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OPERATING OPERATING SYSTEMSSYSTEMS

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CONTENTS

1.1. What is an Operating System?What is an Operating System?2.2. OS FunctionsOS Functions3.3. OS ServicesOS Services4.4. Structure of OSStructure of OS5.5. Evolution of OSEvolution of OS

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SOFTWARE CLASSIFICATION

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WHAT IS OS?It is the software that enables all the programs we use.

The OS organizes and controls the hardware (CPU, RAM, I/O)

Security

System interface

Application interface

OS acts as an interface between the application programs and the machine hardware.

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Operating System SoftwareExamples: Microsoft Windows, Linux, Unix, Mac OS X, iOS, Android.

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What is an operating system (OS)?

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OS FUNCTIONSThe main functions of operating systems are:

• Manages and Interacts with Computer Hardware• Process the tasks • Provides the User/ Computer Interface (CLI / GUI)• Provides the Interface for Application Software • Input / Output Operations• Error Detection, Resource Allocation, Security and

Protection

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OS SERVICES

Operating system services such as,

• Input / Output Operations ,Controlling and Allocating memory,

• Work as a Resource Allocator ,Prioritizing system requests, File System Manipulation, Error Detection,

• Controlling input and output devices, Communication• Managing file systems, protection• CPUs have this capability to use at least two modes:

protected mode and supervisor mode.

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Manages the CPU

- by CPU scheduling algorithms.

Manages RAM

- What is processing in RAM - Virtual memory.

Provides the Interface for Application Software

-Operating systems are designed and developed for a specific CPU or “family of CPUs”

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Provides the interface for storage devices and manages how data is stored on those devices.

Provides the Interface for I/O Devices

• device drivers• keyboard, mouse, printer

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VIRTUAL MEMORY

Virtual Memory (VM) = the ability of the CPU and the operating system software to use the hard disk drive as additional RAM when needed.

Good – no longer get “insufficient memory” error.

Bad - performance is very slow when accessing VM.

Solution = increase size of RAM.

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VIRTUAL MEMORY

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PROCESSESScheduler – Maintains a record of the processes.

• Adds new processes when launched.

• Removes old processes when completed.

• Uses a process table.

Process Table – Information about each process including:

• Main memory cells (RAM)• Priority• Running or waiting (input from

user or saving to disk)

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Viewing processes with Microsoft Windows

Ending a process – Killing the process

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PROCESSESDispatcher - Oversees the execution of each process by the CPU.

• Gives each process a time slice of CPU time.

• Changes between processes.

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MULTIPROCESSING WITH MULTIPLE CPU’S

Asymmetric Multiprocessing = Tasks are assigned to a specific CPU and each CPU has its own RAM memory

Symmetric Multiprocessing = Tasks are assigned to any available CPU and CPU’s can share RAM memory

Rick Graziani graziani@cabrillo.edu 16

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STRUCTURE OF OPERATING SYSTEM:

Application Programs

System Programs

Software (Operating System)

HARDWARE

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STRUCTURE OF OPERATING SYSTEM

The structure of OS consists of 4 layers:

1. Hardware Hardware consists of CPU, Main

memory, I/O Devices, etc,

2. Software (Operating System) Software includes process management

routines, memory management routines, I/O control routines, file management routines.

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STRUCTURE OF OPERATING SYSTEM

3. System programs

This layer consists of compilers, Assemblers, linker etc.

4. Application programs

This is dependent on users need. Ex. Railway reservation system, Bank database management etc.,

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EVOLUTION OF OS:

Evolution of an OS from simple Batch processing to today’s OS-

• Batch Operating System• Multiprogramming Operating System• Time Sharing or Multitasking Operating System• Real-Time Systems• Distributed Operating System

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BATCH PROCESSING:• In Batch processing same type of jobs batch (BATCH- a

set of jobs with similar needs) together and execute at a time.

• The OS was simple, its major task was to transfer control from one job to the next.

• The job was submitted to the computer operator in form of punch cards. At some later time the output appeared.

• The OS was always resident in memory.

• Common Input devices were card readers and tape drives.

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Common output devices were line printers, tape drives, and card punches.

Users did not interact directly with the computer systems, but he prepared a job (comprising of the program, the data, & some control information).

OS

User program area

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MULTIPROGRAMMING:

• Multiprogramming is a technique to execute number of programs simultaneously by a single processor.

• In Multiprogramming, number of processes reside in main memory at a time.

• The OS picks and begins to executes one of the jobs in the main memory.

• If any I/O wait happened in a process, then CPU switches from that job to another job.

• Hence CPU in not idle at any time.

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MULTIPROGRAMMING

OS

Job 1

Job 2

Job 3

Job 4

Job 5

• Figure dipicts the layout of multiprogramming system.

• The main memory consists of 5 jobs at a time, the CPU executes one by one.

Advantages:

•Efficient memory utilization

•Throughput increases

•CPU is never idle, so performance increases.

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TIME SHARING SYSTEMS:

• Time sharing, or multitasking, is a logical extension of multiprogramming.

• Multiple jobs are executed by switching the CPU between them.

• In this, the CPU time is shared by different processes, so it is called as “Time sharing Systems”.

• Time slice is defined by the OS, for sharing CPU time between processes.

• Examples: Unix.

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PROCESS STATES FOR TIME SHARING

Ready Running

Blocked

New Job Execution Terminated

Wait for I/O CompletionI/O Completed

Time Slice over

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REAL-TIME SYSTEMS:

• A real-time operating system is a multitasking operating system intended for applications with fixed deadlines.

• Such applications include some small embedded systems, automobile engine controllers, industrial robots, spacecraft, industrial control, and some large-scale computing systems.

• An early example of a large-scale real-time operating system was Transaction Processing Facility developed by American Airlines and IBM for the Sabre Airline Reservations System.

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These are of two types: a) Hard Real Time,

b) Soft Real Time.

a) These OS guarantee that critical tasks be completed within a certain range of time.

Ex: A complete car welding by robot hardly on the time.

a) These OS provides some relaxation in time limit.

Ex: Multimedia systems, digital audio system.

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DISTRIBUTED SYSTEMS:

• A distributed computer system is a collection of autonomous computer systems capable of communication and cooperation via their H/w and S/w interconnection.

• The distributed operating system provides a illusion to its users that it has a single uniprocessor system, although it is actually consisted of multiprocessors.

• Distributed OS provide the means for system-wide sharing of resources such as computational capacity, files and I/O devices.

• Ex: UNIX, LINUX.

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SUMMARY• HOW AN OS WORKS

Interpreting commands, enabling the user to communicate with the computer.

• OS IS SOFTWARE

• COMPUTER WORKS WITH THE HELP OF OSWhen a computer is turned on it searches for

instructions in its memory. These instructions tell the computer how to start up. Usually, one of the first sets of these instructions is a special program called the operating system .

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Thanks