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Module I

Operating Systems�Course Code: BCS303�

Latharani T R

Assistant Professor

Dept. of Computer Science and Engineering

Jain Institute of Technology

Davangere.

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Course outcomes �

CO1: Explain the structure and functionality of Operating system.
CO2: Apply appropriate CPU scheduling algorithms for the given problem.
CO3: Analyse the various techniques for process synchronization and deadlock handling.
CO4: Apply various techniques for memory management.
CO5: Explain file and secondary storage management strategies.
CO6: Describe the need for information storage mechanisms.

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Textbooks & Reference Books �

1. Abraham Silberschatz, Peter Baer Galvin, Greg Gagne, Operating System Principles, 10th edition, Wiley-India, 2018

2. D.M Dhamdhere, Operating Systems: A Concept Based Approach, 3rd Ed, McGraw- Hill, 2013.

3. William Stallings, Operating Systems: Internals and Design Principles, 9th Edition, Pearson.

4. Andrew S.Tanenbaum, Modern operating Systems, fourth Edition, PHI Learning Pvt.Ltd., 2008

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WHY STUDY OPERATING SYSTEMS

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Almost all code runs on top of an operating system (OS).
Knowledge of how operating systems work is crucial to proper, efficient, effective, and secure programming.
Understanding

The fundamentals of operating systems,
How they drive computer hardware, and
What facilities they provide to the applications

is essential for both users and programmers of the OS.

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Definition of Operating Systems�

An operating system is software that manages a computer’s hardware.
Acts as an interface between a computer user and computer hardware.
They are capable of accomplishing these tasks in a wide variety of computing environments.
An operating system is called as a resource manager as it manages all the resources of a computer system efficiently like

CPU time,
Memory space,
Input/output devices,
Disk space,
Network bandwidth and
Other hardware resources.

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Performs all the basic tasks like

File management,
Memory management,
Process management,
Handling input and output, and
Controlling peripheral devices such as disk drives and printers.

Some popular Operating Systems include

UNIX
Linux,
Windows,
Android,
iOS,
Ubuntu etc.

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Operating systems are everywhere, from

Home appliances that include “Internet of Things” devices,
Smart phones,
Personal computers,
Enterprise computers,
Mainframe computers,
Cloud computing environments,
Automobiles,
TV,
Toys etc.

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Computer System Components�

A computer system can be divided roughly into four components:

The hardware,
The operating system,
The application programs, and
A user.

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Abstract view of the components of a computer system

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Two viewpoints of OS

User view

System View

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User View

User's view of the computer varies based on the interface.
Different devices offer diverse user experiences.

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Traditional Computer Systems

PC and laptop setup with monitor, keyboard, and mouse.
Designed for single-user resource utilization.
Emphasis on ease of use over resource optimization.

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Shift to Mobile Devices

Increasing use of smartphones and tablets.
Connected through wireless technologies.
Touch screen interfaces replace traditional input methods.

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Mobile User Interface

Touch screen interactions (finger touch, press hold, swiping).
Voice recognition interfaces (e.g., Siri, Alexa).
Reflects the trend towards intuitive and user-friendly designs.

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Embedded Systems

Some computers lack a traditional user view.
Examples: home devices (washing machines, microwaves), automobiles etc.
Designed for autonomous operation with minimal user intervention.

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System View

The operating system plays a crucial role from the computer's perspective.
It serves as the interface between hardware and software.
Modern OS allows multiple programs to run simultaneously.
Manages the user programs’ execution.

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Resource Allocation

OS deeply involved with hardware.
Viewed as a resource allocator.
Allocates CPU time, memory space, storage, and I/O devices efficiently.
Ensures optimal utilization of available resources.
Balances efficiency and fairness in resource allocation in case of conflicting resource requests.

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Computer-System Organization

A modern general-purpose computer system consists of

1. one or more CPUs and

2. a number of device controllers connected through a common bus that provides access to shared memory

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Each device controller is in charge of a specific type of device (for example, disk

drives, audio devices, and video displays).

∙ The CPU and the device controllers can execute concurrently, competing for memory

cycles.

✔ For a computer to start running—for instance, when it is powered up or rebooted—

The initial program, or bootstrap program, is executed when the system is powered on.
Bootstrap program is stored in read-only memory (ROM) or electrically erasable

programmable read-only memory (EEPROM).

The bootstrap program must load the operating system

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Then the OS starts executing the first process such as init and waits for some event to occur.
When the CPU is interrupted, it stops what it is doing and immediately transfers execution to interrupt service routine (ISR).
The ISR executes; on completion, the CPU resumes the interrupted computation.

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Interrupt time line for a single process doing output.

