CS160 Exploring Computer Science

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CS160 Exploring Computer Science
Review of Chapters 3 4

• "The learned say that your lights will one day be
  no more." said the firefly to the stars. The
  stars made no answer.
• -- Rabindranath Tagore, "Stray Birds"

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Chapter 3 Operating Systems and Networks

• OS
• Application System Software
• OS Components and Functions
• Key OS terminology
• Bootstrapping, Virtual Memory
• OS Process Administration in Time-share mode
• Handling competition among processes
• Communicating with other devices (Device
  Controllers)

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Software Classification

• Software
• Applications System
• MS-Excel
• Quake II Utility Operating System
• WordPerfect Windows
• Linux
• Solaris
• Shell Kernel

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Application and System Software

• Application Software
• User needs
• System Model
• Source Code
• Machine instructions
• Programs to help USER perform tasks that utilize
  the machines capabilities

• System Software
• Machines needs
• Services
• Hardware management
• Machine instructions
• Programs that help the MACHINE perform tasks that
  manage the machines functions

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User environments

• Single-tasking vs. Multi-tasking
• Single-user vs. Multi-user
• Batch vs. Time-sharing
• How can we accommodate such varied functionality?
• Design a set of layered software that can
• manage processes
• manage users
• manage resources
• manage hardware
• control overall working of the machine

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Why Layered Software?

• Why is OS designed as a layered software?
• It breaks a larger problem to smaller, more
  manageable, subtasks
• Each subtask can be implemented independently
• A layer can be replaced with a different
  implementation without affecting other layers
• Example Command layer can be upgraded or
  modified with a different implementation without
  affecting the Service layer functionality
• Each layer considers the other layer as an
  abstract tool

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Boot Strapping

• ROM A portion of main memory is constructed such
  that information is permanently burned in and is
  known as Read-Only Memory
• The start-up program for the machine resides in
  this ROM portion of memory which has the same
  cell address that CPU is hard-wired to look for
  when power is turned on
• When CPU is turned on, it looks for this small
  program at its hard-wired cell address always,
  which happens to be the ROM portion of memory
  that always retains its contents and cannot be
  changed
• The small program that is permanently stored in
  ROM is called the Bootstrap.
• Bootstrap programs in PCs usually instruct the
  machine to load the OS from floppy first, and if
  it fails load it from hard-drive.

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Bootstrapping Step1
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Bootstrapping Step 2
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Operating System Components

• CPU Management
• Process Management
• Memory Management
• File Management
• I/O System Management
• Interface to the User
• Shells
• Graphical User Interface
• Persistent Storage Management
• Distributed Systems and Networks

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Functional Overview of Modern OS
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Key OS Terminology

• Kernel and Shell
• Process and Program
• Scheduler and Dispatcher
• CPU Interval Timer and Time slices
• Interrupt and Interrupt Handler
• Process Administration
• Interprocess Communication

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Process Management

• A process is a program in execution. A process
  needs certain resources, including CPU time,
  memory, files, and I/O devices, to accomplish its
  task.
• The operating system is responsible for the
  following activities in connection with process
  management
• Process creation and deletion
• Process suspension and resumption
• Provision of mechanisms for
• Process synchronization
• Process communication

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Processes Administration Time-sharing
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Handling competition among processes

• Semaphores
• Critical Region A set of instructions that can
  be executed by only one process at a time
• Mutual Exclusion Requirement that only one
  process execute such Critical Region instructions
  at any time
• How to implement Mutual Exclusion?
• Use a Semaphore, as a single-instruction
  Test-and-Set flag
• Check for the semaphore flag GO before entering
  the Critical Region set the flag to STOP as
  soon as you enter re-set the flag to GO when
  you exit.

