1

Unit 1 : Introduction to Operating

System

School of Computer Science

UPES, Dehradun
India
 Table of Contents
1. System boot
2. System Calls
3. Types of System Calls (Windows and Unix System Calls examples),
4. Open Source Operating Systems
 Learning & Course Outcomes

Learning Outcomes
LO1: Understand the concept of System call in Operating
System
LO2: Understand booting process, different types of system
calls and open-source operating systems

Course Outcomes
CO1: Demonstrate a comprehensive understanding of operating
systems
 System Booting
Booting is the process of starting a computer. It can be initiated by hardware such as a
button press or by a software command.
After it is switched on, a CPU has no software in its main memory, so some processes
must load software into memory before execution. This may be done by hardware or
firmware in the CPU or by a separate processor in the computer system.

Booting Operation Sequence:

• Booting is a start-up sequence that starts
the operating system of a computer when
it is turned on.
• A boot sequence is the initial set of
operations that the computer performs
when it is switched on. Every computer Fig 1. Booting Operation Sequence
has a boot sequence.
 System Booting
When a computer or any other computing device is in a powerless state, its operating system
remains stored in secondary storage like a hard disk or SSD. But, when the computer is started, the
operating system must be present in the main memory or RAM of the system.

When a computer system is started, there is a

mechanism in the system that loads the operating
system from the secondary storage into the main
memory, or RAM, of the system. This is called the
booting process of the system.

Fig 2. System Booting architecture

 System Booting: Process
The following components are involved during booting
process
1. Power Supply
2. CPU
3. BIOS (ROM)
4. CMOS
5. Master Boot Record
6. RAM
7. Boot Loader
8. Storage Drives

Fig 3. Components of Booting

 System Booting: Process
The booting process is as follows:
1. Power Supply
• The first step to switch on the power supply.
• This power supply provides electricity to all motherboard components like CPU, Hard disk etc.
2. CPU
• CPU loads the BIOS (Basic Input output System) to execute instructions stored in it.

3. BIOS
• When we turn on the computer, so the CPU looks for
another program, called the BIOS (Basic Input/Output
System), and runs it. The BIOS is a firmware that is
located on the motherboard and is run by the CPU to
start the booting sequence
Fig 4. ROM and BIOS Program
 System Booting: Process
3. BIOS Initialization, RUN Test and Initialization Hardware
• BIOS settings are stored in a non-volatile memory called, CMOS ( Complementary metal oxide
semiconductor).
• BIOS loads the setting from CMOS
• CMOS is backed by CMOS battery to provide it continuous
power supply even after system is shutdown or restart.
• Now BIOS program is loaded with setting received from CMOS
• BIOS program initializes POST test, called Power on Self-Test.
• POST contains some test cases for each hardware to ensure whether each
Fig 5. CMOS battery
Connected device working properly or not.
• Printer, Keyboard, Mouse, Speaker, RAM etc.

4. BIOS Sends Control to Boot Device

• Once the BIOS finished all initialization and hardware test. It will find the Boot Device to load
operating system
• Now it looks for the actual program that will search and run the operating system
• Boot device contains the instruction for this actual program called boot loader.
 System Booting
Boot Devices
• The boot device is the device from which the operating system is loaded. A modern PC BIOS
(Basic Input/Output System) supports booting from various devices.
• These devices includes
• Local hard disk drive,
• Optical drive,
• Floppy drive,
• A network interface card,
• A USB device.
• The BIOS will allow the user to configure a boot order. If the boot order is set to:
• CD Drive
The BIOS will try to boot from the CD drive first, and if that fails, then it will try to
• Hard Disk Drive boot from the hard disk drive, and if that fails, then it will try to boot from the
• Network network, and if that fails, then it won't boot at all.
 System Booting