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Storage Structure �

Computer programs must be in main memory to be executed.
Main memory is the only large storage area that the processor can access directly.
Interaction is achieved through a sequence of load or store instructions to specific memory addresses.
The load instruction moves a word from main memory to an internal register within the CPU, whereas the store instruction moves the content of a register to main memory

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We want the programs and data to reside in main memory permanently.
This arrangement usually is not possible for the following two reasons:

Main memory is too small to store all needed programs and data permanently.
Main memory is a volatile storage device

Most computer systems provide secondary storage as an extension of main memory.
The main requirement for secondary storage is that it be able to hold large quantities of data permanently.
The most common secondary-storage device is a magnetic disk, which provides

storage for both programs and data.

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The higher levels are expensive, but they are fast.
As we move down the hierarchy, the cost per bit decreases, whereas the access time increases.

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Storage-devices Hierarchy

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Computer programs must be in main memory to be executed.
Main memory is the only large storage area that the processor can access directly.
Interaction is achieved through a sequence of load or store instructions to specific memory addresses.
The load instruction moves a word from main memory to an internal register within the CPU, whereas the store instruction moves the content of a register to main memory

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I/O Structure

A computer system consists of CPUs and multiple device controllers that are connected through a common bus.
Each device controller is in charge of a specific type of device.
A device controller maintains some local buffer storage and a set of special-purpose

registers.

The device controller is responsible for moving the data between the peripheral devices that it controls and its local buffer storage.
Typically, operating systems have a device driver for each device controller.
This device driver understands the device controller and presents a uniform interface to the device to the rest of the operating system.

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To start an I/O operation,

1.The device driver loads the appropriate registers within the device controller.

2.The device controller, in turn, examines the contents of these registers to determine what action to take (such as "read a character from the keyboard").

3.The controller starts the transfer of data from the device to its local buffer.

4.Once the transfer of data is complete, the device controller informs the device driver via an interrupt that it has finished its operation.

5.The device driver then returns control to the operating system, possibly returning the data or a pointer to the data if the operation was a read.

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This form of interrupt-driven I/O is fine for moving small amounts of data but can produce high overhead when used for bulk data movement such as disk I/O.
To solve this problem, direct memory access (DMA) is used.
After setting up buffers, pointers, and counters for the I/O device, the device controller transfers an entire block of data directly to or from its own buffer storage to memory, with no intervention by the CPU.

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Shows the interplay of all components of a computer system.

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Computer-System Architecture

Single-Processor Systems
Multiprocessor Systems
Clustered Systems

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Single-Processor Systems

Combine a main CPU with special-purpose processors to enhance performance and efficiency.

Main CPU:

Executes a general-purpose instruction set.
Handles both system and user processes.
Central unit for processing tasks.

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Special-Purpose Processors:

Dedicated to specific tasks (e.g., disk, keyboard, graphics controllers).
Examples:

Disk controllers manage disk operations and scheduling.
Keyboard processors convert keystrokes to signals.

Benefits:

Offload tasks from the CPU.
Improve overall system efficiency.

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Multiprocessor Systems

Key Advantages:

Increased Throughput: More processors result in faster task completion.
Economy of Scale: Sharing of peripherals, storage, and power supplies reduces costs.
Increased Reliability: Failure of one processor doesn't halt the system, just slows it down.

Graceful Degradation:

Ability to continue operation at a reduced level after a component failure.

Fault Tolerance:

Some systems go beyond and can operate seamlessly even after a single component failure.

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Types of Multiprocessing Systems

Asymmetric Multiprocessing (AMP):

Each processor is assigned a specific task.
Master-Slave Relationship:

Master processor controls the system.
Slave processors execute predefined tasks or await instructions.

Example: SunOS Version 4.

Symmetric Multiprocessing (SMP):

All processors are peers, with no master-slave relationship.
Each processor performs all tasks within the OS.
Typical SMP setups are balanced and efficient for multitasking.

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Symmetric multiprocessing architecture.

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Modern Trends in Multiprocessing

Multi-Core Processors:

Definition: Multiple compute cores on a single chip, essentially a multiprocessor on one chip.
Examples:

Dual-Core Design: Two cores on the same chip, each with its own registers and cache.
N-Way Chips: More cores on a chip, becoming common in high-end systems.

Core Design Variations:

Local Cache: Each core has its own cache.
Shared Cache: Cores share a cache.
Combination: Mix of local and shared caches.

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A dual-core design with two cores placed on the same chip

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Clustered Systems

Consist of multiple CPUs gathered together to perform computational tasks.
Composed of two or more individual systems connected via a Local-Area Network (LAN) or high-speed interconnects like InfiniBand.

