The Linux boot process, a chart

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
BIOS		EFI
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
GRUB(v1)	GRUB(v2)		(E)LILO
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
Initd	Systemd	Upstart	Launchd
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.