A little hardware; a little software

CS 139 – 08/30/06

Algorithm ReviewIn lab yesterday, we built algorithms.Which was the best algorithm?Why?Four characteristics of good algorithms

PreciseSimpleCompleteCorrect

Next weekWe will work more on making the language of algorithms more precise. We may be able to do some of this today.

This sessionWe want to explore some of the environment in which we will implement our algorithms….the computer.Parts of the computerData storageOSFile Systems

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

Stored-Program Concept

Figure 5.1 The von Neumann architecture

The Fetch-Execute CycleFetch the next instructionDecode the instructionGet data if neededExecute the instruction

Memory

Memory is a collection of cells, each with a unique physical address

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RAM and ROMRAM 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 MemoryThe contents in locations in ROM cannot be changed

RAM is volatile, ROM is notThis means that RAM does not retain its bit configuration when the power is turned off, but ROM does

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

Magnetic DisksA read/write head travels across a spinning magnetic disk, retrieving or recording data

Figure 5.5 The organization of a magnetic disk

Compact DisksA CD drive uses a laser to read information stored optically on a plastic diskCD-ROM is Read-Only MemoryDVD stands for Digital Versatile Disk

Computer ComponentsConsider the following ad

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Sizes in Perspective

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File SystemsA file is a named collection of related dataA file system is the logical view that an operating system provides so that users can manage information as a collection of filesA file system is often organized by grouping files into directories

Directory Trees

Figure 11.4 A Windows directory tree

Directory TreesA 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 treeThe directory at the highest level is called the root directory

Figure 11.5 A Unix Directory Tree

Directory TreesAt 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

Path NamesTo 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 fileAn absolute path name begins at the root and specifies each step down the tree until it reaches the desired file or directoryA relative path name begins from the current working directory

Path NamesExamples of absolute pathC:\Program Files\MS Office\WinWord.exeC:\My Documents\letters\applications\vaTech.docC:\Windows\System\QuickTime

Suppose the current working directory isC:\My Documents\letters

Then the following relative path names could be usedcancelMag.docapplications\calState.doc

Exercise 1Open a Command PromptWhat is your working directory?Is there a config.sys file?What is in it?Change to \temp?Make a config.sys using edit.How can you refer to the other config.sys?Change to a different drive.

Text and Binary FilesIn a text file the bytes of data are organized as characters from the ASCII or Unicode character setsA binary file requires a specific interpretation of the bits based on the information in the file

Text and Binary FilesThe terms text file and binary file are somewhat misleadingThey seem to imply that the information in a text file is not stored as binary dataUltimately, 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

File TypesMost 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

File TypesFile names are often separated, usually by a period, into two parts

Main nameFile extension

The file extension indicates the type of the file

Figure 11.1 Some common file types and their extensions

File ProtectionIn multiuser systems, file protection is of primary importanceWe don’t want one user to be able to access another user’s files unless the access is specifically allowedA file protection mechanism determines who can use a file and for what general purpose

File ProtectionA file’s protection settings in the Unix operating system is divided into three categories

OwnerGroupWorld

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Problem SolvingProblem solving is the act of finding a solution to a perplexing, distressing, vexing, or unsettled question

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?

AlgorithmsAn algorithm is set of instructions for solving a problem or subproblem in a finite amount of time using a finite amount of dataThe instructions are unambiguous

Computer Problem-Solving

Figure 6.2 The computer problem-solving process

Figure 6.3: The Interactions Between Problem-Solving

Phases

Following an AlgorithmPreparing a Hollandaise sauce

Figure 6.4

Following an Algorithm (cont.)

Preparing a Hollandaise sauce

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A Computer ExampleProblem

Create an address list that includes each person’s name, address, telephone number, and e-mail address

This list should then be printed in alphabetical order

The names to be included in the list are on scraps of paper and business cards

A Computer Example

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A Computer Example

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A Computer Example

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A Computer Example

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