Os-unit1-Introduction to Operating Systems.pdf

Unit 1: Introduction to Operating Systems
History of OS; simple batch systems; multiprogrammed
batch systems; time-sharing systems; distributed systems
and real-time systems; operating system structures;
services; system calls; system programs and virtual
machines.

Introduction to Operating Systems
An operating system is a program that acts as a link between the
user and the computer system. The presence of an OS is very
important as without it other programs can’t run. Thus, it manages
the resources on a system and they cannot function when the OS is
absent. It has existed since the first generation of computers and has
been evolving with time

An operating system is a software programme that is used to manage and control
computing devices such as smartphones, tablets, computers, supercomputers, web
servers, automobiles, network towers, smartwatches, and other similar devices.
It is the operating system which eliminates the requirement to understand a coding
language in order to interface with computers.
It’s a graphical user interface (GUI) layer that serves as a bridge between the user and
the computer hardware.
The operating system regulates programme execution and administers the software
side of a computer.

Services provided by an operating system :
Program Execution
The OS allows you to run programs, ensuring they execute smoothly. It loads the program
into memory, starts it, and manages its operation.
I/O Operations
The OS handles Input/Output devices like keyboards, printers, and monitors, allowing
programs to read data (input) and display results (output).
File System Manipulation
It manages files and directories on storage devices. You can create, delete, read, write, and
organize files efficiently.
Communication
Programs can communicate with each other via the OS. This is useful for networked
applications or processes working together.

Error Detection
The OS monitors the system for errors (e.g., hardware issues, software bugs) and takes
steps to correct or report them.
Resource Allocation
It distributes system resources like CPU, memory, and storage among running programs
to ensure fairness and efficiency.
Protection
The OS safeguards data and resources by controlling access, ensuring only authorized
programs and users can interact with the system.

Diving into the History of Operating Systems
1. In 1956, twenty years after the Z1 computer, which was the first computer to
run without an operating system, the first-ever operating system was built to
manage tape storage.
2. After that, Bell Labs created UNIX, the first multitasking operating system.
3. In 1977, Apple Dos 3.3 was released, which was the first disc operating
system.
4. In 1981, Microsoft was the next company to develop a DOS operating
system.
5. MS-DOS was combined with Graphics User Interface in 1985. (GUI).
6. As of October 2021, the most recent version of Windows for PCs and tablets
is Windows 11.

Os-unit1-Introduction to Operating Systems.pdf

Between hardware and
software, operating systems are
a middleman. Without an
operating system, hardware
and software cannot function
properly.
The operating system consists
of two components:
1. Shell
2. Kernel

In a computer system, both of
these are utilized to establish
communication and operate the
system. However, there is a
significant distinction between
shell and kernel. A shell is a type
of interface that connects the
kernel and the user. It enables all
of the system’s users to
communicate with the kernel. A
kernel is the fundamental heart
of a typical operating system. Its
purpose is to manage all of the
duties associated with a system.

Simple batch systems
Batch processing is a technique in which Operating System collects one programs and data
together in a batch before processing starts. Operating system does the following activities
related to batch processing.A simple batch system is a basic type of operating system that
doesn't allow direct communication between users and the computer:
● User preparation: Users prepare a job on control cards that includes the program,
data, and control information.
● Job submission: Users submit the job to a computer operator on punch cards or tape.
● Batch creation: The computer operator groups jobs with similar requirements into
batches.
● Batch execution: The computer runs the batches one after the other.
● Output: The output from the jobs is produced after a certain period of time, which
could be minutes, hours, or days.
● Batch monitor: The batch monitor manages the execution of each program in the
batch.

Advantages
● Batch processing takes much of the work of the operator to the computer.
● Increased performance as a new job gets started as soon as the previous job
finished without any manual intervention.
Disadvantages
● Difficult to debug program.
● A job could enter an infinite loop.
● Due to lack of protection scheme, one batch job can affect pending jobs.

Multiprogrammed batch systems
Multiprogramming is a variation of batch processing in which the CPU is kept
busy at all times. CPU time and IO time are two forms of system time required by
each process. When a process completes its I/O in a multiprogramming
environment, the CPU can begin the execution of other processes. As a result,
multiprogramming helps in improving the system’s efficiency.(non preemptive)
Multiprogramming operating systems are divided into two categories, and they
are:
● Multitasking Operating System
● Multiuser Operating System

Multitasking Operating System
A multitasking OS allows two or more programs to run
simultaneously. This is accomplished by the operating system
transferring each program into or out of memory one by one.
Multiuser Operating System
A multiuser operating system allows multiple users from various
terminals to share the processing time on a certain powerful central
machine. The operating system achieves this by frequently switching
between terminals, each of which is allotted a certain amount of
processor time on a central computer.

