How a system boots
The sequence from power-on to the login screen: firmware, bootloader, kernel and initramfs, first process, services, and the messages that tell where it breaks.
- 27 cards
- 5 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Questions
- Sort and link
- Scenarios
- Understand
A few cards from the deck
BIOS: what is it?
Show the answer
The PC's historic firmware that tests the hardware at power-on then reads a disk's first sector to find something to boot.
It works with an MBR partition table and is limited in disk capacity and boot options.
What is the main difference between BIOS and UEFI?
- UEFI reads an EFI partition and handles secure boot; the BIOS, one sector
- UEFI skips the hardware self-test; the BIOS runs it at every power-on
- UEFI is installed on the disk; the BIOS is burnt into the motherboard
- UEFI only boots Windows and its tools; the BIOS boots Linux and the BSDs
Show the answer
UEFI reads an EFI partition and handles secure boot; the BIOS, one sector
Both are firmware stored on the motherboard and run before any system; UEFI replaces the boot sector with files on a dedicated partition, with optional signature checking.
The boot order is: firmware, …, …, first process, …, login screen.
Show the answer
The boot order is: firmware, bootloader, kernel, first process, services, login screen.
Each stage prepares the next: the firmware starts the loader, which finds the kernel, passes it its parameters and hands it control.
UEFI: what is it?
Show the answer
The modern firmware that replaces the BIOS, reads bootloaders from a dedicated system partition (EFI) and handles secure boot.
It is usually paired with the GPT partition table. Secure boot only accepts signed bootloaders.
After an update, the screen stops at
grub rescue>. Which stage failed, and what has to be repaired?- The kernel crashed while initialising: the kernel package has to be completely reinstalled
- The disk is destroyed: the whole disk has to be replaced and then a recent backup restored
- GRUB can no longer find its files: reinstall it and regenerate its configuration
- The display service did not start: systemd has to be restarted from a terminal as root
Show the answer
GRUB can no longer find its files: reinstall it and regenerate its configuration
This prompt is GRUB's own: the firmware did start it, but it cannot find the partition or the /boot/grub folder that holds its modules (a missing grub.cfg alone would give the grub> prompt). Boot from a rescue system, then run grub-install and update-grub.