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Virtual memory

• In a multi-user, multi-tasking environment, main
  memory space is limited and cannot accommodate
  all the programs
• OS creates an illusion of larger memory
• Memory manager divides the required space into
  pages and store these pages in the hard disk
• As and when the next page is needed, memory
  manager will exchange the new page for the old
  one that is not needed anymore for program
  execution
• Thus programs continue to execute as though there
  was no space crunch to begin with
• Such an illusionary memory space is called
  Virtual Memory

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Communicating with other devices

• CPU and Main Memory together form the Core of a
  machine, which we will call the computer, or the
  processor
• How does the computer communicate with other
  devices?
• Through intermediary devices called Controllers

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Device Controllers

• Each device has its own controller by design
• In modern PCs, the controllers plug into
  motherboard and connect via cables to their
  devices
• Each controller if often a computer in its own
  right, with a CPU and memory circuitry
• Controllers communicate with the processor via
  dedicated controller bus or system bus
• Controllers pass messages and data between their
  respective devices and the processor by
  converting the messages and data into forms
  compatible with the main processor design

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Pictorial View Device Controllers
CPU
Cache
Processor
Printer
Network Medium
Disk
Disk
DiskController
NetworkController
PrinterController
Main Memory
System Bus
CD-drive Controller
Modem Controller
CD-drive
Modem
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Summary

• OS is a system software designed to manage the
  various components of a computer system with
  kernel being its core and shell being its
  interface to the user
• OS Kernel has several components like memory
  manager, i/o manager, file manager that perform
  the appropriate functions
• Bootstrapping loads the OS into processor main
  memory at start-up
• Process administration in time-share systems
  involve scheduler, dispatcher and interrupts set
  up by interval timer
• Competing processes can be handled via Deadlock
  detection and avoidance, or Semaphores
• Device Controllers handle communication between
  processor and other devices attached to the system

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Chapter 3 Networks

• Classification of Networks based on Topology,
  Ownership, Geography and Transmission Mode
• Network Protocols The Internet
• IP layers Application, Transport, Network, Link
• TCP vs. UDP

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Computer Network Fundamentals

• A Computer Network is a
• group of computers connected such that they allow
  for remote communication among themselves

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Network Terminology

• Node
• anything connected to the network,
• usually a computer, but it could be a printer or
  a scanner
• Link
• physical medium such as coax cable, fiber optic
  cable etc. connecting the machines
• Segment
• any portion of a network that is separated by a
  switch, bridge or a router from another part of a
  network.
• Backbone
• the main physical cabling connecting all the
  segments
• holds and supports the physical network, just as
  the name suggests
• Topology
• The way each node is physically connected to the
  network

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Network Classifications

• Computer Networks are classified based on
• Topology
• The way in which they are physically connected
• Ownership
• Private (closed network), Public (Open network)
• Geography
• WAN, LAN, MAN
• Transmission
• Circuit switched, Packet Data, Asynchronous
  Transfer Mode (ATM), Frame Relay
• For example A subset of PCC computers could be
  configured in a star topology with private LAN
  using Frame Relay mode of transmission

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Main Network Classifications

• Two main classifications we will look at are
• Topology
• Ring, Bus, Star, Tree, Mesh, Hybrid
• Geography
• LAN often machines hooked up within a building
• WAN connects cities domestically and across the
  world

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Connecting Networks Bridge

• Two compatible network topologies can be directly
  connected using a Bridge to form a larger network

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Connecting Networks Routers

• Non-compatible networks are connected such that
  they form a network of networks, referred to as
  an internet, using a Router which knows how to
  handle each of the two connecting networks they
  remain independent and can communicate only via
  the Router.

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The Internet

• An internet is just an interconnection of two
  independent networks in two topologies
• The Internet (with capital I) is a special
  network of networks involving LANs and WANS all
  over the world, connecting millions of machines,
  using specialized network protocols

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Internet Topology

• Many colleges and universities collaborated on
  research and set up their own indigenous
  internets leading to the establishment of
  conceptual domains
• This led to haphazard growth making it necessary
  to standardize connections and addressing
• Internet is a mish-mash of domains having Bus,
  Star, Mesh, some hybrid topology interconnected
  by gateway routers

