The Linux boot process, a chart

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The Linux boot process, a branching chart. For the most part, the levels do not depend on column. LILO does not require EFI, nor does GRUB require BIOS. EFI and BIOS support are now included in both bootloaders. Yes, there is some simplification here. EFI is very featured, but hasn't been commonly exercised to its full extent. Eventually EFI could possibly replace GRUB/LILO, but that day hasn't come yet.

Pre-Boot Enviroinment
  • POST
  • Read bootable media
  • Load Master Boot Record
  • Execute MBR
  • POST
  • Read bootable media
  • Load the GPT table
  • Mount the EFI system-partition
  • Load EFI-drivers for the system
  • Execute MBR
Bootstrap Environment
  • Stage 1 loaded into MBR/EFI and gets executed by BIOS/EFI
  • Stage 1.5 loaded by Stage 1, including critical drivers
  • Stage 2, in the boot filesystem, executes
  • Stage 2 loads the kernel
  • Stage 1 loaded into MBR/EFI and gets executed by BIOS/EFI
  • Load first sector of core.img
  • Continues loading core.img
  • Loads GRUB config
  • Loads the kernel
  • Stage1 loaded into MBR (or EFI by ELILO) and executed by BIOS/EFI
  • Stage 1 loads the first cluster of Stage 2, and executes.
  • Loads LILO information.
  • Loads the kernel
Kernel Load
  • The kernel uncompresses into memory
  • If configured, the kernel mounts the Initial Ramdisk, which contains needed modules to load the rest of the OS
  • Mounts the root filesystem, loading any needed modules from initrd
  • Swaps / from initrd to the actual root filesystem
  • Executes the specified init process
UID 1 process
  • Checks /etc/inittab for loading procedures
  • Runs scripts specified by inittab
    • Mounts needed filesystems
    • Loads needed modules
    • Starts needed services based on runlevel
    • Finishes setting up userspace
  • Reads /etc/system.conf
  • Mounts needed filesystems
  • Loads needed modules
  • Starts services as needed
  • Runs startup events listed in /etc/events.d based on runlevel.
  • Loads needed modules
  • Mounts needed filesystems
  • Starts needed services
  • Reads /etc/launchd.conf for config details
  • Reads /etc/launchd.plist for per-driver/service details

I did this because Things have Changed from the last time I really studied this. Back when I started it was BIOS, LILO, and Initd. I never did bother to wrap my head around Grub, mostly because the automatic tools have gotten a lot better so knowing it just to install isn't needed, and I haven't had the right kind of boot problems where I'd learn it through troubleshooting. I've also yet to run into EFI in the wild (I think...). Now that OpenSUSE is actively considering moving over to SystemD and Ubuntu having thrown initd over the side some time ago in favor of Upstart, it's time to revisit.

I'm still fuzzy on SystemD so that's probably wrong.

1 Comment

You could add a horizontal bar similar to the Kernel Load line between the BIOS/EFI row and the bootloader row. Either leave the bar empty as simply a way to indicate that the columns above don't directly feed into the column below, like you said, or label the bar as "Bootloader".

Nice chart.