More on Kernel

The kernel code contains both machine dependent as well as machine independent code. This is because the same kernel will be used on a variety of devices which uses different processor architecture.

The following section describes what the machine dependent and machine in-dependent code deals with

Machine dependent code

• Low level system startup function
• Traps and fault handling
• Low level manipulation of runtime context of a process i.e., context switching
• Configuration and initialization of hardware devices
• Runtime support for I/O devices

Machine independent code

• System call handling
• The file system: files, directories, pathname translation, file locking and I/O buffer management
• Terminal handling support: the terminal-interface driver and terminal line disciplines.
• IPC facilities
• Network communication support

Terminal Line Discipline

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Terminal line disciplines refer to the terminal type (TTY) that is to be used - RawTTY & CookedTTY

• Cooked Terminal Type
  • This is also called canonical mode.
  • The terminal driver is, by default a line-based system i.e., characters are buffered internally until a carriage return (Enter or Return) is passed to the program - this is called cooked i.e., the terminal driver "cooks" the characters before serving them up.
  • This allows certain characters to be processed, such as Ctrl+D, Ctrl+S, Ctrl+U, Backspace);
• Raw Terminal Type
  • This is also called non-canonical mode.
  • The terminal can be placed into raw mode where the characters are not processed by the terminal driver i.e., special characters are not processed, but are sent straight through.

<aside> 💡 The architecture dependent code can be accessed at /usr/src/<linux-kernel>/arch.

</aside>

Booting Procedure

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The booting procedure involves the following steps -

1. Power On - BIOS

   • BIOS (Basic Input Output System) is a software program comes pre-built in a motherboard chipset.
   • BIOS loads and scans for devices such as Hard Disk, CD-ROM, RAM, etc.
   • BIOS searches for MBR of the primary hard drive, it scans for 1st stage loader and hands over the responsibility to MBR.

2. Master Boot Record (MBR)

   • MBR contains the information of loader of most operating system.
   • MBR involve a mini executable programs and a table specify the primary partitions.
   • The BIOS surrender rights to the first stage boot loader, which then scans partition table and finds second stage boot loader on the partition configured as bootable.

3. Boot Loader

   • The 1st stage loader a.k.a boot loader loads itself into RAM.
   • The default stage 2 boot loader is a GRUB (Grand Unified Boot Loader) or LILO (Linux Loader)
   • Once GRUB is loaded into RAM, then it’s search for the location of Kernel i.e., it searches for vmlinuz bootable file under /boot directory and loads its.

   (A brief video explanation about the Boot Loader on Youtube)

4. Initializing the Kernel

   • All of the kernels are in a self-extracting, compressed format to save space. The kernels are located in the /boot directory, along with an initial RAM disk image, and device maps of the hard drives.
   • After the selected kernel is loaded into memory and begins executing, it first extracts itself from the compressed version of the file.
   • Once the kernel has extracted itself, it loads systemd process and turns control over to it. systemd is a replacement to the old init (a.k.a SysV init).

5. The systemd process

   • systemd is the mother of all processes and it is responsible for bringing the Linux host up to a state in which productive work can be done.
   • Some of its functions are mounting filesystems, starting and managing system services required to have a productive Linux host.
   • The systemd process boots the system into the run level as specified in /etc/inittab

6. Run Level

   • A runlevel is one of the modes that a Unix-based OS will run on.
   • Each runlevel has a certain number of services stopped or started, giving the user control over the behavior of the machine. Conventionally, seven runlevels exist, numbered from zero to six.

   Runlevels