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BIOS
firmware image on ROM chip
ROM is either a dual in-line package or plastic leaded chip carrier
POST
initializes HW, e.g. memory, video card, keyboard, USB hosts
finds a bootable device (HDD, ODD, USB flash drive, etc.)
reads MBR and executes stage 1 boot loader
boot loader, e.g. GNU GRUB
may offer user OS list and/or some other interactive IX (e.g. CLI)
will load a kernel, e.g. Linux, into memory and begin its execution
Linux
initializes memory, interrupts, etc.
may mount initial ramdisk
loads and schedules init and kernel threads
initial ramdisk
finds root filesystem
loads required drivers and runs required utilities to mount root fs
e.g. for RAID, LVM2, ext4
checks for a suspend-to-disk virtual memory image and resumes if found
look at initrd on Debian:
$ gunzip -c /boot/initrd.img-`uname -r` | cpio --extract
warning: will extract to current working directory
init
multiple implementations
GNU/Linux:
System V-style init
systemd
Upstart
runit
OpenRC
Initng
BSDs:
own lightweight and modular daemon
BusyBox/Linux:
own lightweight daemon
SysV-style:
runlevels
0 - halt
1 - single-user mode
2-5 - multi-user mode
6 reboot
reads /etc/inittab
default runlevel
actions of runlevels
Ctrl+Alt+Del behavior
programs to control TTYs (e.g. /sbin/getty)
etc.
runs initialization scripts
`/etc/rcN.d/* start`, where N = runlevel
links to /etc/init.d/*
cron, servers, ACPI daemon, Network Manager, DMs, etc.
getty
prints /etc/issue, unless disabled
prompts for username
calls /bin/login
login
prompts for password
loads user environment
starts user's default shell (e.g. /bin/bash)
display manager
starts X.org server
prompts for user name and password
starts window manager and desktop shell
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