Linux Booting

1
Linux Booting

• The booting procedure.
• Logging in to the machine.
• Control flow in the machine.

2
Linux Booting

• The booting procedure.
• Logging in to the machine.
• Control flow in the machine.

3
The booting procedure.

• Every computer has sequence of instructions that
  are executed when computer is powered up.
• determined by the hardware designers
• Every OS has a sequence of instructions that it
  executes prior to normal operation.

4
The booting procedure.

• i386 machines incorporate the Basic Input/Output
  System (BIOS)
• stored at a prespecified location in system ROM.
• when computer started, begins executing BIOS code
• BIOS code executes the Power-On Self Test (POST)
• POST performs various diagnostic tests to check
  memory and devices for their presence and correct
  operation.
• when POST is completed, machine ready to load and
  start OS.

5
The booting procedure.

• The boot sector.
• OS loaded from the systems boot disk.
• i386 treats A as default disk if it contains a
  diskette.
• Otherwise depends on how youve set the BIOS.

6
The booting procedure.

• The boot disk
• must contain a boot record at the disks first
  logical sector.
• The boot record fits onto a single 512-byte disk
  sector (called the boot sector)
• 0x000 0x002 lta jump instr to 0x0XXgt
• 0x003 . Disk parameters (used by BIOS) program
• 0x0XX 0x1fd Bootstrap program
• 0x1fe 0x1ff 0xaa55 (the magic number for BIOS)

7
The booting procedure.

• After POST BIOS copies the boot record from disk
  into memory
• Bios then reads boot record parameters
• Boot record contains parameters that describe
  disks physical layout
• Parameters are stored in well-know locations in
  boot record
• Parameters include number of tracks and number of
  sectors per track.

8
The booting procedure.

• After parameters read BIOS branches to the first
  location in program
• then immediately branches to location 0x0XX
• the value XX for Linux is 3e
• the value XX for MS-DOS is 1e
• The small program at address 0x0XX loads a more
  sophisticated loader from other sectors on the
  disk
• The sophisticated loader loads the OS

9
The booting procedure.

• Disk partitioning
• can have up to 4 primary partitions on one
  physical disk
• each primary disk can have logical sub-partitions
• each partition treated like a physical disk below
  the abstract machine that accesses the physical
  disk (eg, BIOS in MS-DOS)
• if partition is bootable then its logical sector
  number 0 will be a boot sector.

10
The booting procedure.

• Disk partitioning
• when hardware is powered up goes to head 0, track
  0, sector 1 and begins executing code.
• an unpartitioned disk has a jump instruction at
  head 0, track 0, sector 1 which jumps to location
  0x0XX and starts the execution of the boot
  program
• in a partitioned disk, physical head 0, track 0,
  sector 1 (first logical disk sector) describes
  how hard disk is partitioned into logical disks.

11
The booting procedure.

• A partitioned disk has a 446-byte bootstrap
  program starting at first byte in the sector.
• next is a 65-byte partition table for the disk
• Table contains the 4 partition entries, each of
  which describes the portion of the physical disk
  that is used for its partition (starting sector
  of the partition, ending sector, number of
  sectors, etc.)
• last 2 bytes of partition sector contain a magic
  number, 0xaa55 to identify the partition sector.

12
Partitioned Disk
446-byte bootstrap program
4 partition entries, each describes the portion
of the physical disk used for its partition
65-byte partition table
2 bytes magic number, 0xaa55 for MS-DOS
13
The booting procedure.

• Partitioning with different OSs
• i386 can be booted with Linux from boot floppy.
• contains a Linux boot program.
• Can have different OS in different partitions
• Windows handles by designating one partition as
  an active partition. System always boots from
  this partition.

14
The booting procedure.

• Partitioning with different OSs
• can have a special program, Linux Loader (LILO)
  in system boot record.
• program allows user to choose at boot time which
  partition is to be the active partition.
• LILO is loaded in the boot record so that BIOS
  runs it after the POST.

15
Starting the Kernel

• machine is powered up
• hardware fetch-decode-execute cycle begins.
• called the hardware process
• this is not a Linux process Linux has not yet
  started.
• POST is completed
• boot record read
• loader places OS into primary memory
• the booting procedure begins to run kernel code
  to initialize the computers hardware.

16
Starting the Kernel

• Computer then starts kernel by setting supervisor
  mode and branching to main entry point in kernel.
• main entry point is not a C program.
• kernel initializes
• trap table
• interrupt handler
• scheduler
• clock
• modules
• etc.

17
Starting the Kernel

• near end of kernel initialization start process
  manager
• now ready for normal process abstraction
• PM starts the initial process
• initial process is first entry in kernels
  process descriptor table
• referred to as process 0, task0, or INIT_TASK
• initial process then creates first useful Linux
  process to run the init program
• after init runs it executes an idle loop
• only other duties of init is to use idle CPU
  time.runs when no other process wants CPU.

18
Starting the Kernel

• init process initializes system
• starts daemons
• starts file manager
• creates system console
• runs other init programs from /etc, /bin and/or
  /sbin, and runs /etc/rc

19
Logging in to the machine

• during initialization kernel creates one process
  on each communication port that can be used to
  support user logins
• uses the getty program
• getty
• waits for a user to begin using the port
• then runs login program
• login expects user id on first line, passwd on
  next
• login verifies user by looking in /etc/passwd
  file

20
Logging in to the machine

• getty
• /etc/passwd file. Each entry separated by colon
• jbarreNcrYpt123334835John
  Barr/home/barr/bin/bash
• jbarr account name
• eNcrYpt123 encrypted password
• 3348 and 35 user ID and group ID
• John Barr real name
• /home/barr users home directory
• /bin/bash path to shell command preferred by
  user

21
Logging in to the machine

• getty
• once login verifies it changes current directory
  to users home directory
• and executes the specified shell program
• now user interacts directly with login process
  via shell.
• user who logs in is using a process that was
  created when the machine was started
• starting shell is complicated data and stack
  segments created etc.
• data includes users environmental variables (eg,
  PATH)
• when user logs off, return from shell call to the
  login process

22
Logging in to the machine
6
Serial Port A
Serial Port B
getty
Serial Port C
Serial Port Z
/etc/passwd
23
Control Flow
hardware process starts
Idle Proc
kernel
ISRs
Proc i
Proc j
program
hardware process
context switch