Key Advantage:

High Availability: Service continues even if one or more systems in the cluster fail, achieved through redundancy.

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General structure of a clustered system

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How Clustered Systems Work

Cluster Software Layer:

Runs on all nodes within the cluster.
Monitoring:

Each node monitors others via the network.
If a node fails, another node takes over its storage and restarts its applications.

Types of Clustering:

Asymmetric Clustering:

One machine is in hot-standby mode, monitoring the active server.
If the active server fails, the standby machine takes over.

Symmetric Clustering:

Multiple machines run applications and monitor each other.
More efficient as it utilizes all available hardware.

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Types of Operating Systems

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Batch Systems

Designed for sequential job management.
Jobs processed in bulk with predetermined input.
Minimal operator intervention during processing.
Ideal for handling large volumes of similar jobs with common requirements efficiently.

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Batch Processing Operating Systems

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Results in speed enhancement.

Key benefit: Can handle multiple users simultaneously.
Suitable for large projects with numerous tasks.

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Widely used in business and scientific applications.
Maximizing resource utilization (CPU time, memory, storage).
Efficient scheduling and processing of jobs in batches.

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Multiprogrammed Systems

Ability to run multiple programs is crucial for optimal system performance.
Users want to run more than one program simultaneously.
Single programs may not keep CPU or I/O devices consistently busy.

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Multiprogramming for CPU Utilization

Modern OS employs multiprogramming for CPU utilization.
Allows several jobs in memory simultaneously.
CPU always has a job to execute, eliminating idle time.
Multiprogramming increases CPU utilization.
CPU scheduling ensures efficient process execution.

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Processes may wait for tasks like I/O operations to complete.

CPU switches to another process during waiting periods.

Memory layout for a multiprogramming system

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Multitasking Systems�

It is a logical extension of multiprogramming.
In multitasking systems, the CPU executes multiple processes by switching among them,
Switches occur frequently, providing the user with a fast response time.
CPU switches to another process during waiting periods.

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Time-Sharing Systems

Logical extension of multiprogramming for fair user resource access.
More complex than multiprogramming.
Enables multiple users to access computer resources concurrently.
Ensures fair and impartial resource allocation.

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Users get CPU attention in time slices, slots, or quantum.
Typically 10 to 100 milliseconds duration.
Primary goal: Provide each user with the illusion of sole control.
Multiple jobs kept in memory simultaneously.
Requires advanced mechanisms for fair user access.

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Time-Sharing Systems

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Real-Time Systems (RTS)

RTS prioritize meeting of specific timing constraints or deadlines.
Unlike traditional systems, emphasis on delivering results within a specified time frame.
Widely used in critical applications where timely responses are crucial.

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Real-Time Systems

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Two main types:

Hard Real-Time Systems
Soft Real-Time Systems

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Hard Real-Time Systems

Deadline adherence is critical.
Used in safety-critical applications like medical devices and aerospace systems.

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Soft Real-Time Systems

Timing constraints exist, but occasional deadline misses are acceptable.
Found in applications like multimedia, video streaming, and online gaming.

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Application Areas

Embedded systems commonly run real-time operating systems.

Scientific experiments,
medical imaging,
industrial control, and
display systems.
Specific applications in automobile-engine fuel injection,
home appliances, and
weapon systems.

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Operating System Operations

Dual-Mode Operation

Purpose:

To distinguish between the execution of operating-system code and user-defined code.

Modes of Operation:

User Mode
Kernel Mode (also known as Supervisor Mode, System Mode, or Privileged Mode)

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Mode Bit:

A hardware bit added to indicate the current mode:

Kernel Mode (0)
User Mode (1)

When executing user applications, the system is in User Mode.
When a system call is made, the system switches to Kernel Mode.

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System Operations and Transitions
System Boot:

Hardware starts in Kernel Mode.
The operating system is loaded, and user applications start in User Mode.

System calls Interrupts and Traps:

Cause the system to switch from User Mode to Kernel Mode.
Operating system takes control in Kernel Mode.
System switches back to User Mode when control is passed to a user program.

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Transition from user to kernel mode

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Protection Purpose:

To protect the operating system from errant users and users from one another.

Privileged Instructions:

Certain machine instructions that may cause harm are designated as privileged.
These can only be executed in Kernel Mode.

Illegal Instructions:

If a privileged instruction is attempted in User Mode, it is trapped by the hardware and treated as illegal.

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Timer

Prevent user programs from getting stuck, running indefinitely without returning control to the operating system.
A timer is set to interrupt the system after a specified period.
Period can be fixed or variable.

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Variable Timer Implementation:

Fixed-Rate Clock ticks consistently.
Counter decrements with each tick.
When counter reaches 0, an interrupt occurs.