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Internet Domains

• A domain is an autonomous system configured and
  controlled by its owner
• Each domain has a name as well as an address
  which needs to be registered with ICANN
• A newly registered domain can attach to the
  Internet via gateways
• Gateways are essentially routers with certain
  special functionality

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Network Protocols

• Network Protocols are a collection of rules
  governing network activities to be followed by
  all computers in the network, such as
• Data/Message format for sending and receiving
• Addressing scheme to locate networks and machines
  in the network
• Transmission format and privileges
• Routing format and specifications
• Because there is no centralized governing agency
  or board of directors or regents, protocols play
  a vital role in the smooth and efficient
  operation of networks

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Common Network Protocols

• Token Ring Protocol
• For controlling transmission rights in a network
  configured in ring topology
• Internet Protocol (IP)
• For formatting, controlling, and regulating
  Internet traffic
• File Transfer Protocol (FTP)
• For controlling the transferring of files across
  the Internet
• HyperText Transfer Protocol (HTTP)
• For transferring hypertext (text, images, audio,
  video) across the Internet
• Simple Mail Transfer Protocol (SMTP)
• For transferring email across the Internet
• Post Office Protocol (POP)
• For accessing local mail server and transferring
  email

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Internet Protocol IP

• Each computer on the network is identified by a
  unique address, known as the IP address
• Each message, be it text, image, or sound, is
  broken up into packets of about 1500 characters
  and transmitted packet-by-packet by the sender or
  source machine
• Each packet has headers showing source IP
  address, destination IP address, and its Sequence
  Number
• Headers help Routers or Hosts route the packets
  to the destination machine
• Destination machines reassemble all the packets
  it receives using another set of rules, the
  Transmission Control protocol (TCP) that verifies
  if any packets got lost along the way

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Name Resolution DNS

• Humans dont like to deal with numbers a lot, so,
  we set up names that are easier to work with
• TLD Top-level domains that we already know are
  .com, .edu, .org, .net, .gov
• Domain names need to be registered with ICANN,
  that associates a name with an IP address for
  that domain.
• Example pcc.edu
• Individual machines within that domain can then
  have another identifying name
• Example faculty.pcc.edu
• DNS Domain Name Servers maintain a look-up table
  associating IP address with host names.

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Internet Architecture

• 4-Layered Architecture
• Application
• Transport
• Network
• Link
• Study what each layer does.

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TCP and UDP

• TCP establishes connection with destination first
  before sending any packets
• Reliable TCP works at both ends of the
  connection the sender end waits for
  acknowledgement from receiver end and retransmits
  any lost packets
• Slightly slower and less efficient as a result of
  retransmission

• UDP is connectionless just blindly sends the
  data even if the destination machine is not
  operational
• Not Reliable Does not wait for acknowledgement
  and retransmit lost packets
• More streamlined

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Summary

• Networks are a group of computers physically
  linked to share data and resources
• Networks can be classified based on topology,
  geography, ownership, transmission
• Network Protocols define how networks should
  handle data as there is no centralized
  controlling authority
• The Internet is a special network of networks
• 4-layered software architecture
• Uses TCP/IP protocol suite for transmission of
  data
• Special form of addressing, known as IP
  addressing

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Chapter 4 Algorithms

• Main components of an algorithm
• Variables and values, Instructions, Sequences,
  Procedures, Selections, Repetitions,
  Documentation
• Algorithm representation using Pseudocode
  notation
• Algorithm Analysis

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Algorithms Definition

• An algorithm is an 1ordered set of 2unambiguous,
  3executable steps that define a 4terminating
  activity
• Brookshear
• The term Algorithm differs from terms like
  Method, or Technique as algorithm attempts to
  encode intelligence in some way.

