1. Schedule Wrap up chapter 3 Chapter 9 Wednesday: Friday:
Quiz over chapter 1-3
Chapter 9 (cont.)
Friday:
Wrap up chapter 9
Brief overview of Office products

3. The World Wide Web
Some computers connected to the Internet are Web servers
Computers programmed to send files to browsers running on other computers connected to the Internet.
These Web servers and their files comprise the World Wide Web (WWW)

4. The World Wide Web Those files are Web pages
Web servers store and send other kinds of files, too
The files are often used to:
Create the Web page (images or animations)
Help with other Web services (play audio or video)

5. Requesting a Web Page Web requests use client/server interaction
Requesting a Web page means your browser is a client asking for a file from a Web server
The file can be found in looking at the URL (Universal Resource Locator)
Web browsers and Web servers both “speak” HTTP

6. Requesting a Web Page The URL has three main parts:
The URL has three main parts:
Protocol. tells the computers how to handle the file
Server computer’s name or the name given by the domain hierarchy
Page’s pathname. tells the server which file (page) is requested and where to find it

7. Describing a Web Page
Servers do not store Web pages in the form seen on our screens
The pages are stored as a description of how they should appear on the screen.
The browser receives the description/source file and creates the Web page image that is described

8. Describing a Web Page
There are two advantages to storing and sending the source rather than the image itself:
A description file usually requires less information
The browser can adapt the source image to your computer more easily

9. The Internet and the Web
Some Web servers have www as part of their domain name, some don’t
Some Web servers seem to add the www if you leave it out
Some Web servers work either way (both and moma.org display the same Web site)
When is the www required and when is it optional?

10. The Internet and the Web
Remember that web addresses are simply names
All computers connected to the Internet (including Web servers) have IP addresses
The DNS server requires you to give the name (URL) exactly because the DNS responds to that exact name

11. The Internet and the Web
An incorrect name/URL and you either access the wrong IP address or the DNS lookup fails (“404 Not Found”)
Computers can be programmed to notice and to redirect you automatically to the correct page
Web administrators may also register all forms of a URL (with and without the “www”)

12. File Structure Directory Hierarchy Think of any hierarchy as a tree
Remember that folders can contain folders as well as files
This scheme is called the file structure of the computer and forms the directory hierarchy
Think of any hierarchy as a tree
folders are the branch points
files are the leaves

13. File Structure Directory Hierarchy
All hierarchies have branch points and leaves
Hierarchy trees are often drawn sideways or upside down
Two terms are standard, however:
Down in the hierarchy means into subfolders (towards the leaves)
Up in the hierarchy means into folders (toward the root)

14. File Structure
Part of the directory hierarchy is shown in the pathnames of URLs:
The page is specified by a pathname that tells the computer how to navigate through the directory hierarchy to the file
Each time there is a slash (/), we move into a subfolder or to the file
We go down in the hierarchy

15. Organizing the Folder
Normally the last item in the sequence is a file name
This is not always necessary or true
When a URL ends in a slash, the browser automatically looks in that folder for a file called index.htm
The index.html file exists only if it was built

16. Organizing the Folder Why have a hierarchy?
Most people build hierarchies to organize their own thinking and work
Directories cost nothing
There is no reason not to use them
It is highly recommended

17. Summary
In this chapter we discussed the basics of networking, including the following:
Basic types of communication: point-to-point, multicast, broadcast, synchronous, and asynchronous.
Networking, including IP addresses, domains, IP packets, IP protocol, WANS and LANS, Ethernet protocol, ISPs, enterprise networks, and wireless networks.

18. Summary
In this chapter we discussed the basics of networking, including the following:
The difference between the Internet and the World Wide Web.
File hierarchies in preparation for our further study of HTML.

