walked through this operationally, as one screen in an installer wizard, without explaining the mechanism underneath it. That mechanism is now on the table.

Hands-On Exercises