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Components of OS

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Components of OS

  1. Kernel :- A kernel is that part of the operating system which interacts directly with the hardware and performs the most crucial tasks.

    • Heart of OS/Core component
    • Very first part of OS to load on start-up.

  2. User space :- Where application software runs, apps don’t have privileged access to the underlying hardware. It interacts with kernel.

    • GUI
    • CLI

shell/ Command Interpreter

A shell, also known as a command interpreter, is that part of the operating system that receives commands from the users and gets them executed.

CPU execution modes 🚦

There are two modes in which CPU can execute code:

  1. User mode

    • In user mode, the CPU executes application-level code (non-privileged code).
    • Programs running in this mode have limited access to system resources and hardware.
    • If a program needs to access hardware resources or perform privileged operations (e.g., writing to disk), it makes a request to the operating system via system calls.

  2. Kernel mode

    • In kernel mode, the CPU executes code with full access to all hardware resources.
    • The operating system (OS) and low-level system processes run in kernel mode, where they can control hardware directly, manage memory, schedule processes, etc.
    • Code running in kernel mode can execute any CPU instruction and reference any memory address.

Kernel mode

When you are typing mkdir newDir in terminal, the mkdir command is running in user mode, but the mkdir command is calling the mkdir system call which is running in kernel mode.

  • CPU execution switches from user mode to kernel mode.
  • This is done by system interrupts.
  • The switching takes some time, so it's not good to switch frequently.
Non-privileged code means 😧

In non-privileged mode, the code :-

  • Cannot directly access hardware devices (e.g., disks, network cards).
  • Cannot modify critical system data or configuration.
  • Cannot directly access memory regions that are protected by the OS.
  • Must rely on the OS to manage resources, schedule tasks, and handle exceptions.

Functions of Kernel

1. Process management

  • Scheduling processes and threads on the CPUs.
  • Creating & deleting both user and system process.
  • Suspending and resuming processes
  • Providing mechanisms for process synchronization or process communication.

2. Memory management

  • Allocating and deallocating memory space as per need.
  • Keeping track of which part of memory are currently being used and by which process.

3. File management

  • Creating and deleting files.
  • Creating and deleting directories to organize files.
  • Mapping files into secondary storage.
  • Backup support onto a stable storage media.

4. I/O management

  • to manage and control I/O operations and I/O devices
  • Buffering (data copy between two devices), caching and spooling.
    • Within differing speed two jobs.
    • Eg. Print spooling and mail spooling.
  • Buffering
    • Within one job.
    • Eg. Youtube video buffering
  • Caching
    • Memory caching, Web caching etc.
what is spooling?
  • Spooling is a process in which data is temporarily held to be used and executed by a device, program or the system. It is a place where data is stored to be processed or printed later.
  • In modern systems, spooling is often used when data needs to be transferred from a program to a peripheral (such as a printer).

Types of Kernels 🤖

1. Monolithic Kernel

  • All functions are in kernel itself. 
    - Bulky in size.
- Memory required to run is high.
- Less reliable, one module crashes -> whole kernel is down.
+ High performance as communication is fast. (Less user mode, kernel mode overheads)
  • Eg. Linux, Unix, MS-DOS.

2. Micro Kernel

  • Only major functions are in kernel.
    • Memory management.
    • Process management.
  • File management and IO management are in User-space. 
    + smaller in size.
+ More Reliable
+ More stable
- Performance is slow (bcoz of switching between user mode and kernel mode)
- Overhead switching b/w user mode and kernel mode.
  • Eg. L4 Linux, Symbian OS, MINIX etc.

3. Hybrid Kernel

  • Advantages of both worlds. (File management. in User space and rest in Kernel space. ) 
    + Combined approach.
+ Speed and design of mono.
+ Modularity and stability of micro.
  • Eg. MacOS, Windows NT/7/10
  • IPC also happens but lesser overheads

4. Exo kernels

  • Smallest in size.
  • Only basic functions.
  • Rest in user space. 
    - has fewest hardware abstractions as possible.
- It allocates physical resources to applications.

5. Nano Kernel

  • It is the type of kernel that offers hardware abstraction but without system services.
  • Micro Kernel also does not have system services therefore the Micro Kernel and Nano Kernel have become analogous.
  • The main difference between the two is that the Nano Kernel is designed to be as small as possible.
  • Micro kernels which actually are micro in size are also called nano kernels.
Nano/Exo kernels

a type of operating system kernel that has a minimal and streamlined design, with a focus on minimalism and efficiency. The goal of a nano kernel is to provide only the essential functions required for the operation of a system while delegating other functions to user-space processes.

Communication b/w User mode and Kernel mode 📡

Q. How will communication happen between user mode and kernel mode?
  • Inter process communication (IPC). This can be done in two ways:
    • Shared memory:
      • A region of memory that is shared between two processes.
      • One process writes to the shared memory and the other process reads from it.
    • Message passing:
      • A logical channel is established between two processes.
      • A process sends a message to another process and the other process receives it.
      • Eg. pipes, sockets, message queues, signals, semaphores, shared memory, and message passing.