Operating System Role:

OS sets the timer before handing control to the user program.
Timer ensures the program does not exceed allowed runtime.
If the counter reaches zero or negative, the OS terminates the program.

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Process Management

A process is a program in execution.
Resources Required:

CPU time, memory, files, and I/O devices.

Process Lifecycle:

When a process terminates, the OS reclaims any reusable resources.

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Program:

A passive entity, such as a file stored on disk.

Process:

An active entity in execution.

Single-Threaded Process:

One program counter specifying the next instruction to execute.

Multithreaded Process:

Multiple program counters, each pointing to the next instruction.

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OS Responsibilities in Process Management

Creating and Deleting Processes:

Manages both user and system processes.

Suspending and Resuming Processes:

Temporarily halts and restarts processes.

Process Synchronization:

Ensures proper execution sequence among processes.

Process Communication:

Facilitates inter-process communication.

Deadlock Handling:

Manages situations where processes are stuck waiting for each other.

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Memory Management

Main Memory is a large array of words or bytes that serves as a repository for quickly accessible data.

Shared by: CPU and I/O devices.

Memory Access:

Instruction-Fetch Cycle: CPU reads instructions from memory.
Data-Fetch Cycle: CPU reads and writes data to memory.

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Importance of Memory Management

Direct Access:

Main memory is typically the only large storage device that the CPU can directly address and access.

Need for Memory Management:

To improve CPU utilization and system responsiveness, multiple programs must be kept in memory.
This necessitates effective memory management by the operating system.

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OS Responsibilities in Memory Management

Tracking Memory Usage:

Monitoring which parts of memory are in use and by whom.

Deciding Memory Allocation:

Determining which processes and data to move in and out of memory.

Allocating and De-allocating Memory:

Dynamically allocating and freeing memory space as needed.

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Storage Management

1. File-System Management

2. Mass-Storage Management

3. Caching

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File-System Management

File - collection of related information defined by its creator.
They represent programs (both source and object forms) and data.
Data files maybe numeric, alphabetic, alphanumeric, or binary.
Normally organized into directories to make them easier to use.
When multiple users have access to files, it's important to manage who can access them and define the specific actions they can perform, such as reading, writing, or appending data.

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OS Responsibilities in File Management

1. Creating and deleting files

2. Creating and deleting directories to organize files

3. Supporting primitives for manipulating files and directories

4. Mapping files onto secondary storage

5. Backing up files on stable (nonvolatile) storage media

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Mass-Storage Management

Programs are stored on a disk until loaded into memory.
Disks serve as both the source and destination for processing data.

OS Responsibilities:

Efficient management is crucial for overall system performance.

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Key Activities in Disk Management

Free-Space Management:

Keeping track of available storage space.

Storage Allocation:

Determining how disk space is allocated to files and programs.

Disk Scheduling:

Optimizing the order in which disk operations are performed for efficiency.

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Secondary Storage:

Frequently used for active processing; needs to be fast and efficient.

Tertiary Storage:

Slower, used for backups, seldom-accessed data, and long-term archival.
Examples: Magnetic tape drives, CDs, DVDs.

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Caching

Caching is the process of storing copies of data in a faster storage system (the cache) temporarily.
Purpose: To speed up data access by checking the cache before fetching from the main storage.

Process:

Check if the data is in the cache.
If present, use the cached data.
If not, fetch from the source, cache it, and then use it.

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Importance of Caching

Performance Boost:

Without caching, the CPU would need to wait several cycles for data from main memory, slowing down processing.

Storage Hierarchy:

Main Memory as Cache: Main memory acts as a fast cache for secondary storage, bridging the speed gap between the CPU and slower storage systems.

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Caching Example & Data Propagation

Increment Operation Example:

I/O Operation: Copy the disk block containing data A to main memory.
Copying A: Transfer A from main memory to the cache and an internal register.
Data Distribution: A exists on the magnetic disk, in main memory, in the cache, and in an internal register.
Inconsistency Post-Operation: After incrementing A in the internal register, A's value differs across storage systems.

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Migration of integer A from disk to register

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I/O Systems

The I/O subsystem consists of several components:

1. A memory-management component that includes buffering, caching, and spooling

2. A general device-driver interface

3. Drivers for specific hardware devices.

Only the device driver knows the peculiarities of the specific device to which it is assigned

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Protection and Security

Mechanism for controlling the access of processes or users to system resources.
Defense against internal and external attacks on the system.

Purpose of Protection:

Access Control: Ensures that only authorized users or processes can access certain resources.
Error Detection: Improves system reliability by detecting latent errors at the interfaces between subsystems.