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Algorithms Definition

• Ordered algorithms specify the sequencing of
  steps in a process
• Unambiguous algorithms specify with clarity
  what activity or action should occur
• Executable algorithms specify action that can,
  in fact, be done
• Terminating there is a well-defined end point

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Ambiguity

• Ambiguity is inherent in the English Language
• Understand the problem clearly and eliminate
  ambiguity when defining the problem
• Computer Scientists have developed formal
  languages and methods to deal with ambiguity

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Algorithm Discovery

• How are algorithms found in the first place?
• The Art of Problem Solving
• Can the process of problem-solving be reduced to
  an algorithm itself?
• This has been shown to be impossible
• (See Chapter 11)
• Ability to solve problems remains more of an
  artistic skill to be developed rather than a
  precise science to be learned

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Getting a foot in the door

• Working Backwards
• Knowing what the output needs to be, work back up
  to the given input
• Adapt an existing solution
• Understand the solution to a related problem and
  apply it to the problem at hand
• Stepwise Refinement
• Decompose the given problem into several sub
  problems and solve them progressively, leading to
  refinement of solution

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TD and BU Methodologies

• Top Down Methodology
• General to Specific
• Example, Stepwise Refinement
• Bottom Up Methodology
• Specific to General
• The two methodologies complement each other and
  are often used in conjunction.
• TD Stepwise Refinement is usually guided by the
  problem solvers intuition, working in the BU
  mode.

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Algorithm Representation

• Before learning to code a program in any specific
  language, we will use a less formal notational
  system for writing algorithms viz., Pseudocode
• Pseudocode
• A Consistent,
• concise notation for representing
• recurring semantic structures,
• which in turn will become the primitives for
  expressing future ideas.

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Components of an Algorithm

• Variables and values
• Instructions
• Sequences
• Procedures
• Selections
• Repetitions
• Documentation

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Variables

• Are containers for values places to store
  values
• Example

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Procedure

• A named sequence of instructions
• So that you can
• Refer to it collectively (by name)
• ...instead of individually (by each instruction
  in the sequence)
• Example
• Drive_To_PCC

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Selection

• An instruction that decides which of two possible
  sequences is executed
• The decision is based on a single true/false
  condition
• Examples we saw in class
• Odd/Even test
• Letter grade based on points earned
• Reciprocals

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Relational Operators

• Relational Operators are used to compare two
  quantities

EQUAL TO NOT EQUAL TO LESS THAN LESS THAN OR
EQUAL TO GREATER THAN GREATER THAN OR EQUAL TO
! (or, ltgt) lt lt gt gt
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Testing for Equality

• To test for equality, use the operator.
• if ( grade 100 )
• printf ( Perfect Score! )
• To test for inequality, use the ! operator
• if (grade ! 100)
• printf (Not perfect!)

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Selection Summary

• Written in the form of If (condition) Else
  clause
• Condition should evaluate to true or false
• Condition can be an expression or a relational
  operation
• Boolean operations can be used to combine
  multiple conditions into one expression,
  evaluated as a whole
• If Else clauses can be nested, in which case,
  the else belongs to the nearest if, unless
  specifically indicated otherwise by the use of
  brackets

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Repetition

• Repeat an instruction...
• ...while (or maybe until) some true or false
  condition occurs
• Test the condition each time before repeating the
  instruction
• Also known as iteration or loop
• Example we saw in class
• Counting and printing the numbers 1 to 10
  consecutively

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Documentation

• Records what the algorithm does
• Describes how it does it
• Explains the purpose of each component of the
  algorithm
• Notes restrictions or expectations

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Algorithm Analysis Overview

• Once an algorithm has been designed (somehow),
• and tested for accuracy with several test cases
  of input,
• how much in the way of resources such as time or
  space (memory) does it require?
• An algorithm that solves the problem accurately
  but takes a year to complete the solution is
  hardly of much use
• An algorithm that gives a quick solution but
  needs several million gigabyte (or, unlimited)
  memory and resources is not of much use either

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Algorithm Analysis Overview

• How can we estimate the time required for an
  algorithm ?
• How can we reduce the running time of an
  algorithm from days or years to fractions of
  seconds ?
• The analysis required to estimate the resources
  used by an algorithm is generally a theoretical
  issue with well-formulated mathematical methods
• Note that this analysis is not based on any
  program written in a specific language nor does
  it depend on the processor the program is run on
  those are separate issues.