The boot process¶

Exercise storage-boot-annex.pdf · PDF

You press the power button. For a few seconds you see a logo, maybe a black screen, then your login. It feels like one step. It is actually a relay race of five or six runners, each handing the baton to the next.

Why should you care? Because the day a computer "does not start", you need to know which runner dropped the baton — the power supply, the firmware, the disk, or the operating system. Every technician's first question is: how far did it get?

The bootstrap problem¶

Here is a riddle. The CPU only does one thing: fetch → decode → execute instructions from RAM (Computer architecture). The operating system is a program. So to start the operating system, the CPU needs it in RAM. But RAM is volatile: at power-on it is empty. And copying the operating system from the disk into RAM... is a job that needs a program.

Who runs the first program?

The answer is a small program stored in a chip that does not forget: the firmware. Starting a computer is called booting, from "pulling yourself up by your own bootstraps" — an impossible trick that computers solve with this tiny, permanent first program.

In the kitchen: every morning the countertop (RAM) is empty and the chef (CPU) has no recipe. Taped to the wall next to the door there is one short card that never goes away. It says: "Check the kitchen. Then go to the pantry and fetch the recipe book." That card is the firmware.

The relay race¶

flowchart LR
    PSU["<b>1 · Power supply</b><br>stable voltage,<br>'power good'"] --> FW["<b>2 · Firmware</b><br>BIOS or UEFI,<br>from a flash chip"]
    FW --> POST["<b>3 · POST</b><br>self-test: CPU,<br>RAM, graphics..."]
    POST --> BL["<b>4 · Bootloader</b><br>GRUB or Windows<br>Boot Manager"]
    BL --> K["<b>5 · Kernel</b><br>the OS core takes<br>control of the hardware"]
    K --> U["<b>6 · Login</b><br>services start,<br>you log in"]

1. Power supply¶

The power supply unit (PSU) turns the wall's alternating current into the low, steady voltages the components need (12 V, 5 V, 3.3 V). Only when those voltages are stable does it send a "power good" signal to the motherboard. Until then, the CPU is held in reset. Nothing has run yet.

2. Firmware¶

Firmware is software stored permanently in a chip on the motherboard. It is the first code the CPU runs.

The CPU is wired to start fetching from one fixed address, and that address points into the firmware chip (flash memory — the same technology as an SSD, see Storage). There are two generations:

Every PC sold today uses UEFI. People still say "go into the BIOS" out of habit — and many UEFI setups even keep a compatibility mode (CSM, Compatibility Support Module) that pretends to be a BIOS. Windows 11 requires UEFI with Secure Boot support.

You enter the firmware setup by pressing a key right after power-on — usually Del or F2, depending on the brand. There you can change the boot order: the list of devices the firmware tries, in turn (internal disk, USB stick, network...). That list is how you boot from a USB stick to install an operating system — you will do exactly that in Unit 2.

3. POST¶

POST (Power-On Self-Test) is the firmware's quick health check of the essential hardware.

The firmware checks that the CPU works, that RAM is present and answers, that there is a graphics output, a keyboard... If something essential is missing, it cannot even show an error on the screen — so it beeps a pattern, or blinks a coloured LED on the motherboard. The meaning of the beeps depends on the manufacturer: look it up in the motherboard manual.

4. Bootloader¶

POST passed. The firmware now walks the boot order and finds a disk with something bootable on it. What it loads is not the operating system yet, but a tiny program whose only job is to load it:

A bootloader is a small program that finds the operating system's kernel on the disk, loads it into RAM, and jumps to it.

• On Linux the usual one is GRUB (GRand Unified Bootloader). It is also the menu you see when a computer has both Linux and Windows installed: which one do you want?
• On Windows it is Windows Boot Manager.

On a UEFI machine, the bootloader is an ordinary file (for example \EFI\Microsoft\Boot\bootmgfw.efi) stored in the EFI System Partition, a small FAT32 partition on the disk. On an old BIOS machine, the first stage had to squeeze into the MBR: the very first 512-byte sector of the disk.

5. Kernel¶

The kernel is the core of the operating system: the program that controls the hardware and shares it among all the other programs.

The bootloader loads the kernel into RAM and hands over the CPU. From this moment the firmware has finished its job. The kernel detects the hardware, loads its drivers (see Peripherals), mounts the disk, and starts the first process, which starts everything else: network, sound, the login screen. On most Linux systems that first process is systemd.

In the kitchen: the card on the wall has been read, the recipe book is on the countertop, and the head chef is now in charge. The card will not be needed again until tomorrow morning.

Secure Boot¶

A bootloader runs before the operating system — before any antivirus, before any password. Malware that replaces the bootloader (a bootkit) would control the machine from its very first instruction, invisible to everything that loads after it.

Secure Boot is a UEFI feature: the firmware only runs bootloaders that carry a valid digital signature from a trusted key.

It was added to the UEFI specification and became widely used with Windows 8 in 2012. The firmware stores a list of trusted keys (Microsoft's among them); an unsigned or tampered bootloader is refused. Most big Linux distributions boot with Secure Boot on, through a small bootloader signed by Microsoft (the shim).

A digital signature is like a wax seal on a letter: anyone can check it, only the owner of the key can make it. You will meet signatures again in the cybersecurity part of the course.

"It doesn't start": how far did it get?¶

Key takeaways¶

• Booting solves a chicken-and-egg problem: RAM is empty at power-on, so the first program must live in a permanent chip — the firmware.
• The relay: PSU → firmware → POST → bootloader → kernel → login.
• UEFI replaced the old BIOS: bigger disks, GPT, a boot file in the EFI System Partition, and Secure Boot.
• The bootloader (GRUB, Windows Boot Manager) loads the kernel into RAM and hands it the CPU.
• Secure Boot only runs signed bootloaders, blocking bootkits that would load before any defence.
• To diagnose a PC that won't start, ask: how far did it get?

Quick check¶

The kernel is now in control — but it still has to talk to your keyboard, screen and headphones. Next: Peripherals.