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Viruses and Worms: Malicious software that can harm the system.
Denial of Service (DOS): Overwhelming the system to make it unavailable.
Identity Theft: Stealing personal information to impersonate someone.

User Authentication:

Systems maintain user names and IDs for distinguishing users.
During login, the system assigns the appropriate user ID, which is linked to all processes and threads initiated by that user.

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Distributed Systems

Collection of physically separate, heterogeneous computer systems that are networked to provide the users with access to the various system resources
Access to a shared resource increases

Computation speed
Functionality
Data availability and
Reliability

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A network is a communication path between two or more systems.

Networks vary by the
Protocols used
Distances between nodes and
Transport media.

Common network protocol are

TCP/IP (Transmission Control Protocol/Internet Protocol )
ATM (Asynchronous Transfer Mode)

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Networks are characterized based on the distances between their nodes.

A local-area network (LAN) connects computers within a building.
A wide-area network (WAN) usually links buildings, cities, or countries.
A metropolitan-area network (MAN) could link buildings within a city.

The media to carry networks are

Copper wires,
Fiber strands, and
Wireless transmissions

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Special-Purpose Systems

1. Real-Time Embedded Systems

2. Multimedia Systems

3. Handheld Systems

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Real-Time Embedded Systems

Found in devices like car engines, manufacturing robots, VCRs, and microwave ovens.
Designed for dedicated tasks with limited, focused functions.

System Characteristics:

Operate on simple hardware with minimalistic operating systems.
Primarily focused on monitoring and managing hardware, like automobile engines and robotic arms.

Real-Time Operating Systems (RTOS):

Systems with rigid time constraints for processing.
Tasks must be completed within set time limits to avoid system failure.
Embedded systems often rely on RTOS to meet precise timing requirements.

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Multimedia Systems

Multimedia data consist of audio and video files as well as conventional files.
Multimedia data must be delivered(streamed) according to certain time restrictions.
Multimedia describes a wide range of applications. These include

1. Audio files such as MP3

2. DVD movies

3. Video conferencing

4. Live webcasts of speeches

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Handheld Systems

Handheld systems include

PDAs and
Cellular telephones.

Main challenge - Limited size of devices.
Because of small size, most handheld devices have a

Small amount of memory,
Slow processors, and
Small display screens.

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Computing Environments

1. Traditional Computing

2. Client-Server Computing

3. Peer-to-Peer Computing

4. Web-Based Computing

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Traditional Computing

Office Environment:

PCs and Networks: Personal computers connected via a network with servers providing file and print services.

Home Networks:

Single Computer Setup: Typically, a single computer connected through a slow modem.
Security Measures: Some homes employ firewalls to protect against security breaches.

Expansion with Web Technologies:

Portals: Companies use web portals for access to internal servers.
Network Computers: Devices focused on web computing.
Handheld PDAs: Connect to wireless networks for accessing company portals.

System Types:

Batch Systems: Processed jobs in bulk with predetermined input.
Interactive Systems: Wait for user input to process.

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Client-Server Computing

Server Categories:

1) Compute Servers:

Provides an interface for clients to request actions (e.g., read data).
Server executes the requested action and returns results to the client.

2) File Servers:

Offers a file-system interface for clients.
Clients can create, read, and delete files.
Example: A web server that delivers files to clients using web browsers.

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Peer-to-Peer Computing

All nodes are considered peers
Each can act as both a client and a server.
Advantage:
Unlike client-server systems, where the server can be a bottleneck, peer-to-peer systems distribute services across multiple nodes.

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Joining the Network:
A node must join the peer network to participate.
Service Discovery Methods:

Centralized Lookup:

Node registers its service with a centralized lookup service.
Other nodes contact this service to find out which node provides the desired service.

Broadcast Request:

A peer broadcasts a service request to all other nodes.
Nodes providing the service respond to the request.

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Web-Based Computing

This includes

PC
Handheld PDA &
Cell phones

Load balancer is a new category of devices to manage web traffic among similar servers.
In load balancing, network connection is distributed among a pool of similar servers.
Use of operating systems like Windows 95, client-side, have evolved into Linux and Windows XP, which can be clients and servers

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Operating System Services

OS makes certain services available to users and their programs.
These services make the programming task easier for the programmer.
The services provided may differ from one system to another, but we can identify common classes.

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A view of operating system services

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User interfaces

Graphical User Interface (GUI)
Touch-screen interface.
Command-Line Interface (CLI)

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Graphical User Interface (GUI)

Most common UI.
Window system with a mouse as a pointing device.
Users interact through menus, make selections, and input text via a keyboard.

