What is a computer? What is an operating system? What is a file system?

1. What is a computer? What is an operating system? What is a file system? CS 139 – Aug 31, 2005

2. Yesterday’s Lab • You developed an algorithm • Input – an 8 ½ x 11 inch piece of paper • Audience – Fellow classmates • Output – A paper airplane that would fly

3. Problem Solving • Problem solving is the act of finding a solution to a perplexing, distressing, vexing, or unsettled question

4. A good algorithm • Simple – Each step does one thing. No unnecessary steps. • Precise – Unambiguous. Proper sequencing. • Correct – The instructions told you to do the right things in the right order. • Complete – Nothing was left out or left to your imagination. • Abstraction – Probably not. A separate set of instructions that can be “substituted” in the algorithm. Ex. Fold = fold and crease.

5. Ask Questions... • …to understand the problem • What do I know about the problem? • What is the information that I have to process in order the find the solution? • What does the solution look like? • What sort of special cases exist? • How will I recognize that I have found the solution?

6. Algorithms • An algorithm is set of instructions for solving a problem or sub-problem in a finite amount of time using a finite amount of data • The instructions are unambiguous

7. Material from • Computer Science Illuminated – Nell Dale and John Lewis • Chapter 5, Chapter 10, Chapter 11 • Book is on reserve in the CISAT library, second floor of HHS. Use my name to ask for it.

8. Stored-Program Concept Figure 5.1 The von Neumann architecture

9. Flow of Information • The parts are connected to one another by a collection of wires called a bus Figure 5.2 Data flow through a von Neumann architecture

10. Arithmetic/Logic Unit • Performing basic arithmetic operations such as adding • Performing logical operations such as AND, OR, and NOT • Most modern ALUs have a small amount of special storage units called registers

11. Control Unit • Control unit is the organizing force in the computer • There are two registers in the control unit • The instruction register (IR) contains the instruction that is being executed • The program counter (PC) contains the address of the next instruction to be executed • ALU and the control unit called the Central Processing Unit, or CPU

12. The Fetch-Execute Cycle • Fetch the next instruction • Decode the instruction • Get data if needed • Execute the instruction

13. Memory • Memory is a collection of cells, each with a unique physical address Page 122

14. RAM and ROM • RAM stands for Random Access Memory • Inherent in the idea of being able to access each location is the ability to change the contents of each location • ROM stands for Read Only Memory • The contents in locations in ROM cannot be changed • RAM is volatile, ROM is not • This means that RAM does not retain its bit configuration when the power is turned off, but ROM does

15. Secondary Storage Devices • Because most of main memory is volatile and limited, it is essential that there be other types of storage devices where programs and data can be stored when they are no longer being processed • Secondary storage devices can be installed within the computer box at the factory or added later as needed

16. Magnetic Disks • A read/write head travels across a spinning magnetic disk, retrieving or recording data Figure 5.5The organization of a magnetic disk

17. Compact Disks • A CD drive uses a laser to read information stored optically on a plastic disk • CD-ROM is Read-Only Memory • DVD stands for Digital Versatile Disk

18. Sizes in Perspective Page 119

19. File Systems • A file is a named collection of related data • A file system is the logical view that an operating system provides so that users can manage information as a collection of files • A file system is often organized by grouping files into directories

20. Directory Trees Figure 11.4 A Windows directory tree

21. Directory Trees • A directory of files can be contained within another directory • The directory containing another is usually called the parent directory, and the one inside is called a subdirectory • A file system is often viewed as a directory tree • The directory at the highest level is called the root directory

22. Figure 11.5 A Unix Directory Tree

23. Directory Trees • At any point in time, you can be thought of as working in a particular location (that is, a particular subdirectory) • This subdirectory is referred to as the current working directory

24. Path Names • To indicate a particular file using text, we specify that file’s path, which is the series of directories through which you must go to find the file • An absolute path name begins at the root and specifies each step down the tree until it reaches the desired file or directory • A relative path name begins from the current working directory

25. Path Names • Examples of absolute path C:\Program Files\MS Office\WinWord.exe C:\My Documents\letters\applications\vaTech.doc C:\Windows\System\QuickTime • Suppose the current working directory is C:\My Documents\letters • Then the following relative path names could be used cancelMag.doc applications\calState.doc

26. Text and Binary Files • In a text file the bytes of data are organized as characters from the ASCII or Unicode character sets • A binary file requires a specific interpretation of the bits based on the information in the file

27. Text and Binary Files • The terms text file and binary file are somewhat misleading • They seem to imply that the information in a text file is not stored as binary data • Ultimately, all information on a computer is stored as binary digits • These terms refer to how those bits are formatted: as chunks of 8 or 16 bits, interpreted as characters, or in some other special format

28. File Types • Most files, whether they are in text or binary format, contain a specific type of information • For example, a file may contain a Java program, a JPEG image, or an MP3 audio clip • The kind of information contained in a document is called the file type • Most operating systems recognize a list of specific file types and their associated applications

29. File Types • File names are often separated, usually by a period, into two parts • Main name • File extension • The file extension indicates the type of the file Figure 11.1 Some common file types and their extensions

30. File Protection • In multi-user systems, file protection is of primary importance • We don’t want one user to be able to access another user’s files unless the access is specifically allowed • A file protection mechanism determines who can use a file and for what general purpose

31. File Protection • A file’s protection settings in the Unix operating system is divided into three categories • Owner • Group • World Page 356