Boot Loader
• Boot Loader is a software program that is responsible for “actually loading” the operating
system. The Boot Loader is typically a part of the Operating System itself.
• Till the point Boot Loader starts loading the OS, there is nothing in the Main Memory of the
machine.
• After POST test, BIOS search for a place where bootloader is located.
• Boot loader is located at MBR (Master Boot Record) which further exists at 0th index of boot
disk
• BIOS instructs CPU to run bootloader in main memory (RAM) to start running OS
 System Booting: Process
5. Boot loader initialize the OS
• Control is transferred from BIOS to CPU and CPU to boot loader
• Boot loader responsible to initialize the OS into main memory
• Boot loader is not a single program for all kind of OS
• Each operating system has its own instruction for initialization.
• Each OS manufacturer write its own program for bootloader
• Microsoft has written bootmgr.exe bootloader to initialize Windows
• Apple has written boot.efi bootloader to initialize Mac:
• Linux has written Grub bootloader to initialize its OS
• The whole control is managed by bootloader
• Bootloader performs all the checks to initialize OS
 System Booting: Dual Booting
Dual booting is a configuration where two operating systems are installed on a single computer,
allowing the user to select which OS to boot into at startup. This setup is useful for users who
need to use features or applications specific to different operating systems.
Key Concepts
Boot Loader
• A boot loader is a small program that manages the boot process of a computer.
• It allows the user to choose between multiple operating systems at startup.
• Common boot loaders include GRUB (GNU GRand Unified Bootloader), LILO (Linux Loader),
and Windows Boot Manager.
Partitioning
• Hard disk partitioning is crucial for dual booting. Each OS needs its own partition, and
sometimes additional partitions are required for data or swap space.
• A typical dual-boot setup might include partitions for each OS and possibly a shared data
partition accessible by both.
 System Booting: Dual Booting
Key Concepts
File Systems
• Different operating systems often use different file systems. For example, Windows typically
uses NTFS or FAT32, while Linux might use ext4, btrfs, or other file systems.
• Shared partitions should use a file system that both operating systems can read and write,
such as FAT32 or exFAT.
 Dual Booting: Process
Step-by-Step Process
Backup Data
• Before making any changes to the disk, back up important data to prevent data loss.
Partition the Disk
• Use a partitioning tool (e.g., GParted, Disk Management in Windows) to create separate
partitions for each operating system.
• Ensure there is enough space for each OS and its applications.
Install the First Operating System
• Install one of the operating systems first, if it's not already installed.
• During installation, allocate the appropriate partition for this OS.
 Dual Booting: Process
Install the Second Operating System
• Boot from the installation media of the second OS.
• During installation, select the partition designated for this OS. Be careful not to overwrite the
existing OS.
• The installer of the second OS usually detects the presence of the first OS and configures the
boot loader to allow choosing between the two.
Configure the Boot Loader
• If using GRUB, it will usually detect both operating systems and present a menu at startup.
• For Windows Boot Manager, you might need to add the second OS manually using tools like
EasyBCD.
Post-Installation
• Update the OSes and drivers.
• Set up shared partitions or directories if needed.
• Customize the boot loader menu (e.g., setting default OS, timeout duration).
 Example: Dual Booting Windows and Linux
1. Install Windows
• Install Windows first as it tends to overwrite existing boot loaders without asking.
• During installation, create a partition for Windows, leaving space for Linux.
2. Partitioning for Linux
• After installing Windows, boot from a Linux live CD/USB.
• Use a partitioning tool to create partitions for Linux (e.g., root /, home /home, swap).
3. Install Linux
• Install Linux on the newly created partitions.
• The Linux installer will usually detect the Windows installation and add it to the GRUB boot
menu.
4. Boot Loader Configuration
• GRUB will be installed and set as the default boot loader.
• After installation, you can edit the GRUB configuration to set the default OS and timeout.
 System Call
A system call is a mechanism that provides the interface between a
process and the operating system. It is a programmatic method in which
a computer program requests a service from the kernel of the OS.

In other words
A system call is a procedure, written in c, c++ or assembly level
languages for performing various operations. A system call is a method
of interacting with the operating system via programs. A system call is a
request from computer software to an operating system's kernel.

System call offers the services of the operating system to the user
programs via API (Application Programming Interface). System calls are
the only entry points for the kernel system.
Fig 6. System call
 System Call: Several Definitions

• A system call is a programmed request to the kernel of the operating system to perform a
specific task that the program does not have the necessary permissions or capabilities to
execute directly.
• It provides an interface between a process and an operating system to allow user-level
processes to request services of the operating system.
• System call provides the services of the operating system to the user programs via
Application Program Interface(API).
• They allow processes to request services such as file manipulation, process control,
communication, and more.
 System Call: Execution Flow
•User Mode to Kernel Mode Transition: When a system call is made, the CPU switches from user
mode (where the application runs) to kernel mode, where the OS executes the requested service. This
transition is critical for protecting the system from errant or malicious behavior.

•System Call Interface: This is a set of functions exposed by the OS that applications can use to make
system calls. Each system call has a specific number and corresponding kernel routine.

•Trap Instructions: The transition to kernel mode is initiated by a trap instruction, which is a special
CPU instruction that triggers the system call handler.

•Execution in Kernel: The OS kernel processes the system call request, performs the necessary
operations, and returns the result to the user program.

•Return to User Mode: After executing the system call, control is returned to the user mode
application.
 System Call: Implementation
User Mode vs. Kernel Mode:
•Programs run in user mode with limited
access to resources.
•A system call causes a mode switch to
kernel mode, where the OS can perform
privileged operations.
•After the operation is complete, the
Fig 7. Transition from User mode to Kernel mode
system switches back to user mode.