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Touch-screen interface

Phones and tablets utilize a touch-screen interface.
Users interact by sliding fingers across the screen or pressing on-screen buttons.

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Command-Line Interface (CLI)

Alternative to GUI.
Relies on text commands.
Users input commands through a keyboard in a specific format with options.
Some systems offer a combination of GUI, touch-screen, and CLI options.

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Program execution

The OS facilitates the loading of a program into memory and its execution.
The program must be able to end its execution, either normally or abnormally (indicating error).

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I/O operations

A running program may need input/output(I/O) from a file or an I/O device, for the functions such as reading from a network interface or writing to a file system.
Users cannot directly control I/O devices for reasons of efficiency and security.
Therefore, the operating system must provide a means to do I/O.

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File-system manipulation

Programs interact with files and directories using various operations.
Common operations include

Reading,
Writing,
Creating,
Deleting by name,
Searching for specific files,
Setting access permissions, and
Obtaining file information.

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Many operating systems offer a variety of file systems.
Each file system may provide unique functionality or performance characteristics.

E.g

Windows – FAT, NTFS, HPFS
UNIX - EXT, MINIX

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Communications

Information sharing may be necessary between processes running on the same machine or on separate computers connected by a network.
Communications may be implemented via

Shared memory, in which two or more processes read and write to a shared section of memory, or
Message passing, in which packets of information in predefined formats are moved between processes by the operating system.

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Error detection

Operating systems must continuously detect and address errors to ensure system reliability.
Errors may originate in various components:

CPU and memory hardware (e.g., memory errors, power failures).
I/O devices (e.g., network connection failures, printer paper shortage).
User programs (e.g., arithmetic overflow, illegal memory access).

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The operating system must take the correct action for each type of error.
Actions ensure correct and consistent computing.
System Halting

In critical situations, the operating system may have to halt the system.
Halt is a last resort to prevent further damage or data corruption.

2. Process Termination

In certain scenarios, the OS might terminate a process causing errors.
Prevents the propagation of errors and maintains system stability.

3. Error Codes

OS may return error codes to processes for self-detection and correction.

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Resource allocation

When there are multiple processes running at the same time, resources must be allocated to each of them.
The operating system manages many different types of resources such as

CPU cycles,
Main memory,
I/O devices and
File storage.

Operating systems use scheduling and allocation algorithms to allocate these resources.
There may also be routines to allocate printers, USB storage drives, and other peripheral devices.

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Logging

Logging is helpful to keep track of which programs consume what kinds and quantities of computer resources.
Record keeping can either be done just

To collect consumption statistics, or
For accounting purposes (so users can receive bills).

Helpful for system administrators to change the configuration of the system to improve computing services.

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In multiuser or networked systems, owners strive for control over data to prevent

Unauthorized access and
Interference among concurrent processes.

Implementation of user authentication through passwords ensures that only authorized users can access and interact with sensitive information.

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Protection extends to external devices, including network adapters.
Encryption, firewalls, and secure protocols contribute to network security.
Regular patching of vulnerabilities at all system tiers is essential.

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System calls�

Way for programs to interact with the operating system.
A computer program makes a system call when it makes a service request to the operating system’s kernel.
System call provides the services of the operating system to the user programs via Application Program Interface (API).
The API specifies a set of functions that are available to an application programmer.

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A programmer accesses an API via a library of code provided by the operating system.
In the case of UNIX and Linux for programs written in the C language, the library is called libc.
System calls are the exclusive entry points into the kernel system.
They provide a standardized way for programs to access system resources.
A system call can be written in high-level languages like C, C++, Pascal or in assembly language.

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To illustrate how system calls are used, let us take a UNIX command

cp in.txt out.txt

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The handling of a user application by invoking the open () system call

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Types of System Calls

Process Control: e.g. fork, exec, exit, wait
File Management: e.g. open, read, write, close
Device Management: e.g. ioctl, read, write
Information Maintenance: e.g. getpid, time, uname
Communication: e.g. create, socket, send, receive

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Key System Calls:

end, abort – Halt program execution (normal/abnormal termination)
load, execute – Load and run a new program
create process, terminate process – Start or stop a process
get/set process attributes – Manage process details (e.g., priority)
wait for time – Delay execution for a set time
wait event, signal event – Wait for or signal events
allocate and free memory - Manage memory allocation.

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Process Execution Flow

Program Halting:

Execution can halt normally (end) or abnormally (abort).
In case of failure, error messages are dumped for debugging.

Control Flow After Program Execution (Two options):

Return Control to the Invoking Program:

Saves the memory image of the existing program.
Allows the existing program to resume after the loaded program terminates.

Concurrent Execution:

If both programs continue concurrently, a new process is created (multiprogramming).

