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CS212: OPERATING SYSTEM Lecture 1: Introduction 1

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Chapter 1: Introduction What Operating Systems Do Computer-System Organization Computer-System Architecture Operating-System Structure Operating-System Operations Distributed Systems Computing Environments 2

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Objectives To provide a grand tour of the major operating systems components To provide coverage of basic computer system organization 3

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What is an Operating System? A program that acts as an intermediary between a user of a computer and the computer hardware Operating system goals: Execute user programs and make solving user problems easier Make the computer system convenient to use Use the computer hardware in an efficient manner 4

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Computer System Structure Computer system can be divided into four components Hardware provides basic computing resources CPU, memory, I/O devices Operating system Controls and coordinates use of hardware among various applications and users Application programs define the ways in which the system resources are used to solve the computing problems of the users Word processors, compilers, web browsers, database systems, video games Users People, machines, other computers 5

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Four Components of a Computer System 6

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The operating systems Role User View: Single User (personal computer) OS designed for ease of use, with some attention paid to performance. Multiple Users (mainframe or minicomputer) OS designed to maximize resource utilization. Workstations users connected to networks of other workstations and servers OS designed to compromise between individual usability and resource utilization. 7

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The operating systems Role (cont.) System View: OS is a resource allocator Manages all resources Decides between conflicting requests for efficient and fair resource use It is important when many users access the same mainframe or minicomputer OS is a control program Controls execution of programs to prevent errors and improper use of the computer It is especially concerned with the operation and control of I/O devices. 8

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Operating System Definition No universally accepted definition The one program running at all times on the computer is the kernel. Everything else is either a system program (ships with the operating system) or an application program. An operating system is a software that manages the computer hardware, as well as providing an environment for application programs to run. 9

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Computer-System Organization Computer-System Operation Storage Structure I/O Structure 10

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Computer System operation Computer-system operation One or more CPUs, device controllers connect through common bus providing access to shared memory Concurrent execution of CPUs and devices competing for memory cycles 11

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Computer System Operation (cont.) Computer Startup: bootstrap program is loaded at power-up or reboot Typically stored in ROM or EEPROM, generally known as firmware Initializes all aspects of system Loads operating system kernel and starts execution 12

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Computer System Operation (cont.) Interrupt: The occurrence of an event is usually signaled by an interrupt from either the hardware or the software. Hardware interrupts by sending a signal to the CPU through system bus. Software interrupts by executing a special operation called a system call. The interrupt is signal that gets the attention of the CPU and is usually generated when I/O is required. For example, hardware interrupts are generated when a key is pressed or when the mouse is moved. Software interrupts are generated by a program requiring disk input or output. 13

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Storage Hierarchy Storage systems organized in hierarchy Speed Cost Volatility Caching copying information into faster storage system; main memory can be viewed as a last cache for secondary storage 14

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Storage-Device Hierarchy 15

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I/O Structure 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. OS have a device driver for each device controller. This device driver understands the device controller and presents a uniform interface to device to the rest of the operating system. 16

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Computer-System Architecture Multiprocessors systems growing in use and importance Also known as parallel systems, tightly-coupled systems Advantages include 1. Increased throughput 2. Economy of scale 3. Increased reliability fault tolerance Two types 1. Asymmetric Multiprocessing (master-slave relationship) 2. Symmetric Multiprocessing (all processors are peers) 17

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Symmetric Multiprocessing Architecture 18

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A Dual-Core Design Processors were originally developed with only one core. The core is the part of the processor that actually performs the reading and executing of the instruction. Single-core processors can only process one instruction at a time Dual-core processor contains two cores (Such as Intel Core Duo). 19

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Operating System Structure Multiprogramming needed for efficiency Multiprogramming organizes jobs (code and data) so CPU always has one to execute. (Increase CPU Utilization.) A subset of total jobs in system is kept in memory One job selected and run via job scheduling When it has to wait (for I/O for example), OS switches to another job Multiprogramming systems provide an environment in which the various system resources are utilized effectively, Not for user interaction with the computer system 20

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Memory Layout for Multiprogrammed System 21

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Uniprogramming Processor must wait for I/O instruction to complete before preceding 22

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Multiprogramming When one job needs to wait for I/O, the processor can switch to the other job 23

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Multiprogramming 24

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Utilization Histograms 25

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Example 26

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Operating System Structure (cont.) Timesharing (multitasking) is logical extension of multiprogramming in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing Response time should be < 1 second Each user has at least one program executing in memory process If several jobs ready to run at the same time CPU scheduling If processes dont fit in memory, swapping moves them in and out to run Virtual memory allows execution of processes not completely in memory 27

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28 Time Sharing Using multiprogramming to handle multiple interactive jobs Processors time is shared among multiple users Multiple users simultaneously access the system through terminals

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Operating-System Operations Dual-mode operation allows OS to protect itself and other system components User mode and kernel mode Mode bit provided by hardware Provides ability to distinguish when system is running user code or kernel code Some instructions designated as privileged, only executable in kernel mode System call changes mode to kernel, return from call resets it to user 29

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Transition from User to Kernel Mode 30

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Distributed Systems Distributed systems allow users to share resources on geographically dispersed hosts connected via a computer network. The basic types of networks are: Local-area Network (LAN) connects computers within a room, a floor, or a building. Wide-area Network (WAN) links building, cities or countries. A network operating system provides features such as file sharing across the network and includes a communication scheme that allows different processes on different computers to exchange messages 31

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Computing Environments Traditional computing ( Centralized computing) Office environment PCs connected to a network, terminals attached to mainframe or minicomputers providing batch and timesharing Now portals allowing networked and remote systems access to same resources Home networks Used to be single system, then modems Now firewalled, networked Centralized computing is computing done at a central location, using terminals that are attached to a central computer. 32

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Computing Environments (Cont) Client-Server Computing Many systems now servers, responding to requests generated by clients Compute-server provides an interface to client to request services (i.e. database) File-server provides interface for clients to store and retrieve files 33

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Peer-to-Peer Computing Another model of distributed system P2P does not distinguish clients and servers Instead all nodes are considered peers May each act as client, server or both To participate in a P2P system, a node must join P2P network by: Registers its service with centralized lookup service on network, or Broadcast request for service and respond to requests for service via discovery protocol 34

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Silberschatz, Galvin and Gagne 2009 Operating System Concepts 8 th Edition, End of Chapter 1