System Call Interface: Trap Instruction:

•The OS provides a set of APIs (system •A trap is a special CPU instruction that triggers
call interface) that applications use to the execution of a system call.
invoke system calls. •It transfers control from user mode to the kernel
•Each system call is associated with a mode by causing an interrupt, which is handled
unique identifier or number that the by the OS kernel.
kernel recognizes.

21
 Application Program Interface
• Programmer uses API, which further uses system calls to implement services provided by
OS
• API is provided to simplify the concepts of system call.
• A programmer never use system call directly.

Fig 9. API - System call - OS Relationship

 Application Program Interface

• Some of the popular API are as follows:

• POSIX API: This API works with UNIX, LINUX

and MacOS X
• Win32 API: This API works with Windows
• Java API: This API works with Java Virtual
Machine

Fig 11. API and Designated OS

 Standard API: Example
Consider the ReadFile() function in the Win32 API—a function for reading from a file

Parameters passed to ReadFile() function

• HANDLE file: The file to be read
• LPVOID buffer: A buffer where the data will be read into and written from
• DWORD bytesToRead: The number of bytes to be read into the buffer
• LPDWORD bytesRead: The number of bytes read during the last read
• LPOVERLAPPED ovl: It indicates if overlapped I/O is being used
 API – System Call - OS Relationship
Interaction Flow:
• Application to API: An application program makes a call to an API
provided by the OS or a library.
• API to System Call: The API, which runs in user space, translates the
high-level API function call into one or more system calls, which are
the low-level interfaces to the OS.
• System Call to OS Kernel: The system call transfers control to the OS
kernel, which then executes the corresponding kernel function to
perform the requested operation. Fig 10. API - System call - OS
• OS Kernel to Hardware (if needed):The kernel function may interact Relationship
directly with hardware or manage resources such as memory,
processes, or files.
 API – System Call - OS Relationship: Example
Interaction Flow Example:
• Example: An application calls the printf() function from the C Standard Library (API).
• printf() internally calls the write() system call.
• The write() system call invokes the kernel function to write data to the appropriate file
descriptor (e.g., a terminal or a file).
• The OS kernel handles the hardware interaction (if necessary) and completes the request.
 System Call: Importance in OS Operations

•Security: By mediating access to critical resources, system calls ensure that user applications
cannot directly manipulate hardware or sensitive data, which could lead to system instability or
security breaches.

•Abstraction: System calls abstract the complexities of hardware operations, allowing

application developers to write code without needing to understand the underlying hardware
details.

•Resource Management: System calls are essential for managing system resources such as
memory, CPU, and I/O devices, ensuring fair and efficient use.
 System Call: Types

There are five types of System Calls in OS

PROCESS CONTROL FILE MANAGEMENT DEVICE INFORMATION COMMUNICATIONS

MANAGEMENT MAINTENANCE

Fig 15. System call Types

 System Call: Types
This system calls perform the task of process creation, process termination,
etc.
Functions
• End and Abort a process
PROCESS CONTROL
• Load and Execute process
• Create Process and Terminate Process
• Wait and Signal Event
• Allocate and free memory
File management system calls handle file manipulation jobs like creating a
file, reading, and writing, etc.
Functions
FILE MANAGEMENT Create a file, Delete file, Open and close file, Read, write, and reposition
Get and set file attributes
 System Call: Types
Device management does the job of device manipulation like reading from
device buffers, writing into device buffers, etc.
Functions
• Request and release device
DEVICE MANAGEMENT • Logically attach/ detach devices
• Get and Set device attributes

It handles information and its transfer between the OS and the user
program.

Functions
INFORMATION • Get or set time and date
MAINTENANCE • Get process and device attributes
 System Call: Types
These types of system calls are specially used for inter-process
communications.
Functions
• Create, delete communications connections
COMMUNICATIONS • Send, receive message
• Help OS to transfer status information
• Attach or detach remote devices
 System Call: Windows & Unix
Process Windows Unix

Process Control CreateProcess(), ExitProcess(), WaitForSingleObject() Fork(), Exit(), Wait()

File Manipulation CreateFile(), ReadFile(), WriteFile(), CloseHandle() Open(), Read(), Write(),

Close()
Device Management SetConsoleMode(), ReadConsole(), WriteConsole() Ioctl(), Read(), Write()

Information GetCurrentProcessID(), SetTimer(), Sleep() Getpid(), Alarm(), Sleep()

Maintenance
Communication CreatePipe(), CreateFileMapping(), MapViewOfFile() Pipe(), Shmget(), Mmap()

