Computer Booting

Abstract

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• When the power is switched on, BIOS is executed first, followed by Boot Device, lastly Kernel Booting

BIOS

• Contains low-level IO software

• Nowadays, stored in Flash RAM - nonvolatile & can be updated

• The newer standard is called UEFI

  
  

• The first program that starts when booting up a computer

  1. Checks Main Memory capacity
  2. Check IO Device
  3. Scanning IO Bus to detect all devices attached
  4. Determine the Boot Device by trying out a list of devices stored in the CMOS. (Users can press keyboard shortcut - F12, F11, Esc, F8, F9 to enter a Boot Menu and choose the boot device manually)
  5. The Boot loader inside the boot device will carry out the next step

Keyboard shortcut to enter BIOS

Boot Device

• Storage device that contains Master Boot Record and the OS
• Master Boot Record is read into Main Memory to start the Boot Loader

Partition Table

Contains information about how the partitions on the disk are organized. Can be created in 2 disk partitioning schemes - Master Boot Record and GUID Partition Table (Needs UEFI BIOS)

Essential partitions of a Linux Boot Device

/boot partition

• Stores the Linux Kernel, initial RAM disk image (initrd), and Boot loader
• 500MB - 1GB is typically sufficient
• Ext4 is a common choice, but other Linux filesystems can work too

/boot/efi partition

• EFI System Partition (ESP), stores the EFI bootloader files specifically needed for systems using UEFI (most modern systems)
• 100MB - 500MB is usually enough
• Formatted as FAT32 for compatibility across different architectures
• Needs “boot” or “esp” flag
• /boot becomes optional to unified the partition for kernel and boot loaders. However, non-encrypted /boot is required whatever the UEFI choice if the rest of the system is encrypted!

/ partition (root partition)

• Core of your Linux installation. It contains all your system files, user files, program data, etc
• The space of the rest of the boot device
• EXT4 is the most common and reliable option

Optional partitions for Linux Boot Device

These optional partitions provide extra features.

/home partition

• Keeps your user files separate from system files, you can format your / partition without losing personal files and settings, great for Distro hopping
• EXT4 for stability and reliability and XFS for big media files

linuxswap partition

• Swap Space
• Traditionally, double your RAM size was recommended. Modernly, it depends on your usage. If you hibernate often, matching your RAM size is wise. Otherwise, a few GB might suffice if you have ample RAM
• ”linux-swap” File System and no mount point

Boot loader

1. Examines the partition table at the end of boot sector to determine which partition is active. Providing a menu for user to select which OS to boot
2. When an OS is selected, boot loader loads in the corresponding Kernel into the Main Memory
3. Kernel Booting will finish up on the booting of the OS
   

• One common boot loader is Grub2

References

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• How Does Linux Boot Process Work?