Boot strapping process

Question 1

boot strapping process(0)

Answer 1

0. BIOS - The BIOS checks system hardware components. The only responsibility of the BIOS in the boot process is to transfer control to first-stage boot loader on the MBR. Not all machines are PCs, some larger machines have entire firmware operating systems just to initialize and coordinate their
   hardware components.

Question 2

boot strapping process(1)

Answer 2

1. First-stage boot loader - Transfer control to second-stage boot loader. This usually resides on the same file system that the kernel resides in. In UNIX, this is the /boot directory.

Question 3

boot strapping process(2)

Answer 3

2. Second-stage boot loader - The main responsibility of the subsequent boot loader stage/s is to load the kernel into memory and execute it. There might be more stages before the kernel is actually loaded though. For example, a third-stage boot loader might be used to load support for a particular file system. Once the kernel is loaded, the boot loader can do one more optional task - it can set the initial state of a micro-kernel in memory. This is done by loading a snapshot of the kernel with all appropriate modules configured from a ram image file (called initrd in Linux). The first thing the kernel does is mount the top-level file system as a read-only file system. This is because the kernel is not sure that it is safe to modify the file system at this point. Whether there was an initrd image or not, the kernel loads required modules. Once all the modules are loaded and configured, the kernel transfers control to the principal process of the system. The program of that process is called init, and it resides on the top-level (root) file system in this location: /sbin/init.

Question 4

boot strapping process(3)

Answer 4

3. /sbin/init - The init process loads all services that are needed to keep the operating system functioning correctly. Once the operating system is initialised, the init process loads user-space utilities. These processes run in user mode and are required to support users, as opposed to the operating system. The init process looks in a file that lists file systems and corresponding partitions. This file is located in the root filesystem, /etc/fstab. Among the partitions in the list is the root file system which, at this, stage is still mounted in read-only mode. The root file system is re-mounted in read- write mode. Finally, the init process runs one or more getty processes. The getty program presents the user with a login prompt, that, when the user types in the correct username and password, runs a shell process and gives the user input control over that shell.

Question 5

run lvl defintions

Answer 5

The /sbin/init program works in phases, or runlevels, which is as follows:
0 Halt. Stop the system from running by switching off all services. This doesn’t necessarily mean power-down, machine might still have to be switched off, yet, PC hardware does power-down.
1 Single-user text mode. This mode is for the superuser only and is typically only used for severe maintenance operations.
2 Full multi-user text mode.
3 Full multi-user text mode with networking services.
4 Not used (user-definable).
5 Full multi-user graphical mode with networking services.
6 Reboot. This mode shuts down all services and reboots the machine.