Disadvantages
•It’s quite complex and sophisticated.
•CPU scheduling is necessary.
•Since all types of tasks get stored in the main memory, memory management is
required in the operating system.
•The more difficult challenge is managing all procedures and tasks.

Time-sharing system
It allows the user to perform more than one task at a time, each task getting the
same amount of time to execute. Hence, the name time sharing OS. Moreover, it
is an extension of multiprogramming systems. In multiprogramming systems, the
aim is to make the maximum use of the CPU. On the other hand, here the aim is
to achieve the minimum response time of CPU.

Working of Time Sharing Operating System
The operating system performs time sharing through CPU scheduling and multiprogramming.

Distributed systems
A Distributed Operating System (DOS) is software that manages a group of independent
computers (nodes) connected through a network, making them appear and function as a
single cohesive system to users. It enables resource sharing, task distribution, and fault
tolerance while providing transparency about the underlying complexity.

Features:
1. Transparency: Users see the system as one, not as multiple computers.
○ Access transparency: Users access resources without worrying about their
physical location.
○ Location transparency: Users don't need to know where the resource is located.
2. Resource Sharing: Shares hardware (CPUs, memory, storage) and software (files,
applications) across nodes.
3. Fault Tolerance: If one node fails, the system continues working by using other
nodes.
4. Scalability: Can add more computers to handle increased loads.
5. Concurrency: Multiple users can run tasks simultaneously.

Real-time systems(RTOS)
A Real-Time Operating System (RTOS) is a specialized operating system
designed to process data and respond to inputs or events within a strict time
constraint. It is primarily used in systems where timing and reliability are critical,
such as embedded systems, medical devices, robotics, and automotive systems.

Hard Real-Time Operating Systems
These systems guarantee task completion within strict deadlines. Failure to
meet a deadline results in catastrophic consequences.
Characteristics:
○ Deterministic behavior with zero tolerance for delays.
○ Used in life-critical and mission-critical systems.
Examples:
○ Airbag systems in cars.
○ Medical devices like pacemakers.
○ Spacecraft control systems.

Soft Real-Time Operating Systems
Deadlines are important, but occasional delays are acceptable, though they
may degrade performance.
Characteristics:
○ Prioritizes tasks but allows flexibility for lower-priority tasks.
○ Suitable for non-critical but time-sensitive applications.
Examples:
○ Online video streaming.
○ Video conferencing applications.
○ Gaming consoles.

Firm Real-Time Operating Systems
Missing a deadline does not cause catastrophic failure, but the output
becomes less useful if a deadline is missed.
Characteristics:
○ Falls between hard and soft RTOS in terms of timing strictness.
○ Failure to meet deadlines affects system efficiency but not its
functionality.
Examples:
○ Industrial automation systems.
○ Banking and stock trading systems.

Operating system structures
The structure of an Operating System (OS) refers to how its
components are organized to perform various tasks effectively.
Different structures are used based on the OS design and its goals,
such as efficiency, modularity, or simplicity.

Types of Operating Systems Structures
● Simple Structure
● Monolithic Structure
● Micro-Kernel Structure
● Layered Structure
● Virtual Machines

Simple Structure
No Strict Layering
Direct
Communication

Monolithic Structure
A Monolithic OS structure is fast and
efficient for small or less complex systems
but struggles with maintainability and
scalability for larger, modern computing
environments.

Layered Structure
The OS is divided into hierarchical layers, with each layer performing specific tasks and relying on the lower
layers.
Clear separation of functionality.
Easier debugging and
maintenance.
Performance overhead due to
inter-layer communication.

Virtual Machines
Non Virtual Machine Virtual Machine

A hypervisor
(VMM) is a
software that you
can use to run
multiple virtual
machines on a
single physical
machine.

An Operating System provides services to both the users and to the programs. It provides
programs, an environment to execute. It provides users, services to execute the programs in
a convenient manner.
Following are few common services provided by operating systems.
● Program execution
● I/O operations
● File System manipulation
● Communication
● Error Detection
● Resource Allocation
● Protection

Program execution
A process includes the complete execution context (code to execute, data to manipulate,
registers, OS resources in use). Following are the major activities of an operating system with
respect to program management.
● Loads a program into memory.
● Executes the program.
● Handles program's execution.
● Provides a mechanism for process synchronization.
● Provides a mechanism for process communication.
● Provides a mechanism for deadlock handling.

I/O operations
Operating System manages the communication between user and device drivers. Following are
the major activities of an operating system with respect to I/O Operation.
● I/O operation means read or write operation with any file or any specific I/O device.
● Program may require any I/O device while running.
● Operating system provides the access to the required I/O device when required.