Protection SetFileSecurity(), InitializeSecurityDescriptor(), Chmod(), Umask(), Chown()

SetSecurityDescriptorgroup()
System Call: Windows & Unix

Fig 16. System Call, Windows vs Linux

 System Call: Some System Calls
System Explanation System Explanation
Call Call
open() It allows you to access Exec() When an executable file replaces an earlier
a file on a file system executable file in an already executing process,
this system function is invoked.
read() It is used to obtain data from a Close() It is used to end file system access. When this
file on the file system system call is invoked, it signifies that the
program no longer requires the file, and the
buffers are flushed
Wait() It is used to suspend the parent Write () It is used to write data from a user buffer to a
process. Once the child process has device like a file. This system call is one way for a
completed its execution, control is program to generate data
returned to the parent process.
fork() It is used to generate child process Exit() It is used to end program execution.
 Open-Source Operating System
• The term "open source" refers to computer software or applications where the owners
or copyright holders enable the users or third parties to use, see, and edit the product's
source code
• The open-source operating system allows the use of code that is freely distributed and
available to anyone and for commercial purposes.
• Being an open-source application or program, the program source code of an open-
source OS is available. The user may modify or change those codes and develop new
applications according to the user requirement.
 Open-Source Operating Systems: Working
• It works similarly to a closed operating system, except that the user may modify the
source code of the program or application. There may be a difference in function even if
there is no difference in performance.

• For instance, the information is packed and stored in a proprietary (closed) operating
system. In open-source, the same thing happens. However, because the source code is
visible to you, you may better understand the process and change how data is
processed.
• Some Open-Source Operating Systems are as follows:

Linux Kernal Linux Lite Linux Mint Fedora

Solus Chrome OS React OS

 MCQ
Q1. Which component performs the POST (Power-On Self-Test)?
A. CPU B. BIOS
C. RAM D. Bootloader
Q2. Where are BIOS settings stored?
A. RAM B. Hard Disk
C. CMOS D. SSD
Q3. Where is the Master Boot Record (MBR) located?
A. At the beginning of the RAM B. At the beginning of the hard disk
C. In the BIOS D. In the CMOS
Q4. What is a cold boot?
A. Restarting the system without powering off B. Starting the system from a powered-off state
C. Updating the BIOS settings D. Entering the boot menu
 MCQ
Q5. What is dual booting?
A. Running two operating systems simultaneously B. Installing two operating systems on a single
computer
C. Loading the operating system twice D. Booting from two different storage devices
Q6. What is a system call?
A. A hardware function B. A software program that manages computer hardware
C. Interface between a process and D. A type of virus
the operating system
Q7. Which of the following is a popular API used with UNIX, LINUX, and MacOS X?
A. Win32 API B. Java API
C. POSIX API D. Python API
Q8. What does the read() system call do?
A. Writes data to a file B. Reads data from a file
C. Closes a file D. Opens a file
 MCQ
Question No-Answer Option Description
Q1-Answer B BIOS
Q2-Answer C CMOS
Q3-Answer B At the beginning of the hard disk
Q4-Answer B Starting the system from a powered-off state
Q5-Answer B Installing two operating systems on a single computer
Q6-Answer C interface between a process and the operating system
Q7-Answer C POSIX API
Q8-Answer B Reads data from a file
 Summary/Key Points
• Booting is a start-up sequence that starts the operating system of a computer when it is turned
on.
• Booting sequence comprises of three components: Boot Loader, Boot device & Boot sequence
• A system call is a programmatic way in which a computer program requests a service from the
kernel of the operating system it is executed on.
• Process control system calls perform the task of process creation, process termination, etc.
• File management system calls handle file manipulation jobs like creating a file, reading, and
writing, etc.
• Device management does the job of device manipulation like reading from device buffers,
writing into device buffers, etc.
• Information maintenance system call handles information and its transfer between the OS and
the user program.
• Communication system calls are specially used for inter-process communications
• The open-source operating system allows the use of code that is freely distributed and available
to anyone and for commercial purposes.
 Reference Material
• Operating Systems Concepts (10th Ed.) Silberschatz A, Peterson J and Galvin P, John
Wiley & Sons, Inc. 2018.
Topic:
• Open-Source Operating Systems, Page No: 46-47
• System Calls, Page No: 62-65
• Types of System Calls, Page No: 66-73
• System boot, Page No: Page No: 94-95
• Modern Operating Systems (4th Ed.) by Andrew S. Tanenbaum and Herbert Bos, 2007
• Operating Systems: Principles and Practice by Thomas Anderson and Michael Dahlin,
2014
 Coming Up-Next Lecture
• Process: Program and Process concept
• Process in memory
• Process Control Block
• Process States
Thank You

43