Genesis: from raw hardware to processes

Genesis: from raw hardware to processes

• System booting sequence: how does a machine come into life

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?

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.

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.

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

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)

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

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

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.

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).

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

On RedHat, the init do the following (run as a process)

• 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).

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