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.
   • App: GUI, CLI

User Mode & Kernal Mode

• To prevent the application software from accessing the hardware directly
• The modern OS have two basic modes in which they can execute a certain program
  • User-mode
  • Kernel-mode

Kernel-mode

• The OS runs in kernel mode, also known as supervisor mode or privileged mode
• The Kernel mode is the privileged mode where the process has unrestricted access to system resources like hardware, memory, etc
• It will have to first go through the OS kernel by using syscalls

User mode

• The User mode is normal mode where the process has limited access
• All the User application runs in User Mode and direct access to the hardware is prohibited.
• To get access to restricted resources the kernel provides System Call(syscalls) through which a process get the required access
• Mode Detail
  • Moving between the user mode and the kernel mode is referred to as context switching
  • Context switching occurs when a user process makes a request to the underlying system API using syscalls
  • The switch does not occur automatically; rather, an interrupt is generated
  • An interrupt handler then saves the state of the CPU, switches to the kernel mode where the CPU can execute the instructions, and then restores the state of the CPU and returns to user mode.

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.
   • 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.

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 Memory mgmt and Process mgmt are in kernel
   • File mgmt. and IO mgmt. are in User-space
   • Smaller in size.
   • More Reliable
   • More stable
   • Performance is slow.
   • Overhead switching b/w user mode and kernel mode.
   • Eg. L4 Linux, Symbian OS, MINIX etc.
3. Hybrid Kernel
   • Advantages of both worlds. (File mgmt. 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. Nano/Exo kernels

Inter process communication (IPC)

• Two processes executing independently, has independent memory space (Memory protection), But some may need to communicate to work.
• IPC is the way by which multiple processes or threads communicate among each other
• IPC in OS obtains modularity, computational speedup and data sharing.
• Different ways to implement IPC are
  • pipe, message passing, message queue, shared memory
  • direct communication, indirect communication, FIFO.
• It is important to obtain synchronization among processes in IPC to maintain data consistency. Semaphore and mutex are two ways to do so.

System calls

• System calls are implemented in C.
• A system call is a mechanism using which a user program can request a service from the kernel for which it does not have the permission to perform.
• User programs typically do not have permission to perform operations like accessing I/O devices and communicating other programs.
• System Calls are the only way through which a process can go into kernel mode from user mode.

Types of System Calls:

1. Process Control
   • end, abort
   • load, execute
   • create process, terminate process
   • get process attributes, set process attributes
   • wait for time
   • wait event, signal event
   • allocate and free memory
2. File Management
   • create file, delete file
   • open, close
   • read, write, reposition
   • get file attributes, set file attributes
3. Device Management
   • request device, release device
   • read, write, reposition
   • get device attributes, set device attributes
   • logically attach or detach devices
4. Information maintenance
   • get time or date, set time or date
   • get system data, set system data
   • get process, file, or device attributes
   • set process, file, or device attributes
5. Communication Management
   • create, delete communication connection
   • send, receive messages
   • transfer status information
   • attach or detach remote devices

Examples of Windows & Unix System calls

• Process Control
  • Windows: CreateProcess(), ExitProcess(), WaitForSingleObject()
  • Unix: fork(), exit(), wait()
• File Management
  • Windows: CreateFile(), ReadFile(), WriteFile()
  • Unix: open(), read(), write()

BIOS/UEFI

• Basic Input/Output System
  • BIOS chip is a ROM chip found on mother board that allows to access & setup computer system at most basic level
  • BIOS tests and initializes system hardware which is backed by CMOS battery
• Unified Extended Firmware Interface
  • UEFI is an upgraded version on BIOS
  • UEFI can do a lot more than just initialize hardware; it’s really a tiny operating system.
  • For example
    • Intel CPUs have the Intel Management Engine.
    • This provides a variety of features, including powering Intel’s Active Management Technology, which allows for remote management of business PCs

CMOS battery

• CMOS stands for Complementary Metal Oxide Semiconductor
• The CMOS battery powers the BIOS firmware in your laptop
• The CMOS battery gets charged whenever your laptop is plugged in.
• Most batteries will last 2 to 10 years from the date they’re manufactured.

What happens when you turn on your computer?

1. PC On
2. CPU initializes itself and looks for a firmware program (BIOS/UEFI) stored in BIOS/UEFI Chip
3. CPU runs the BIOS which tests and initializes system hardware
   • Bios loads configuration settings.
   • If something is not appropriate (like missing RAM) error is thrown and boot process is stopped.
   • This is called POST (Power on self-test) process.
4. BIOS will handoff responsibility for booting your PC to your OS’s bootloader
5. Bootloader loads the Full OS
   • The bootloader is a small program that has the large task of booting the rest of the operating system
   • Like: Boots Kernel then, User Space
   • Windows uses a bootloader named Windows Boot Manager (Bootmgr.exe)
   • Most Linux systems use GRUB, and
   • Macs use something called boot.efi