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Process Monitoring & Event Management

Process Attribute Control:

Modify job priorities or maximum execution times.

Process Termination:

Terminate if incorrect or no longer needed.

Waiting for Events:

Pause execution until an event occurs; signal when completed

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File Management System calls

create file, delete file
open, close
read, write, reposition
get file attributes, set file attributes

Working procedure:

1) create and delete files.

2) Once the file is created,

open it to use it.
may also read or write.

3) close the file.

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We need to be able to

Determine the values of file-attributes and
Reset the file-attributes if necessary.

File attributes include

File name
File type
Protection codes and
Accounting information.

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Device Management System calls

Request device, release device;
Read, write, reposition;
Get device attributes, set device attributes;
Logically attach or detach devices.

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Resource Allocation & Device Usage

Additional Resources Needed for Program Execution:

Examples include memory, tape drives, or files.
Resources must be requested and allocated to the program.

Resource Availability:

If available, control is returned to the user program.
If unavailable, the program waits until released.

Files as Virtual Devices:

System calls for files also apply to devices due to their similarity.

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Information Maintenance System calls

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

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Most systems have a system call to return

current time and
current date.

Other system calls may return information about the system, such as

number of current users
version number of the OS
amount of free memory or disk space.

The OS keeps information about all its processes, and there are system calls to access this information.

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Communication System calls

create, delete communication connection
send, receive messages
transfer status information
attach or detach remote devices

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Two models of communication.

1) Message-passing model and

2) Shared Memory Model

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Message Passing Model

Open Connection:

Use the open connection system-call.

Identifiers:

get hostid and get processid system-calls translate host/process names into identifiers.

Permission:

Recipient process gives permission with accept connection system-call.
Receiving processes, called daemon processes, wait for connections using wait for connection.

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Message Exchange:

read message and write message system-calls are used to send and receive messages.

Close Connection:

The communication is terminated with the close connection system-call.

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Advantages of Message Passing:

Ideal for transferring smaller amounts of data between processes.
Simpler than shared memory in terms of design and implementation.

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Shared Memory Model

Processes access shared memory using the map memory system-call.
Data exchange is done by reading and writing to the shared memory region.

Process Responsibility:

Processes manage their access and must ensure no simultaneous writes to the same location.

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Advantages of Shared Memory:

Offers faster communication than message passing.

Disadvantages:

Processes are responsible for managing memory access and preventing conflicts.

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System Programs

Integral software designed to administer and regulate computer system functionality at a system level.
Close interaction with the operating system.
Core responsibility: manage and control computer hardware and resources(memory, processors, and various devices.)
Provide user-friendly interface and for enhanced user experience and development and execution.

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System programs in the operating system hierarchy�

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Six Categories of System Programs

File Management:

Programs that create, delete, copy, rename, print, and manipulate files or directories.
Examples: File Explorer, command-line utilities like cp, mv, rm.

Status Information:

Programs that query the system for information like date, time, available memory, disk space, or performance statistics.
Examples: top, df, date, system logs, performance monitors.

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3. File Modification:

Text editors for creating or modifying files, search tools, and text processing commands.
Examples: vim, nano, grep sed

4. Programming-Language Support:

Compilers, assemblers, debuggers, and interpreters for languages like C, C++, Java, Python, etc.
Examples: GCC compiler, Python interpreter, Java SDK, Visual Studio.

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Program Loading and Execution:

Loaders and linkers to load compiled programs into memory for execution.
Examples: ld, absolute loaders, re-locatable loaders, and debugging systems.

Communications:

Tools for creating connections between processes, users, and systems (e.g., messaging, web browsing, file transfer).
Examples: ssh, ftp, telnet, email clients, web browsers.

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Operating system Design and Implementation

Two basic groups of requirements:

User goals and
System goals

User goals

The system should be

Convenient to use
Easy to learn and to use
Reliable, safe, and fast.

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System goals

✔ The system should be

Easy to design, Implement, and maintain
Flexible, reliable, error free, and efficient.

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Implementation

✔ OS's are written in higher-level languages like C/C++

✔ Advantages of higher-level languages:

Faster development and
OS is easier to port.

✔ Disadvantages of higher-level languages:

Reduced speed and
Increased storage requirements.

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Operating-System Structure

The OS is divided into a number of layers.

✔ Each layer is built on the top of another layer.

✔ The bottom layer is the hardware.

The highest is the user interface

✔ A layer is made up of

Data and
Operations that can manipulate the data.

✔ The layer consists of a set of routines that can be invoked by higher-layers.

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Higher-layer

∙ Do not need to know how lower-layer operations are implemented

∙ Needs to know only what lower-layer operations do.