20. Learning Objectives
Describe how the Fetch/Execute Cycle works, listing the five steps
Explain the function of the memory, control unit, ALU, input unit and output unit

21. What Computers Can and Cannot Do
What computers can do
Deterministically perform or execute instructions to process information
Deterministically…the computer determines which instruction to do next…it does only what it is “told” to do
They have no imagination, are not creative or purposeful. No sense of irony, subtlety, etc.
Computers only execute instructions

22. The Fetch/Execute Cycle
“Instruction Execution Engine”…a machine that cycles through a series of operations
Series is called: Fetch/Execute Cycle
Get the next instruction
Figure out what to do
Gathering the data needed to do it
Do it
Save the result, and
Repeat (billions of times/second)!

23. A Five-Step Cycle
These operations are repeated in a never-ending sequence
The step names suggest the operations described in the previous paragraph

24. Anatomy of a Computer
All computers, regardless of their implementing technology, have five basic parts or subsystems:
Memory,
Control unit,
Arithmetic/logic unit (ALU),
Input unit, and
Output unit

25. Principal Subsystems of a Computer

26. 1. Memory
Memory stores both the program while it is running and the data on which the program operates
Properties of memory:
Discrete locations
Memory is organized as a sequence of discrete locations
In modern memory, each location is composed of 1 byte (8 bits)

27. 1. Memory Addresses Values Finite capacity
Every memory location has an address, whole numbers starting at 0
Values
Memory locations record or store values
Finite capacity
Memory locations have a finite capacity (limited size),
Data may not “fit” in the memory location

28. Byte-Size Memory Location
Common visualization of computer memory
Discrete locations are shown as boxes holding 1-byte each
Address of location is displayed above the box and the contents of location is shown in the box

29. Byte-Size Memory Location
That 1-byte memory location can store one ASCII character or a number less than 256
Blocks of four bytes are used as a unit so often that they are called memory words

30. Random Access Memory
Computer memory is called random access memory (RAM)
“Random access” is out-of-date and simply means that the computer can refer to the memory locations in any order
RAM is measured in megabytes (MB) or gigabytes (GB)
Lots of memory is needed to handle the space required of programs and data

31. 2. Control Unit
The control unit of a computer is where the Fetch/Execute Cycle occurs
Its circuitry fetches an instruction from memory and performs the other operations of the Fetch/Execute Cycle on it
A typical machine instruction has the form ADD 4000, 2000, 2080

32. 2. Control Unit
ADD 4000, 2000, 2080
Looks like those three numbers should be added together
What it really means is that whatever numbers are stored in memory locations 2000 and 2080 be added together, and the result be stored in location 4000

33. Illustration of a single instruction

34. 3. Arithmetic/Logic Unit (ALU)
“Does the math”
A circuit in the ALU can add two numbers
The circuit uses logic gates or simpler circuits that implement operations like AND and OR
There are also circuits for multiplying, for comparing two numbers, etc.
The ALU carries out each machine instruction with a separate circuit

35. 4. And 5. Input and Output Units
These two components are the wires and circuits through which information moves into and out of a computer
A computer without input or output is useless

36. The Peripherals
Peripherals connect to the computer input/output (I/O) ports
They provide input or receiving its output
They are not considered part of the computer:
They are only specialized gadgets that encode or decode information between the computer and the physical world

37. The Peripherals
The keyboard encodes our keystrokes into binary form for the computer
The monitor decodes information from the computer’s memory and displays it on a screen
The peripherals handle the physical part of the operation

38. Portable Memory & Hard Drives
Some peripherals are used by computers for both input and output:
USB memory
Hard disks/drives
They are storage devices
The hard disk is the alpha-peripheral, being the most tightly linked device to the computer

39. Hard Disk Hard disk is essential
Programs and their data must reside in the computer’s memory when programs run
The hard disk can be seen as an extension of the computer’s memory
Typically it is a hundred times larger and several thousand times slower

40. A Device Driver for Every Peripheral
Most peripheral devices are “dumb”
They provide only basic physical translation to or from binary signals.
Additional information from the computer is needed to make it operate “intelligently”
Added processing by software called a device driver gives the peripheral its standard meaning and behavior
Every device needs a device driver