Genesis: from raw hardware to processes

1
Genesis from raw hardware to processes

• System booting sequence how does a machine come
  into life

2
How is the First Process Created?

• What happens when you turn on a computer?
• How to get from raw hardware to the first running
  process, or process 1 under UNIX?

3
Booting Sequence

• The address of the first instruction is fixed
• It is stored in read-only-memory (ROM)
• ROM is read-only, permanent memory, and is always
  available to be used.

4
Booting Procedure for i386 Machines

• On i386 machines, ROM stores a Basic Input/Output
  System (BIOS)
• BIOS contains program that knows how to access
  storage devices, keyboard, and graphic card.

5
BIOS Code

• Performs Power-On Self Test (POST)
• Tests the system
• Looks for and checks peripherals
• During this time, you will hear memory counting,
  which consists of noises from the floppy and hard
  drive, followed by a final beep
• Locates a valid device with which to boot the
  system (boot device)
• Floppy ? CDROM ? hard drives

6
After the POST

• The master boot record (MBR) is loaded by BIOS
  from the boot device.
• The MBR is stored at the first logical sector
  (track 0, cylinder 0, and head 0) of the boot
  device (e.g., a hard drive) that
• Fits into a single 512-byte disk sector (boot
  sector)
• Boot loader machine code instructions for
  booting the machine.
• Partition table the physical layout of the disk
  (e.g., number of tracks)

7
After Getting the Info on the Boot Device

• BIOS loads a more sophisticated loader from the
  boot sector and yields control to it.
• Under Linux, this sophisticated loader is called
  LILO (Linux Loader) or Grub (GNU Grub Unified
  Boot Loader).
• The more sophisticated loader loads the operating
  system

8
More on OS Loaders

• LILO
• Is partly stored in MBR with the disk partition
  table.
• A small machine code binary on the MBR whose job
  is to locate the second stage boot loader and
  load the first part of it into memory.
• A user can specify which disk partition and OS
  image to boot
• Every time a configuration change is made, run
  /sbin/lilo to change MBR.
• Windows loader assumes only one bootable disk
  partition
• After loading the kernel image, LILO sets the
  kernel mode and jumps to the entry point of the
  operating system

9
Booting Sequence in Brief

• A CPU jumps to a fixed address in ROM,
• Loads the BIOS,
• Performs POST,
• Loads MBR from the boot device,
• Loads an OS loader,
• Loads the kernel image,
• Sets the kernel mode, and
• Jumps to the OS entry point.

10
Linux Initialization

• Set up a number of things
• Trap table
• Interrupt handlers
• Scheduler
• Clock
• Kernel modules
• Process manager
• Hand-craft environment for the init process (the
  first process, process 1).

11
Process 1

• Is instantiated from the init program
• Is the ancestor of all processes
• Controls transitions between runlevels
• Executes startup and shutdown scripts for each
  runlevel

12

• On RedHat, the init do the following (run as a
  process)
• Run /etc/rc.d/rc.sysinit that sets environment
  path, checks file system, etc (system
  initialization)
• Run /etc/inittab script that decides how system
  should be setup in each runlevel.
• Set the source function library,
  /etc/rc.d/init.d, how to start and kill a
  program, etc
• Start all background processes by looking at the
  rc directories for the runlevel as default in
  /etc/inittab e.g /etc/rc.d/rc5.d for runlevel 5.
  
• Fork /sbin/mingetty to give virtual consoles
  (login prompts).

13
Runlevels

• Level 0 shutdown
• Level 1 single-user
• Level 2 multi-user (without network file
  system)
• Level 3 full multi-user
• Level 5 X11
• Level 6 reboot