✔ Advantages:

1) The layered approach is simple to construct and debug.

2) Simplifies debugging and system verification.

✔ Disadvantages:

1) Appropriately defining the various layers is difficult as a layer can use only lower-level layers, careful planning is necessary.

2) Less efficient than other types.

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Layered operating system.

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Microkernels

Near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS).
Main function-provide a communication facility between client program and various services running in user-space.
Communication is provided by message passing.
All non-essential components are removed from the kernel and implemented as system- & user-programs.

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✔ Advantages:

1) Ease of extending the OS. (New services are added to user space w/o modification of kernel).

2) Easier to port from one hardware design to another.

3) Provides more security & reliability. (If a service fails, rest of the OS remains untouched.).

4) Provides minimal process and memory management.

✔ Disadvantage:

1) Performance decreases due to increased system function overhead.

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Architecture of a typical microkernel

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Modules

The kernel has

∙ Σet of core components and

∙ Dynamic links in additional services during boot time( or run time).

✔ Seven types of modules in the kernel

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The top layers include

Application environments and
Set of services providing a graphical interface to applications.

Kernel environment consists primarily of

Mach microkernel and
BSD kernel.

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Mach provides

Memory management;
Support for RPCs & IPC and
Thread scheduling.

BSD component provides

BSD command line interface
Support for networking and file systems and
Implementation of POSIX APIs

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Virtual Machines

A virtual environment that functions as a separate computer with its own operating system.
Allows multiple operating systems to run concurrently on a single physical machine.
E.g VMware

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The virtual-machine provides
Interface that is identical to the underlying hardware.
Copy of the underlying computer to each process.

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Benefits of virtual machine

Run Multiple OS Simultaneously

VMs allow sharing of hardware resources.
Different operating systems can run on the same hardware without conflict.

Enhanced Security

Host systems are protected from potential malware in guest VMs.
A virus in a VM typically won’t affect the host or other VMs.

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Resource Isolation

No direct sharing of resources between VMs.
Options for sharing file systems or creating a virtual network between VMs.

Ideal for OS Development

Virtual environments allow experimentation with operating system changes without affecting the host system.

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Developer Flexibility

Developers can test and run multiple operating systems concurrently on a single workstation.

Testing in Multiple Environments

QA engineers can run applications in different OS environments without the need for multiple physical machines.

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Data Center Efficiency

Consolidate multiple underutilized systems into fewer physical machines using VMs.
Achieves better resource optimization through physical-to-virtual conversion.

Why Use Virtual Machines?

Improve efficiency, security, and resource management.
Ideal for development, testing, and optimizing data center resources.

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VMware Architecture.

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VMware Workstation: Virtualization of Intel X86 Hardware

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Host OS: Linux
Guest OS: FreeBSD, Windows NT, Windows XP
Virtualization Layer: Abstracts physical hardware into isolated virtual machines.
Virtual CPU: Independently handles each VM's processing tasks
Virtual Memory: Each VM operates with its own allocated memory
Virtual Disk Drives: Simulated disk space for each VM
Virtual Network Interfaces: Independent network connectivity for each VM
Hardware abstraction allows multiple operating systems to run simultaneously on one physical machine.
Each guest OS operates in isolation from the others, ensuring stability and security.

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Operating-System Generation (SYSGEN)

SYSGEN is the process of configuring an operating system for a specific computer site.
Operating systems must be adapted to run on various machines with different configurations.
Operating systems are distributed on disk, CD-ROM, DVD-ROM, or as ISO images.

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Key Information Required:

CPU: Type, options and for multi-CPU systems, each CPU’s details.
Boot Disk: Formatting, partitions, and their contents.
Memory: Amount of available memory, often determined by probing memory addresses.
Devices: Device number, interrupt number, type, and model.
Operating System Options: E.g., CPU scheduling algorithm, max number of processes, and buffer sizes.

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Approaches to System Generation:

Source Code Modification:

Admin modifies the OS source code for specific hardware.
Complete recompilation of the operating system.
Tailored but time-consuming.

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Precompiled Libraries:

Modules from precompiled libraries are selected based on system configuration.
Device drivers are linked in based on need.
Faster but results in a more general system.

Table-Driven Systems:

System components are selected at execution time.
SYSGEN simply involves creating appropriate tables.
Code is generalized but can adapt to hardware changes.

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System Boot

Booting means starting a computer by loading the kernel.
Bootstrap program is a code stored in ROM.
The bootstrap program

locates the kernel
loads the kernel into main memory and
Starts execution of kernel.

The bootstrap program can perform a variety of tasks.
One task is to run diagnostics to determine the state of the machine
OS must be made available to hardware so hardware can start it.

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