File System manipulation
A file system is normally organized into directories for easy
navigation and usage. These directories may contain files and other
directions. Program needs to read a file or write a file.
● The operating system gives the permission to the program for
operation on file.
● Permission varies from read-only, read-write, denied and so
on.
● Operating System provides an interface to the user to
create/delete files.
● Operating System provides an interface to the user to
create/delete directories.
● Operating System provides an interface to create the backup of
file system.

Communication
OS handles routing and connection strategies, and the problems
of contention and security.
● Two processes often require data to be transferred between
them.
● The both processes can be on the one computer or on
different computer but are
connected through computer network.
● Communication may be implemented by two methods
either by Shared Memory or by Message Passing

Error Detection
Error can occur anytime and anywhere. Error
may occur in CPU, in I/O devices or in the
memory
hardware.
● OS constantly remains aware of
possible errors.
● OS takes the appropriate action to
ensure correct and consistent
computing.

Resource Allocation
In case of multi-user or multi-tasking
environment, resources such as main
memory, CPU cycles
and files storage are to be allocated to each
user or job.
● OS manages all kind of resources using
schedulers.
● CPU scheduling algorithms are used for
better utilization of CPU

Protection
Protection refers to mechanism or a
way to control the access of
programs, processes, or users
to the resources defined by
computer systems.
● OS ensures that all access to
system resources is
controlled.
● OS ensures that external I/O
devices are protected from
invalid access attempts.
● OS provides authentication
feature for each user by
means of a password.

System calls
The interface between a process and an operating system is provided by system
calls.
The processes execute normally in the user mode until a system call interrupts
this. Then the system call is executed on a priority basis in the kernel mode. After
the execution of the system call, the control returns to the user mode and
execution of user processes can be resumed.

System calls are required in the following situations −
● If a file system requires the creation or deletion of files.
Reading and writing from files also require a system call.
● Creation and management of new processes.
● Network connections also require system calls. This includes
sending and receiving packets.
● Access to a hardware devices such as a printer, scanner etc.
requires a system call.

Types of System
Calls Windows Linux
Process Control
CreateProcess() fork()
ExitProcess() exit()
WaitForSingleObject() wait()
File Management
CreateFile() open()
ReadFile() read()
WriteFile() write()
CloseHandle() close()
Device Management
SetConsoleMode() ioctl()
ReadConsole() read()
WriteConsole() write()

Types of System
Calls Windows Linux
Information
Maintenance
GetCurrentProcessID() getpid()
SetTimer() alarm()
Sleep() sleep()
Communication
CreatePipe() pipe()
CreateFileMapping() shmget()
MapViewOfFile() mmap()

System programs
System programs provide a convenient environment for program development
and execution.
System calls and system programs are critical to the operation of the OS. These
structures describe the functions that the operating system must carry out. The
system call establishes a connection between the user software and the operating
system's services. In contrast, the system software defines the OS user interface.
The system program also offers a proper environment for the development and
execution of a program.

It traditionally lies between the user interface and system calls so users can only
view up-to-the System Programs he can’t see System Calls.

System Programs can be divided into these categories:
File Management:
Create, delete, copy, rename, print, dump and generally manipulate files and directories.
A file is a collection of specific information stored in the memory of a computer system.
File management is defined as the process of manipulating files in the computer system i.e.
process of creating, modifying and deleting files.
It helps to create new files in the computer system and placing them at specific locations.
It helps in easily and quickly locating these files in the computer system.
It helps to store files in separate folders known as directories.

Status Information:
Information like date, time, amount of available memory, disk space and number of
users etc are provided by such system programs.
Some also provide details like performance, logging and debugging information which
is more complex.
All this information is formatted and displayed on output devices or printed. Terminal
or other devices or files or a window of GUI is used for showing the output of programs.

File Modification:
e.g. text editors to create and modify files.
For searching contents of files or perform transformations of files
we use special commands.

Programming Language Support:
For common programming languages, we use Compilers,
Assemblers, Debuggers and interpreters which are already
provided to users.
It provides all support to users.
We can run any programming language. All languages of importance
are already provided.

Program Loading and Execution:
When the program is ready after Assembling and compilation, it must be
loaded into memory execution.
A loader is part of an operating system that is responsible for loading
programs and libraries.
It is one of the essential stages for starting a program.
Loaders, linkage editors and overlay loaders are provided by the system.

Communications:
Virtual connections among processes, users and computer systems are
provided by the programs.
Users can send messages to another user on their screen, can send email,
browse on web pages, can transfer the files from one user to another etc.

System Call Example
A system call is a request made by a program to the operating system's kernel to perform low-level operations, such as reading a file,
writing data, or creating a process.
Example: Reading a File
In C programming:

System Program Example
A system program provides a user-friendly way to interact with system functionalities like file management, process management, or
system settings.
Example: Copying a File (using a shell command)
This cp command is a system program that uses system calls internally (like open(), read(), and write()) to copy the content of
file1.txt into file2.txt.

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