Beginning Snapshots

Chapter 0

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Objectives

Give an overview of computer science• Show its breadth

Provide context for computer science concepts• Events from the past

Describe basic components, organization of a computer

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Areas of Computer Science

Algorithms and Data Structures

Architecture

Artificial Intelligence and Robotics

Database and Information Retrieval

Human-Computer Communication

Numerical and Symbolic Computation

Operating Systems

Programming Languages

Software Methodology and Engineering

Social and Professional Context

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Important Concepts in Computer History

The mechanism of arithmetic

The stored program

The graphical user interface

The computer network

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Computing Systems

Computers have two kinds of components:Hardware – physical devices such as• CPU• memory• storage devices

Software – programs such as• Operating system• applications• utilities

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Hardware: CPU

Central Processing Unit (CPU):• the “brain” of the machine• Circuitry that performs arithmetic and logical ML

statements

CPU measurement • Speed (roughly) in megahertz

(millions of clock-ticks per second)

Examples• Intel Pentium, AMD K6, Motorola PowerPC, Sun

SPARC,

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Storage

Random Access Memory (RAM)• “Main” memory, which is fast, but volatile...• Analogous to a person’s short-term memory.• Many tiny “on-off” switches

• “on” is represented by 1, “off” by 0.

Each switch is called a binary digit, or bit.• 8 bits is called a byte.• 210 bytes =1024 bytes is called a kilobyte (1K)• 220 bytes is called a megabyte (1M).

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Storage

Secondary Memory (Disk):• Stable storage using magnetic or optical

media.• Analogous to a person’s long-term memory.• Slower to access than RAM.

Examples: • floppy disk (measured in kilobytes)• hard disk (measured in gigabytes (230 bytes))• CD-ROM (measured in megabytes), ...

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Input and Output

Input devices• Instructions and data must be encoded in

binary form and transmitted to the CPU

Examples:• keyboard• mouse, trackball, joystick• scanner• audio, video capture boards

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Input and Output

Output devices• Convert data from binary to another form

Examples• monitors, printers• sound, video• robotics control

Communication between CPU and peripheral devices is through ports• Ports communicate via the system bus

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Software: OS

OS acts as the “manager” of the system, • Making sure that each hardware device

interacts smoothly with the others.

Provides an interface • Enables user to interact with the computer, • Awaits user input if no application is running.

Examples: MacOS, Windows-95, Windows-NT, UNIX, Linux, Solaris, ...

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Software: Applications

Applications are non-OS programs • Perform some useful task• Including word processors, spreadsheets,

databases, web browsers, C++ compilers

Example C++ compilers/environments: • CodeWarrior (MacOS, Win95, WinNT,

Solaris) • GNU C++ (UNIX, Linux)• Turbo/Borland C++ (Win95, WinNT)• Visual C++ (Win95, WinNT)

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Software: User Programs

Programs that are neither OS programs nor applications are called user programs.

User programs are what you’ll be writing in this course.

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What is Programming?

Computer Program• A sequence of statements that instruct a

computer in how to solve a problem.

Programming• The act of designing, writing and maintaining

a program

Programmers • People who write programs

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What kinds of statementsdo computers understand?

A computer only understands machine language statements.

Characteristics of machine language• A sequence of ones and zeros • Cause the computer to perform a particular

action, such as add, subtract, multiply, ...

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Machine Language (ML)

ML statements • Stored in a computer’s memory

Computer memory is a sequence of switches. • an “on” switch is represented by 1, • an “off” switch is represented by 0.

ML thus appears to be binary (base-2):

0010111010110101

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Early Computers

... required a programmer to write in ML...

Problem• Easy to make mistakes!• Such mistakes are hard to find!• Not portable -- only runs on one kind of

machine!

Programming was very difficult!

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A Bright Idea

Devise a set of abbreviations (mnemonics) corresponding to the ML statements

Create a program (the assembler) to translate them into ML.

ADD 34, R1MOVE R1,1200CMPR R1, R2

100111001110010110100110110111

Assembler

Assembly languagestatements (mnemonics)

Machine language statements

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Advantages and Disadvantages

More natural than binary.

Much easier to read programs

Much easier to find and fix mistakes

Still not portable to different machines

Each machine had its own set of mnemonics

Each had its own assembler

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High Level Languages

Improvement on assembly language• Devise a set of statements that are close

to human language (if, while, do, ...)• Create a program to translate them into

ML.

The set of statements is called a high level language (HLL) The translation program is called a compiler.

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Contrast Assembler and HLL Compiler

Assembler translates one mnemonic into one ML statement

Compiler translates one HL statement into several ML statements

1010110011110101

0000000000010000

0010111010110101

0000000000010010

0010111011111101

0000000000010100

z = x + y; Compiler

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Advantage of HLLs

Programs are easier to read

Errors are easier to find

Programs are portable from one computer to another• Assumes the language is standard• Just create a new compiler which does the

translation into the correct ML

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Objectives in Programming A program should solve a problem:

Correctly

Efficiently

Readably

In user-friendly fashion

It actually solves the problem

Without wasting time or space

Understandable by another person

In a way that is easy for its user to use

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Summary of "Levels" of Computer Languages

Low levelML in binary language

Medium Level Assembly language

High Levelas in C++

Hard to read, not portable

Mnemonics, easier to read, still not portable

Reads like English and algebra, portable